462 COMPARATIVE ANATOMY 



Development. Thymus glands first appear in a six-weeks embryo 

 as hollow tubular outpocketings of the third pair of pharyngeal pouches, 

 dorsal to those of the parathyroid glands. As they grow caudally, 

 the anlagen become solid and considerably elongated, the anterior part 

 atrophies, and the posterior enlarges. In the tenth week, after the gland 

 attains its definitive position in the mediastinum, thymic corpuscles 

 appear. By the third month, cortex and medulla are differentiated, and 

 the gland assumes the appearance of a lymph gland. Enlargement 

 continues until puberty. (Fig. 382) 



Evolution. Thymus glands are peculiar to vertebrates but occur 

 in all members of that sub-phylum. In cyclostomes all gill-pouches 

 give rise to thymic tissue and the gland persists dorsal to the gills through- 

 out life. Ventral anlagen have also been described. In fishes there is a 

 reduction in the number of thymus anlagen, since some of the gill-pouches 

 make no contribution to the thymus. In amphibia the thymus is further 

 reduced, and comes to lie at the angle of the jaw. The thymus glands 

 of reptiles are lobular organs located in the sides of the neck. In mam- 

 mals the third pair of pharyngeal pouches, or sometimes the third and 

 fourth, produce the cells which form the thymus. The gland assumes 

 its definitive position in the mediastinum at the base of the neck. The 

 history of the thymus is thus one of reduction in the number of gill-pouches 

 which contribute to the gland. However, in some orders of mammals, 

 such as marsupials, rodents, ungulates and prosimians, a new t>^e of 

 thymus gland appears, the cervical thymus, which is said to be of ecto- 

 dermal origin. It is of relatively small size and of unknown function. 



THE PITUITARY GLAND 



Anatomy. The pituitary gland is a flattened oval body, its longest 

 diameter averaging 10 to 12 mm., located at the base of the brain near 

 the optic chiasma, where it is attached by a stalk to the lower end of the 

 inf undibulum. Lodged in a cavity, the sella turcica, in the sphenoid bone, 

 the pituitary is as safe from injury as possible, (Figs. 285-383) 



Like the suprarenals, the pituitary is formed of two elements of diverse 

 origin and function. The chief parts are a larger anterior glandular 

 lobe, which develops from the ectoderm of Rathke's pouch, and a smaller 

 posterior nervous lobe derived from the floor of the brain and partly 

 surrounded by the anterior lobe. Between the two major lobes are two 

 smaller glandular masses, the pars intermedia and the pars tuberalis. 

 Blood is richly supplied from the adjacent arterial circle, while numerous 

 veins drain the blood into the venous circle and the basilar plexus of 

 veins. Nerves are supplied from the carotid plexus and the infundibulum. 



Histology. The anterior lobe consists of cords of epithelial cells 

 interspersed among numerous blood and lymph spaces. In their staining 



