1242 



PHYSIOLOGY 



herbivora respectively. Forsyth has shown that, in man, the situation of 

 the parathyroids corresponds almost exactly with the places in which are 

 found occasionally accessory thyroids; and according to Edmunds, after 

 excision of the thyroid, the parathyroids undergo histological alteration and 

 are converted into thyroid tissue, the cells taking on an alveolar arrangement 

 and producing colloid material. According to this view the parathyroids 

 would represent simply immature thyroid tissue. On the other hand, it has 

 been suggested (Biedl) that the parathyroids have a function entirely dis- 

 tinct from that of the thyroid gland, removal of the thyroids producing 

 simply a condition of cachexia and the changes associated with myxoadema, 



FIG. 562. Mesial sagittal section through the pituitary body of an adult monkey 

 (semi-diagrammatic). (After HERRING.) 



a, optic chiasma; &, third ventricle; c, tongue-like process of pars intermedia; 

 d, epithelial investment of posterior lobe; e, anterior lobe; /, epithelial cleft; 

 g, pars intermedia ; h, posterior lobe. 



while removal of the parathyroids is responsible for the nervous disturbances 

 and tetany observed after total extirpation of these organs. The matter 

 cannot yet be regarded as definitely settled. 



THE PITUITARY BODY 



The pituitary body consists of two parts which have separate modes of origin. An 

 outgrowth from the buccal cavity in the embryo meets a hollow extension of the anterior 

 cerebral vesicle. The buccal ectoderm gives rise to the anterior lobe and pars intermedia 

 of the pituitary, while the neural epiblast becomes developed into the posterior lobe 

 (Fig. 562). In some animals the posterior lobe remains hollow and retains its primitive 

 connection with the third ventricle of the brain, but in man it becomes entirely solid. 

 The anterior lobe in the adult consists of nests of epithelial cells (Fig. 563), many of 

 which are filled with granules, and is richly supplied with large thin- walled capillary 

 blood vessels. The anterior lobe is separated from the posterior lobe by a cleft which 

 is the remains of the original hollow outgrowth from tin- l>u< < nl cavity. The epithelial 

 tissue immediately surrounding this cleft differs somewhat from that constituting the 



