THE DUCTLESS GLANDS 421 



The nervous portion of the pituitary body is made up of 

 neuroglia cells and fibres. Ependyma cells line the central 

 cavity in the cat, and send long fibres forwards and upwards 

 towards the brain, most of which terminate in the outer part 

 of the neck. There are no true nerve cells, and the nerves 

 supplying the pituitary probably reach it through sympathetic 

 fibres accompanying the blood-vessels. The nervous portion is 

 invaded to a large extent by the epithelial cells of the pars inter- 

 media. Columns of epithelial cells grow into it, especially in 

 the region of the neck, and islets of these cells are frequently 

 found throughout the posterior lobe. 



A substance histologically resembling the colloid of the 

 thyroid gland occurs in the nervous portion of the posterior 

 lobe. It appears to be a product of the epithelial cells, and 

 to be carried by lymphatics into the third ventricle of the 

 brain. 



Development of the pituitary body begins very early in 

 embryonic life. In mammals the epithelial portion is derived 

 entirely from the ectodermic wall of the buccal invagination 

 known as Rathke's pouch. Its origin is single and mesial. 

 The epithelium is early distinguishable into two parts. One of 

 these, the intermediate part, is closely adherent to the wall of 

 the cerebral vesicle; the cells are clear, and tend to form colloid. 

 The other portion of the buccal epithelium gives rise to the 

 anterior lobe proper. Its cells are granular, and form solid 

 columns separated by blood-channels. 



The infundibulum is an invagination of part of the wall of 

 the thalamencephalon, which is adherent to the anterior and 

 upper wall of Rathke's pouch. It therefore possesses an epithe- 

 lial covering derived from the latter. The infundibular process 

 grows backwards, and, in the cat, retains its central cavity. It 

 is lined by ependyma cells, which during development become 

 elongated, so that ependyma fibres run obliquely in its neck. 

 The posterior lobe of the pituitary is, from the first, a composite 

 structure of epithelium of the pars intermedia and of neuroglia 

 and ependyma, and the relations between the two tissues become 

 more and more intimate (30). 



(b) Physiological action of extracts of the pituitary body. — 

 There are two substances contained in an aqueous extract having 

 opposite effects (pressor and depressor) upon the blood-vessels ; 

 but with the first injection the depressor substance is over- 



