The chromatic responses of the chameleon have excited curiosity from the earliest times. They were known to Aristotle, who wrote concerning them: “the change in the colour of its skin takes place when it is filled with air. It can acquire either a black colour like that of a crocodile, or ochreous like that of a lizard, or spotted with black like the panther; for the eyes also change like the rest of the body, and so does the tail1” (Book II, History of Animals).

Nevertheless, the phenomena of colour change have been studied less extensively in Reptiles than in Fishes and Amphibia. Among those who have investigated the physiology of pigmentary response in the chameleon may be mentioned Bert, Brücke, Krukenberg and Keller. The only recent investigations on colour change in Reptiles worthy of note are embodied in the memoirs of Schmidt (1912) on the histology of reptilian pigmentary effectors, and those of Parker (1906) and Redfield (1918) on the behaviour of the Mexican homed “toad” Phrynosoma. Though a variety of pigments occur in the skin of Reptiles, the predominant agents of colour change in all cases are—as in Fishes and Amphibia—the melanophores. The melanophores of Reptiles are unicellular branched cells, and according to the unanimous testimony of Brucke, Keller, Thilenius, Carlton, Parker and Schmidt, their activity depends on the migration of the pigment along the cell processes. Three categories of external stimuli are known to be effective in the determination of colour change—light, temperature and mechanical or other nocuous stimuli. As in Amphibia and Fishes, Reptiles respond to warmth by pallor; but the reaction to bright illumination is darkening of the skin, whereas generally speaking—Necturus being a notable exception—the reverse is the case with Amphibia and Fishes. Both these reactions are essentially local (vide infra) and it has been held by many investigators that they depend on the direct excitation of the melano-phores. In the case of nocuous stimulation however locally applied stimuli evoke generalised pallor, so that a mechanism of co-ordination is evidently involved.

The nature of this mechanism was regarded by earlier workers as nervous. A somewhat different interpretation has been proffered on the other hand by Redfield. The phenomenon of excitement pallor was extensively studied by Redfield in Phrynosoma (1918). As the result of his investigations this worker came to the conclusion that the main factor in distributing the stimulus to chromatic response after nocuous stimulation is the liberation of adrenaline into the circulation. It had long been known that adrenaline induces contraction of the melano-phores of Vertebrates. And the new evidence adduced by Redfield seemed to point very strongly to an endocrine control of excitement pallor. The chief points which he records are : (a) failure of animals to respond to electrical stimulation of the roof of the mouth after section of the cord at the point between the eighth and thirteenth vertebra, (6) failure of animals to respond in most cases after epi-nephrectomy, (c) failure of local section of nerves to selected areas to interfere with the responses in any way.

Following as they did upon Cannan’s researches into the rôle of the adrenals in sympathomimetic accompaniments of excitement in the Mammal, these results were fully consonant with what appeared to be the correct explanation of the consequences of prolonged excitement in warm-blooded Vertebrates. The criticisms that have been brought to bear upon the work of Caiman and his collaborators by Stewart and his colleagues have re-opened the question; and it is now very doubtful whether it is possible to define any specific conditions in which increased liberation of adrenaline from the suprarenal medulla of intact mammals takes place. Consequently the determination of excitement pallor in Reptiles acquires, as Stewart himself has observed, a new interest in relation to the attempt to interpret a physiological, as opposed to pharmacodynamic, rôle for adrenaline in the animal body.

Phrynosoma, though displaying the chromatic reaction more noticeably than most other common lizards of the North American continent, is by no means the most felicitous type to select for the study of this phenomenon. As is proverbial, the chameleons among Lacertilia afford the most striking display of colour change, and this family is well represented in South Africa. The viviparous species Chamaeleo pumilus is particularly abundant in the Cape Peninsula, and was the form selected by the writers for investigation. There does not seem to have been any investigations on the control of colour response in the chameleon, since the intervention of endocrine agencies (cf. Hogben and Winton, 1922–23 and Hogben, 1924) in the co-ordination of pigmentary effector activity has been established.

Individual chameleons vary considerably in colour, a fact that had given rise to the popular misconception that a single individual can change from any one colour to any other. Needless to say any given individual can only change from a definite light to a definite dark tint. Selected individuals which in the dark condition were a deep olive, changing through green to yellow, were employed in the experiments recorded below. All the individuals were females. It is a curious fact that of about two hundred animals collected for these experiments, not more than half a dozen were males.

As indicated above, the main purpose of the investigation was to determine whether the phenomenon of excitement pallor provides evidence of conditions under which the adrenals discharge their active product into the blood stream in increased amount. It was necessary however for this purpose to confirm earlier observations on the normal sequence of pigmentary changes in the species investigated. As regards the influence of light, there is no doubt about the salient facts which have been established with a few exceptions for reptiles in general, viz. (1) that the exclusion of the organs of vision does not prevent the darkening which accompanies exposure to bright illumination at ordinary temperatures (10—20 ° C.) (Bert, Keller, Krukenberg); (2) that areas locally illuminated respond locally by darkening (Bert, Brücke, Keller, Parker, Redfield). The conclusion drawn from these facts by most investigators has been that reptilian melanophores respond directly to light without the intervention of the nervous system, and Redfield (1918) upholds this view with experiments in which peripheral sections of the nerve supply of local areas was carried out. Brücke however regarded the phenomenon as dependent on nervous co-ordination, a possibility not to be excluded in view of the evidence brought forward by Parker (1906) to show that the skin of Amphibia possesses photo-receptors of a simple kind. Brücke based his view on the effects of spinal transection. His results have been criticised by Fuchs on the grounds that his preparations had not recovered effects of “shock.” In the course of the present investigations chameleons were kept alive after section of the cord or cord and sympathetic chain, for more than a week, and in all cases where the region posterior to the point of section failed to respond to stimulation of the mouth, the exclusion of light only resulted in pallor anterior to the point of section. Since these animals responded by pallor to electrical stimulation of the cloaca on the side posterior to the point of section, the results of shock may be said to have been adequately eliminated. Similarly with regard to temperature, exposure to a source of warmth in chameleons in which the cord and chain had been cut at say the fourteenth vertebra resulted in complete pallor anterior to the point of section only. These data might be interpreted in the sense defined above, as inferred by Brücke. His interpretation may be put explicitly as follows : (1) that the melanophores of the chameleon are normally maintained in a state of “tone” controlled by a centre in the anterior part of the c.N.s. ; (2) that light acts reflexly through this centre, by releasing them from this state of tone; (3) that warmth reflexly augments the tonic control of the melanophores. On the other hand, the data could equally well be harmonised with Redfield’s view by different assumptions, namely, that the state of expansion of the melanophores at any given moment is due partly to a more or less continuous state of tone, partly to the direct action of light in releasing them from this extrinsic control, partly to the effect of warmth in directly influencing them so as to augment cumulatively the tone effect. On this view the pallor that normally occurs in darkness would result from the fact that the tonic control is no longer antagonised by light while the failure when darkened to exhibit pallor in the region posterior to the point of section of the cord would be due to the fact that the impulses tending to keep the melanophores in a state of sustained contraction no longer reach the area in question. Without cutting all dorsal roots of the spinal nerves, a well-nigh impossible operation, there is no apparent method of distinguishing between the two alternatives experimentally. The latter is however the more economical hypothesis.

All the experiments which follow deal with the analysis of excitement pallor. Chameleons do not readily respond to rough handling by pallor; and even electrical stimulation of the skin usually evokes only localised effects. On the other hand the application of a faradic current from an ordinary shocking coil to the roof of the mouth or merely mechanical stimulation of the cloaca calls forth a generalised lightening of tint. Mechanical stimulation of the roof of the mouth failed to evoke the response. The time relations of excitement pallor are worth noting in contrast with the phenomena of colour change in Amphibia. They are illustrated in the following protocol :

In general it was found that pallor was complete within from half to two minutes from the beginning of stimulation. All experiments in relation to this question were performed on chameleons kept in bright light at room temperature (18–21° C.). The conditions were such as to ensure that the skin remained dark during the course of the operations except when stimulated.

The generalised pallor which follows after stimulation of the roof of the mouth or cloaca in the manner indicated implies the intervention of a co-ordinating mechanism. To ascertain how far nervous and endocrine agencies enter into this phenomenon, the first question to be investigated was the effect of section of the c.N.s. at different levels; and as there was little difficulty in obtaining large supplies of chameleons, it was possible to investigate this issue extensively by experiments involving (1) section of the cord alone at different levels, (2) section of the cord and both of the sympathetic chains simultaneously at different levels, (3) section of the cord with unilateral section of the chain, (4) section of the chain alone.

(a) Section of the cord alone

When the cord is cut at any level in the trunk region, stimulation of the cloaca produces pallor only on the side posterior to the cut (Text-fig. 3). The two regions as in all the segmental effects hereunder described are very sharply defined (Text-fig. 2). When the cord is cut at any level anterior to a point corresponding to the tenth or eleventh vertebra—in extreme cases the critical point varied individually from behind the ninth to the twelfth in one instance—stimulation of the roof of the mouth results in complete pallor on the side anterior to the point of section, while the region behind remains completely dark (see Text-fig. 1, a, b, c). How striking is the localisation of the response may be inferred from an actual photograph here reproduced (p. 300). Behind this region—from the eleventh or twelfth vertebra backwards that is to say—the effects of section of the cord alone were quite different (Text-fig. 1, d). On stimulation of the roof the mouth in animals so treated, the result obtained was a generalised pallor affecting the whole body with the exception in nearly all cases of a short region at the distal extremity of the tail. These experiences, obtained repeatedly on a large series of animals, showed conclusively that a nervous agency of some kind is the main factor in the co-ordinating mechanism; but they leave open the possibility that adrenal secretion plays an adjuvant rôle in the phenomenon. For, if the nerve supply of the adrenals emerges just in front of the tenth or eleventh vertebra, it is possible that the pallor of the posterior half of the animal when the cord is cut behind this point results from liberation of adrenaline into the circulation. This possibility did not seem at the outset a likely one in view of the consequence obtained from cloacal stimulation after transection of the cord in the same region of the trunk, and in view of the fact that the generalised pallor resulting from stimulation of the mouth after transection at the level of e.g. the thirteenth or fourteenth vertebra was perfectly uniform both as regards its extent and rate of development. As will be seen, the effects of section of the cord and chain simultaneously provide conclusive evidence for rejecting the view that adrenal secretion has any significant influence in contributing to excitement pallor. One fact however is worthy of mention here. In order to exclude the possibility that adrenaline secretion may reinforce the effects of continued stimulation, stimulation of the mouth or cloaca in cases, where purely segmental responses occurred, was in some cases protracted for a period of ten minutes or even a quarter of an hour.

(b) Section of the cord and chain

When in addition to cutting the cord, the sympathetic chain is cut on both sides at the same level, the result of stimulating the mouth is a pallor confined to the region anterior to the point of section, even when the cut is made in the region behind the eleventh vertebra (Text-fig. 4). In two experiments in which the cord was cut at the level of the twelfth and thirteenth vertebra, and the chain was cut on both sides at the level of the sixteenth vertebra, pallor extended after stimulation of the roof of the mouth beyond the point of transection of the cord as far as the level, where the chain was cut. In this case the chain was cut behind the region where the adrenals are located.

Fig. 1.

Effects of stimulation of mouth after section of cord only at different levels. (The shaded area remains dark.)

Fig. 1.

Effects of stimulation of mouth after section of cord only at different levels. (The shaded area remains dark.)

Fig. 2.

Photograph of chameleon showing pallor anterior to point of section at tenth vertebra after stimulation of roof of mouth.

Fig. 2.

Photograph of chameleon showing pallor anterior to point of section at tenth vertebra after stimulation of roof of mouth.

Fig. 3.

Effects of stimulation of cloaca after section of cord only in posterior region (12th and 15th vertebra).

Fig. 3.

Effects of stimulation of cloaca after section of cord only in posterior region (12th and 15th vertebra).

Fig. 4.

Effects of stimulation of mouth after section of cord and sympathetic chain of both sides in posterior region.

Fig. 4.

Effects of stimulation of mouth after section of cord and sympathetic chain of both sides in posterior region.

(c) Section of the cord with unilateral section of the chain

The most conclusive evidence in favour of a purely nervous interpretation of the determination of excitement pallor in the chameleon lies in the effects of spinal transection with unilateral section of the sympathetic chain (see Text-fig. 5). In this case the region posterior to the point of section remained dark on the side on which the chain was cut, while the side on which the chain was intact exhibited generalised pallor after stimulation of the roof of the mouth.

Fig. 5.

Effects of stimulation of mouth after section of cord and unilateral section of sympathetic chain :

I a. Left side: chain cut at same level as cord (13th vertebra).

I b. Right side: chain intact.

II a. Left side : chain intact.

II b. Right side : chain cut at same level as cord (12th vertebra).

Fig. 5.

Effects of stimulation of mouth after section of cord and unilateral section of sympathetic chain :

I a. Left side: chain cut at same level as cord (13th vertebra).

I b. Right side: chain intact.

II a. Left side : chain intact.

II b. Right side : chain cut at same level as cord (12th vertebra).

(d) Section of the chain only

Section of the chain only does not interfere with the generalised pallor which follows stimulation of the roof of the mouth. But in one experiment (Text-fig. 6) in which the cut went through a large ganglion in the posterior region of the trunk a narrow band of skin remained dark in the segment of the cut.

Fig. 6.

Effect of stimulation of the mouth after section of chain involving partial destruction of a ganglion in the region of the thirteenth vertebra without section of cord.

Fig. 6.

Effect of stimulation of the mouth after section of chain involving partial destruction of a ganglion in the region of the thirteenth vertebra without section of cord.

The evidence brought forward in section 3 clearly indicates that the pallor resulting from nocuous stimulation is predominantly, if not wholly, nervous. Two alternatives now present themselves : is this control exercised through direct innervation of the pigmentary effectors, or is the production of pallor due to some chemical conditions such as oxygen-want, etc., resulting from extreme constriction (or dilation) of the peripheral arterioles? As has been pointed out by the senior author, the latter possibility has too frequently been overlooked. In this case it can be rejected on experimental grounds by cutting out the influence of the circulation. If the body is divided into strips by section at right angles to the caudal cephalad axis, pallor can always be induced in isolated segments of the trunk by electrical stimulation of the caudal end of the C.N.S. The experience can more-over be repeated again and again on the same strip. It is very difficult to believe that a reversible effect of this kind could be repeated after the circulation had been stopped, unless the pigmentary effector organs were in direct connection with the central nervous system. And it thus seems legitimate to draw the conclusion that the production of excitement pallor depends on the direct innervation of the melanophores in Reptiles. It should be borne in mind that there is no available histological evidence for the direct innervation of the pigmentary effectors in Reptiles and Amphibia, though the nervous connections of the Melanophores of Fishes have been observed by more than one investigator.

With this direct evidence available, it is possible to map the innervation of the pigmentary effector system in the chameleon on the basis of the experiments re-corded in the preceding section. All the results therein recorded are represented diagrammatically in Text-fig. 7. The experiments show that the efferent impulses are distributed by the sympathetic nervous system, and from the fact that stimulation of the cloaca after transection of the cord at any level results in pallor posterior to the cut only, it may be concluded that post-ganglionic neurones supplying each segment of the trunk are connected by pre-ganglionic neurones to the cord in the same segment; and further, that there are no ascending nerve paths in the chain. The same assumption explains why the section of the chain alone does not prevent the production of pallor behind the cut, when the mouth is stimulated. The segmental character of the response to stimulation of the mouth after section of the cord in front of the tenth vertebra indicates that up to this level there are no descending pre-ganglionic neurones in the sympathetic chain; while the fact that generalised pallor follows after the stimulation of the mouth when the cord alone is cut behind the 11th–12th vertebra, indicates that there pass into the chain about the level of the 10th–12th vertebra some pre-ganglionic neurones which have a descending course, distributing impulses leaving the cord at this point to segments posterior to it. The fact that the tip of the tail remains dark after section of the cord in the region behind the 11th–12th vertebra is explicable on the assumption that these descending neurones terminate in front of the region where the post-ganglionic neurones supplying the pigmentary effector organs at the extremity of the tail arise. The same assumptions account for the effects of unilateral section of the chain at the same level as the cord and of section of the chain at a lower level than that at which the cord is cut in the same region.

Fig. 7.

Diagrammatic representation of the nerve paths involved in the control of the pigmentary effector system of the chameleon on the basis of the experiments recorded in the text.

For the purpose of diagrammatisation the number of ganglia is reduced, and the ascending and descending afferent paths from cloaca and mouth respectively are represented in each case by a single neurone. Section of the cord alone anterior to A restricts the pallor following stimulation of the mouth to the region in front of the cut. After section of the cord alone at the level indicated by B, generalised pallor involving the hind extremities with the exception of the tip of the tail follows stimulation of the roof of the mouth.

Fig. 7.

Diagrammatic representation of the nerve paths involved in the control of the pigmentary effector system of the chameleon on the basis of the experiments recorded in the text.

For the purpose of diagrammatisation the number of ganglia is reduced, and the ascending and descending afferent paths from cloaca and mouth respectively are represented in each case by a single neurone. Section of the cord alone anterior to A restricts the pallor following stimulation of the mouth to the region in front of the cut. After section of the cord alone at the level indicated by B, generalised pallor involving the hind extremities with the exception of the tip of the tail follows stimulation of the roof of the mouth.

Some further light is thrown on the possibility that adrenaline secretion enters significantly into the phenomenon of excitement pallor in the chameleon by a study of the minimal effective dose. In the first two experiments synthetic adrenaline (suprarenin hydrochloride synth. Hoechst) was employed. In all cases the injection was intraperitoneal to ensure rapid absorption, and the dosage indicates the equivalent amount in 1 c.c. of saline solution.

Experiment I

Two medium sized chameleons were injected with 1 : 10,000. General pallor resulted within five minutes. The pallor was more intense, and the hue more yellowish than in any case of pallor which supervened after nocuous stimulation. The condition persisted for about 36 hours.

A chameleon of the same size (as nearly as possible) became completely pale within ten minutes after an injection of 1 : 50,000. The condition persisted for the period of three hours during which it was under observation. The pallor in this case was accompanied by the same intensely yellow tint. Three other chameleons of about the same size received respectively 1 : 150,000, 1 : 300,000 and 1 : 500,000. An incomplete and somewhat patchy pallor in no case extending over the whole body ensued in all three cases, and subsided within an hour. Four chameleons which received a dosage of 1 : 1,000,000, 1 : 5,000,000, 1 : 10,000,000, 1 : 50,000,000 displayed no response whatever.

Experiment II

A second series of chameleons were injected via the peritoneum with 1 c.c. in each case of synthetic adrenaline in the following dilutions 1 : 25,000, 1 : 50,000, 1 : 100,000, 1 : 200,000, 1 : 400,000,1 : 800,000,1 : 1,000,000. Those that received 1 : 100,000 or the stronger solutions displayed complete pallor after 15 minutes. The pallor was still maximal after two hours. When examined ten hours later, the pallor had not completely disappeared. Those that received doses of 1 : 200,000 and 1 : 400,000 did not display complete pallor, but gave a noticeable reaction persisting at least two hours. With solutions of 1 : 800,000 and 1 :1,000,000 the results were entirely negative.

Experiment III

Seven chameleons were injected (intraperitoneal) each with 1 c.c. of the following solutions of Parke Davis’ adrenaline 1 :100,000,1 :150,000, 1 : 200,000, 1 : 300,000, 1 : 400,000, 1 : 500,000, 1 : 600,000. After 15 minutes the first three were incompletely pale. The last showed no reaction. A slight pallor was displayed by the remainder. After 30 minutes, Nos. 1, 2, 3 were completely pale, Nos. 4, 5, 6 still displayed a slight degree of pallor, and No. 7 a doubtful reaction. After 114 hours, Nos. 1,2,3 showed complete pallor, Nos. 4,5,6 slight pallor. After ten hours, Nos. 1,2,3 were still noticeably paler than the controls and No. 4 showed a slight pallor. Nos. 5, 6, 7 were all completely dark.

From these observations it is seen that there is a fairly definite threshold effective dose for the adrenaline pallor reaction, that the minimal quantity is from a physiological standpoint very considerable, and that this quantity produces a response which persists for a period which is very protracted in comparison with the effects observed in connection with the phenomenon of excitement pallor. It is clear that neither of these conclusions reinforce the likelihood that adrenal secretion plays any significant part in the production of excitement pallor.

The persistence of the response to adrenaline secretion is in keeping with the effects of adrenaline on other effector systems in cold-blooded animals. Thus the pressor effect which in mammals is so characteristically evanescent may be prolonged over a period of over a quarter of an hour in the case of the tortoise (Hogben and Schlapp, 1924). Whatever explanation may be given to this persistence, it implies that if adrenal secretion entered into the phenomenon of excitement pallor, prolonged nocuous stimulation should evoke a more lasting pallor than stimulation for shorter periods. Whether such is actually the case may be inferred from the following protocol :

Thus in the case of a stimulus applied for one minute recovery took three minutes. With a stimulus of five minutes duration, recovery took place in two and a half minutes, and with more prolonged stimulation recovery supervened before the cessation of stimulation.

Effects of Epinephrectomy

The effects of removal of the adrenals were investigated on a few acute preparations only. Had the results been otherwise the conclusion drawn from them would be of doubtful value unless confirmed by experiment on chronic preparations. Unfortunately, although the adrenals are in an eminently accessible situation for operative procedure, the chameleon did not prove to be a very viable animal for laboratory purposes, and preparations that apparently recovered well immediately after the operation did not survive till the wound was healed. However the application of nocuous stimuli to the mouth in animals in which the cord was intact or cut at the level of the thirteenth to fourteenth vertebra produced generalised pallor, except in cases where the chain which lies very close to the adrenals was cut or damaged in the course of removing the latter. It was thus possible to find no evidence of participation of increased adrenal secretion in the phenomenon of excitement pallor in the chameleon.

Far too much importance has been attached by comparative physiologists to the action of drugs, as indicative of nervous control of effector structures, apparently under the misapprehension in some cases that drug action is a good deal more specific than is actually the case. It cannot be too strongly emphasised that most of the well-known drugs have a general protoplasmic action, and that their selective action is generally speaking quantitative rather than qualitative. Conclusions based on this line of evidence as brought forward in a recent paper by Carter (1925) on the supposed innervation of the cilia of veliger larvae are of little significance, if unsupported by more direct forms of experimental procedure. In the experiments which have been conducted by us an attempt was therefore made to discriminate between peripheral and central action. The results are essentially what would be expected from the direct evidence that the melanophores of the chameleon are under c.N.s. control, tending to keep them in the contracted state. The following experiments show that strychnine and caffeine act through the C.N.S. by augmentation of the activity of the chromatic centre, and that the equally characteristic action of curare and atropine in the opposite sense is due to paralysis of nerve endings. Taken in conjunction with the foregoing experiments, the new data reinforce conclusions already inferred from an independent source.

A few data regarding the action of drugs on the pigmentary effector system of Reptiles are available from the work of previous authors, who did not attempt to identify the seat of action, except in the case of curare, where the darkening previously observed by Bert to follow injection was shown conclusively by Krukenberg to be due to peripheral action on the nerve endings. Pallor following the injection of strychnine is recorded by Brücke, Krukenberg and Keller, and an identical response to caffeine is recorded by Krukenberg, who also observed darkening after administration of atropine.

Action of Strychnine

Conclusive proof that the pallor which accompanies the excited state following strychnine injection acts through the C.N.S. is afforded by experiments of which the following protocol is typical :

Action of Cocaine

The action of cocaine at first raised the hope of discriminating between the alternative hypotheses to account for the light reaction adumbrated in § 2. A dose of 0.001 gm. injected into the cord in the tail region produced pallor which in a few seconds manifested itself near the seat of injection and gradually crept forwards till maximal pallor of that intense yellowish tint so characteristic of the response to adrenaline was general within 3—5 minutes later. Since cocaine acts specially on the sensory nerve endings, and since the propagation of the effect suggested a nervous route, the inference at first appeared to justify the view that light inhibits the chromatic centre reflexly through photo-receptors in the skin (cf. § 2 above). This conclusion did not prove to be sustained by further examination, as is shown by the following facts:

  • (1) Injection of cocaine into a chronic preparation with section of cord and chain at about the level of the thirteenth vertebra produced generalised pallor.

  • (2) In chameleons injected with cocaine subsequent section of the cord and chain did not affect the generalised character of the response, nor did complete destruction of the cord in the posterior part of the trunk region.

It is thus clear that the cocaine pallor is not due simply to interference with afferent paths, since it still occurs when all efferent routes are destroyed.

Action of Histamine

Histamine has a very similar action to that of adrenaline and cocaine. An injection of 0.001 gm. (intraperitoneal) produces complete generalised pallor within ten minutes accompanied by twitchings with complete recovery within 12 hours. The influence of histamine is not upon the chromatic centre since section of the cord and chain does not affect the character of the response as is the case with strychnine.

Action of Caffeine

Injection of 1 c.c. 1 per cent, solution of caffeine produced pallor within ten minutes as would be expected. After section of the cord and chain at about the thirteenth vertebra injection of the same quality produced pallor anterior to the point of section. The following experiment illustrates both the well-known action of caffeine on the higher centres of the c.N.s. and the decussation of fibres below the chromatic centre. The right side of the brain of a chameleon was destroyed after section of the cord about the level of the ninth vertebra. Injection of the minimal effective dose of caffeine then produced pallor on the right side of the body anterior to the point of section of the cord.

Action of Curare and Atropine

An injection of 1 c.c. 1 per cent, curare in pale chameleons kept in darkness produces melanophore expansion with general mortu-paralysis. The same quantity of atropine produced darkening without paralysis. Stimulation of the cord after darkening produced by atropine and curare did not result in pallor over the region innervated by the part of the c.N.s. stimulated.

The main outcome of this investigation may be stated as follows :

  • (1) The pigmentary effector system of the chameleon is directly innervated through the C.N.S., and the production of excitement pallor is determined through this agency.

  • (2) The light and heat reactions are not wholly independent of the C.N.S. A chromatic centre in the brain at least plays a static rôle in tending to keep the melanophores in a state of contraction, except when released from that condition by external stimuli.

  • (3) While a number of circumstances make it unlikely that adrenaline plays any part in the production of excitement pallor, no evidence could be found to support the view that excitement pallor is conditioned or reinforced by active liberation of adrenaline into the circulation.

The authors are unable to offer any explanation of the difference between their own findings and those of Redfield. It seems unlikely, in view of the striking uniformity of the characteristic features of Reptilian pigmentary effector activity, that different mechanisms are involved in the production of excitement pallor in the chameleon and in Phrynosoma. Until the influence of adrenal secretion has been demonstrated in material more favourable for the study of colour response than in Phrynosoma, it is legitimate to express the doubt that the phenomena of colour response in Reptiles provides conclusive evidence of the possibility of defining conditions in which the liberation of adrenaline into the vertebrate circulation in increased quantity takes place.

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1

Cresswell’s translation.