. The endocrine organs; an introduction to the study of internal secretion . s epithelium becomes broken up and modified to form the reticulum and concentric corpuscles of the medulla. In Petromyzon the thymus retains its original connexion with the branchial clefts, but in all other vertebrates this connexion becomes lost, and in mammals the organ takes up a position either in the neck or in the anterior mediastinum. In birds it forms a series of isolated masses on each side of the neck. Small portions of thymus tissue are frequently found embedded in the thyroid, these having developed from
. The endocrine organs; an introduction to the study of internal secretion . s epithelium becomes broken up and modified to form the reticulum and concentric corpuscles of the medulla. In Petromyzon the thymus retains its original connexion with the branchial clefts, but in all other vertebrates this connexion becomes lost, and in mammals the organ takes up a position either in the neck or in the anterior mediastinum. In birds it forms a series of isolated masses on each side of the neck. Small portions of thymus tissue are frequently found embedded in the thyroid, these having developed from the sprouts of the fourth branchial pouch ; on the other hand, an accessory thyroid is often met with in the substance of the thymus. The thymus is singularly susceptible to theeffects of inanition; even a short period of fastingproduces a considerable diminution in its size. FUNCTIONS OF THE THYMUS The functions of the thymus are more obscurethan those of most of the endocrine organs:indeed there is room for question whether itshould be included amongst these bodies. But. FIG. 31.—Section of developingthymus. (From Prenant,Bouin, and Maillard.) c., cortex; m., medulla. At thisstage the medulla is continuousthroughout the gland. the mutual relations which appear to be estab-lished between it and the generative glands inthe male sex perhaps entitle it to occupy a place amongst them. Injection of extracts of the gland seems to have no specific results of feeding with the thymus appear also to be non-specific;such as are obtained seem to be dependent upon the large amount ofnuclein it contains. But according to Gudernatsch, tadpoles which arefed with thymus do not undergo metamorphosis, although they increasegreatly in size: the same observer describes exactly the opposite effectas the result of thyroid feeding. Noel Paton and Goodall, who removedthe thymus in a number of guinea-pigs, were not able to establish anyrelation of the gland to the rate of growth of these a
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