460 COMPARATIVE ANATOMY 



back as far as the posterior border of the thyroid gland. The migration 

 of the cells of the fourth pouch is less extensive, and they usually come to 

 lie near the anterior border of the thyroid. The epithelial character of 

 the cells of the parathyroid is retained during the histogenesis of the 

 gland. Mesenchyme cells work their way into the anlage, forming blood 

 vessels and breaking up the cellular mass into cords. (Fig. 381) 



Evolution. Parathyroid glands occur in all vertebrates except fishes, 

 making their first appearance in Amphibia in which the gills have begun 

 to degenerate. The factors involved in the emergence of their endocrinal 

 function are at present speculative, since to say that they owe their origin 

 to gill degeneration means little. Comparison of lower with higher 

 vertebrates reveals a progressive reduction in number but apparent 

 increase in endocrinal importance. 



THE ULTIMOBRANCHIAL BODIES 



Among the pharyngeal derivatives which may have an embryonic 

 endocrinal function are the ultimobranchial bodies, called also post- 

 branchial and suprapostcardial bodies. These are paired glands which 

 arise from or near the fifth pair of gill-pouches. They come to lie near 

 the posterior border of the thyroid and attain a vesicular structure. 



Evolution. Ultimobranchial bodies occur in all gnathostomes except 

 possibly teleostome fishes. In Amphibia, the gland may be paired or 

 unpaired. It is not unlikely that, as its name suggests, it represents a 

 posteriormost pair of gill-pouches modified as an endocrinal organ. In 

 man it persists only as a transient embryonic structure the function of 

 which has become that of an endocrinal organ. (Fig. 381) 



THE THYMUS GLAND 



Anatomy. The thymus glands or throat sweetbreads are paired 

 organs located where the throat joins the chest. They are pinkish in 

 color in childhood, but become yellow in old age from fatty degeneration. 

 They enlarge up to puberty and thereafter gradually shrink. Contrary 

 to earlier opinion, the gland does not completely disappear in the adult, 

 but persists as a shrunken remnant within the mediastinum. 



Branches of the subclavian or of the internal mammary artery supply 

 the gland with blood. Its numerous veins drain into the internal mam- 

 mary or the inferior thyroid. Lymph vessels are abundant. The nerves 

 are sympathetic nerves derived from the vagus. 



Histology. From the connective tissue capsule of the gland, partitions 

 extend inwards and divide it into lobes and lobules. In each lobule a 

 cortex and medulla are distinguishable as in a lymph node, but the large 

 lymph sinuses in the medulla, and the germinal centers which are present 

 in lymph nodes, are lacking in the thymus. The resemblance of the 



