The primitive condition of the muscles related to the visceral arches of the gnathostome fishes was, as Vetter long ago concluded, a simple constrictor muscle in each arch, and associated with this muscle there was a branchial bar which lay internal to the muscle.

From this simple primitive condition two distinctly different lines of descent are indicated by later differentiations of the muscles, and these differing differentiations are associated with, and caused by, two distinctly different forms of branchial bar in the branchial arches. One of these two lines of descent is represented by the Teleostomi and the other by the Plagiostomi, the Holocephali and Dipneusti apparently occupying somewhat intermediate positions.

In the Teleostomi, the four typical elements of each branchial bar of recent fishes lie, approximately, in a single plane, and this must have been their primitive relation to each other. Primarily this plane must have been transverse to the axis of the body, but it later became inclined to that axis. Associated with this form of arch the branchial filaments of the gill-bearing arches are supported by cartilaginous or osseous rods. In the hyal arch there are, in addition, osseous branchiostegal rays which lie anterior to the modified constrictor of the arch.

In the Plagiostomi, the dorsal and ventral elements of each branchial bar are directed postero-mesially at a marked angle to the middle elements of the bar, these latter elements lying, as in the Teleostomi, in a plane inclined to the axis of the body. A sigina form of bar is thus produced, and associated with it there are cartilaginous branchial rays in all the gill-bearing arches. These cartilaginous rays all lie, primarily, posterior to the constrictor muscle of the related arch, but the muscle fibres may later become in part inserted on them.

In the Holocephali and Dipneusti, the dorsal elements of the branchial bars are directed postero-mesially, as they are in the Plagiostomi, while the ventral elements are directed antero-mesially, as in the Teleostomi. In the Holocephali there are, according to Vetter, cartilaginous rays both in the hyal and the branchial arches, and the visceral muscles as described by him seem plagiostoman in character. In the Dipneusti there are cartilages in the hyal arch that are considered by Fürbringer to be branchial rays, but there are neither branchial rays nor supporting rods to the branchial filaments in the branchial arches; and the branchial muscles are teleostoman in character.

The constrictor muscle is found in a more primitive condition in the Selachii than in any others of the gnathostome fishes. Because of the sigma form of branchial bar in these fishes, the proximal (anterior) edge of the constrictor of each branchial arch has slipped forward over the anterior edge of the middle, posteriorly-directed angle of the sigma, and backward over the posterior edges of the dorsal and ventral, anteriorly-directed angles of the sigma; and from the parts of the constrictor that cross or span these three angles are differentiated, respectively, the adductores arcuum branchialium, the arcuales and interarcuales dorsales, and the coracobranchiales of Dohrn's descriptions of embryos. These latter muscles are simply the proximal (anterior) portions of the ventral ends of the primitive constrictores of the branchial arches, they are of branchial origin, are innervated by branches of the nervus vagus of the arch to which they belong, and they coexist, in the adult, with the coracobranchiales of Vetter's descriptions. The latter muscles are said, by both Dohrn and Edgeworth, to be derived from the ventral ends of the branchial myotomes, but their innervation, in the adult, by spinal or spino-occipital nerves, their relations to the other hypobranchial muscles, and the marked want of accord in-the descriptions of their development, all warrant the conclusion that they must be of spinal origin.

The distal (posterior) portion of the constrictor muscle of each branchial arch of the Selachii, the so-called constrictor superficialis, lay primarily not only on the anterior surface of the branchial rays of its arch, but also on that surface of the extrabranchials of its arch; and, in the adults of recent fishes, its dorsal and ventral ends turn posteriorly, to a greater or less extent, across the dorsal and ventral edges, respectively, of the next posterior gill-pouch. When the constrictor contracted, the muscle was accordingly stretched across the extrabranchials of its arch, and certain of the muscle fibres, in certain fishes, were there cut in two by acquiring insertion on the extrabranchials. Other fibres simply became tendinous where they passed over the extrabranchials, and so there gave rise to more or less pronounced linear aponeuroses, or so-called septa. That part of each constrictor that lay between the dorsal and ventral extrabranchials of its arch thus became more or less cut out of the primarily continuous constrictor, and formed the muscules interbranchialis. This muscle is never found definitely differentiated in the hyal arch, but indications of the beginnings of its differentiation may there be found.

In the hyal arch of the Selachii, an adductor muscle was probably developed exactly as in the branchial arches, but it was later transformed into the inferior postspiracular ligament. Arcualis and interarcualis muscles are not found in this arch of the adult, but Dohrn says that they are represented, in embryos, by ligaments, which he does not, however, describe. The coracobranchialis of Dohrn's descriptions of the branchial arches is not differentiated in this arch. The dorsal and ventral portions of the constrictor tend to separate into deeper and superficial layers, as Ruge has stated, the deeper layer retaining its primitive insertion on the cartilaginous bar of its arch, while the superficial layer acquires a secondary insertion on the cartilaginous bar of the mandibular arch.

In the Batoidei, an adductor muscle was not differentiated in the hyal arch, and there is accordingly, in these fishes, no inferior postspiracular ligament. The proximal (anterior) portion of the primitive constrictor of this arch is differentiated into the levator and depressor hyomandibularis, and probably also the depressor mandibulæ, these muscles replacing the adductor, arcualis, interarcualis, and Dohrn's coracobranchialis of the Selachii. The so-called septa in these fishes are probably similar to those in the Selachii, but this cannot be definitely determined from existing descriptions.

In the mandibular arch of the Selachii, a small adductor muscle was probably developed exactly as in the more posterior arches. Later, because of the suppression of the branchial diaphragm related to this arch, excepting as it may be represented in the anterior wall of the spiracular canal, the entire constrictor muscle was forced, in its middle portion, onto the anterior surface of the cartilaginous bar of its arch, and, acquiring insertion on the palatoquadrate and mandibula, where it crossed their lateral edges, was added to the small, pre-existing adductor, and so gave rise to the large adductor mandibulæ actually found in the adult. From those portions of the primarily continuous constrictor that lay dorsal and ventral, respectively, to the palatoquadrate and mandibula, the musculi levator maxillæ superioris and intermandibularis were developed. The musculus levator anguli oris was probably derived from the anterior edge of the united levator and adductor muscles before they became separated from each other. The musculus intermandibularis underwent relative reduction, and, in the adults of recent fishes, is largely crowded out and replaced by those superficial fibres of the hyal constrictor that have secondarily acquired insertion on the mandibula. The relations of the nervus hyoideus facialis to the muscle fibres thus inserted on the mandibula is against the view that those fibres that are of mandibular origin have lost their primitive innervation by the nervus trigeminus and secondarily acquired innervation by the nervus facialis.

The posteriorly directed dorsal and ventral ends of the hyal and branchial constrictores of the Selachii always overlap, to a greater or less extent, the next posterior constrictor. Where the ends of the constrictores are strongly inclined posteriorly, they may overlap two or more posterior constrictores, the fibres of the muscles then crossing the extrabranchials of those arches and there tending to become tendinous exactly as they do where they cross the extrabranchials of their own arches; a series of tendinous aponeuroses thus being formed in each constrictor. The overlapping muscles then fuse more or less completely with each other, and, as the linear aponeuroses related to each extrabranchial are superimposed and transverse to the muscle fibres, the continuous muscle-sheet formed by the fusion of the several constrictores is cut up into what have heretofore been considered to be separate segments, one related to each branchial arch and developed entirely from the myotome of that arch. These segments are, however, each formed by muscle fibres derived from two or more consecutive constrictors, and hence from a similar number of consecutive myotomes.

The dorsal portion of the constrictor of the ultimate branchial arch undergoes excessive development and becomes the musculus trapezius.

In the Teleostomi, each branchial bar. although inclined to the axis of the body as in the Selachii, continues to lie, approximately, in a single plane, and the dorsal and ventral ends of the constrictores do not turn posteriorly as in the Selachii. The pull of the constrictor, when contracting, did not, accordingly, tend to make the muscle slip, in the middle of its length, over the anterior edge of the branchial bar of its arch, but in certain of the branchial arches of the Ganoidei the proximal edge of the muscle slipped over the posterior edge of the branchial bar, and there gave rise to an adductor that is the functional equivalent but not the homologue of tho adductor of the Selachii. The remaining fibres of the middle portion of the constrictor either later aborted or, possibly, became modified to form the radially arranged muscles related to the supporting rods of the branchial filaments. The dorsal and ventral ends of the constrictores became the levatores and the transversi and obliqui dorsales and ventrales. The levator of the ultimate arch is a slender muscle, and may secondarily acquire insertion on the shouldergirdle. It is the homologue of the large musculus trapezius of the Selachii.

In the hyal arch of the Teleostomi, the constrictor persists to a greater extent than in the branchial arches. Its dorsal portion becomes the adductor hyomandibularis and the adductor and levator operculi, these muscles, together, being the equivalent of the levatores of the branchial arches of the Teleostomi and of the dorsal ends of the constrictores of the Selachii.

The ventral portion of the constrictor of the ultimate, or fifth, branchial arch of the Teleostomi is modified to form the musculi coracobranchiales or pharyngoclaviculares, these muscles of these fishes thus being branchial muscles, and hence probably not the homologues of the coracobranchiales of the Selachii. They always retain, in all fishes that I have been able to examine, their primitive innervation by branches of the nervus vagus. The dorsal portion of the constrictor of this arch forms, as already stated, the fifth levator muscle, which is the homologue of the musculus trapezius of the Sclachii.

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