II30 



HUMAN ANATOMY. 



within the pituitary fossa on the base of the skull, just in advance of the dorsum sellae 

 (Fig. 996). Above, the fossa is closed by a special partition of dura, the diaphragma 

 sellcE, through an opening in which the infundibulum passes to the mushroom-shaped 

 organ. The pituitary body consists of two principal parts, of which the so-called 

 anterior lobe is much the larger and of a darker grayish red color. The boundary 

 between the anterior and posterior lobes is occupied by a zone of modified glandular 

 tissue, the pars intermedia, which extends for a variable distance along the ventral 

 surface of the posterior lobe towards the infundibulum. The two lobes are not only dis- 

 tinct as to structure and probably function, but are developed from entirely different 

 regions. The anterior lobe is formed as an outgrowth from the oral diverticulum, 

 while the posterior lobe first appears as a ventral evagination from the diencephalon 

 (Fig. 1532). The anterior lobe, glandular in character, has been described in con- 

 nection with the Accessory Organs of Nutrition (page i8o6j and, therefore, calls 

 for no further consideration in this place. 



Fig. 977. 



Pars intermedia 



Posterior or cerebral lobe 



.Blood-sinus 



Anterior or 

 oral lobe 



Connective-tissue 

 trabecula 



.Capsule 

 Transverse section of pituitary body, showing relation ol anterior (oral) and posterior (cerebral) lobes. X 7. 



The posterior lobe of the pituitary body is lighter in color and softer in con- 

 sistence and directly attached to the fioor of the third ventricle by means of its stalk, 

 the infundibulum. During the early stages of its development, this lobe is repre- 

 sented by a tubular outgrowth whose walls partake of the general character of the 

 adjacent brain-vesicle. Later the lumen within the lower end of the diverticulum dis- 

 appears in consequence of thickening and approximation of its walls, a funnel-shaped 

 recess of variable depth within the infundibulum alone remaining. In the adult con- 

 dition, the posterior or cerebral lobe retains few histological features suggesting its 

 nervous origin. Of the demonstrable interlacing fibres, with fusiform enlargements 

 and elongated nuclei, none can be identified as nerve-fibres, while of the numerous 

 cells which the lobule contains, only a few of large size and pigmented cytoplasm 

 uncertainly resemble nervous elements. With the exception of possibly neurogliar 

 cells, the existence of definite nervous tissue within the cerebral lobe of the mature 

 human hypophysis is doubtful. 



The optic tracts and commissure are elsewhere described (page 1223), 

 sufifice it at this place to mention their relation to the interpeduncular structures. 

 The optic tracts diverge backward and wind around the ventral surface of the cere- 

 bral peduncles (Fig. 993). Their medial ends are fused into a transversely flattened 

 white band, the optic commissure or chiasm. The latter is connected with the front 

 surface of the tuber cinereum, whilst above the chiasm the anterior wall of the ventricle 

 consists of a delicate sheet of gray matter, the lamina cinerea (lamina terminalis). 

 This structure lies in the mid-line, passes almost vertically upward, with a slight 

 forwardly directed curve, and becomes continuous with the rostrum of the corpus 



