The anatomical relations of the thymus gland in mice have been studied in serial sections throughout the entire thoracic and posterior cervical regions. No evidence was obtained that the thymus has a secretory function under the control of the nervous system, and no nerve fibres or endings were observed in the proximity of the thymocytes, Hassall's corpuscles, or reticular cells. The thymic innervation resembles that of the lymph nodes rather than that of the adrenal gland or of the spleen and the neuro-histological picture was unchanged during accidental or age involutions. The thymic innervation consists entirely of autonomic vasomotor nerves originating in an outlying sympathetic plexus--the ‘thymic plexus’, and it is suggested that thymus functions are controlled by vasodilatator and vasoconstrictor mechanisms with the possible aid of thymotropic hormones carried in by the blood-stream. The thymus may aid in the regulation of the lymphocyte content of the blood, acting as a lymphocytic reservoir instead of as a lymphocytopoietic organ.

The source of thymic innervation could not be determined solely by anatomical methods and the majority of current histological techniques were found to be unsuitable for differentiating between nervous and non-nervous argyrophilic fibres. The basic techniques of Holmes and Romanes appeared to satisfy the necessary criteria and Steedman's ester wax method was successfully used in conjunction with these techniques.

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