Histological Appearances of the Mammalian Pituitary Body 141) 



infundibulum ; but they may occur at any part, and are not infrequent in 

 the middle of the posterior lobe, lying partly in the cavity. The islets vary 

 in size from a few cells to twenty or more, and are usually compact, but 

 in the neck of the infundibulum have a looser structure, and individual 

 epithelial cells occur among the fibres of the nervous portion. These are 

 probably the cells referred to by Berkley, but they are not nerve cells; 

 they are derived from the epithelial investment. Occasionally a pituitary 

 body is met with in which the posterior lobe is quite transformed by the 

 ingrowth of epithelium. One of the most remarkable features about the 

 epithelial investment of the pituitary is the property it possesses from a 

 ver}^ early stage of development of spreading over and around the structures 

 with which it comes into contact. It also tends to inv^ade them, and may 

 even spread for some distance into the base of the brain in some animals. 

 In the monkey (cf. tig. 5) the cells often penetrate towards the cavity of 

 the third ventricle behind the neck of the infundibulum. The same is true 

 in the case of other animals ; and it is not confined to mammals — a similar 

 invasion is common in birds. The extent of the ingrowth shows great 

 variations in different individuals of the same species. Careful and good 

 fixation of the tissue is necessarj^ to show it ; for, if any shrinkage takes 

 place, the epithelial cells are more difficult to distinguish from the other 

 cells of the posterior lobe, and are easily overlooked. When the islets are 

 large and deeply placed they are readily seen. 



In well-fixed preparations, and especially after fixation by Flemming's 

 solution, the posterior lobe is found to contain small hyaline bodies, highly 

 refractive when unstained. These bodies lie scattered throughout the 

 nervous substance of the lobe, and stain indifferently with eosin or 

 methylene blue. They are not so prominent in formalin fixed preparations, 

 but can be seen. They are not found in the pituitary of the new-born 

 kitten, but in adult pituitaries are invariably present, and in all animals 

 examined. They occur in the bod}^ of the lobe and in the neck, extending 

 for some distance upwards towards the brain, but not into the brain itself. 

 Their appearance suggests that of cells which have undergone hyaline 

 degeneration ; no nucleus is present, the outline is often irregular, and no 

 structure can be made out in them. Sometimes, however, they have a 

 distinctl}' granular appearance, and are not unlike the granules of the cells 

 of the anterior lobe, but do not stain so deeply. The significance of these 

 bodies is difficult to determine. They seem to be of the nature of a 

 secretion, and are not unlike diluted colloid material. In some situations 

 the substance lies in what look like lymph spaces lined with endothelium. 

 The question naturally arises as to whether this material represents the 

 physiologically active principle of tlie posterior lobe ; its situation certainly 

 agrees with the position in which that is found. The substance often lies 

 between the i^pendyma cells near the central cavity, and may possibly be 

 a secretory ])r()duct of these cells, in which case the nervous part of the 

 pituitary might be regarded as a glandular structure, but it is not alwaj'-s 



