THE BRAIN AND ITS MEMBRANES. 751 



The pituitary body ( hypophysis cerebri) is a small reddish-gray vascular mass 

 weighing from five to ten grains, and of an oval form, situated in the sella Tur- 

 cica, in connection with which it is retained by a process of dura mater derived 

 from the inner wall of the cavernous sinus. This process covers in the pituitary 

 fossa, enclosing the pituitary body, and having a small hole in the centre through 

 which the infundibulum passes. The pituitary body is very vascular, and consists 

 of two lobes, separated from one another by a fibrous lamina. Of these, the ante- 

 rior is the larger, of an oblong form, and somewhat concave behind, where it 

 receives the posterior lobe, which is round. The two lobes differ both in develop- 

 ment and structure. The anterior lobe, of a dark, yellowish-gray color, is devel- 

 oped as a tubular prolongation of the epiblast of the buccal cavity, and resembles 

 to a considerable extent, in microscopic structure, the thyroid body. It consists 

 of a number of isolated vesicles and slightly convoluted alveoli lined by epithe- 

 lium aud united together by connective tissue. The epithelium is columnar, and 

 occasionally ciliated. The alveoli sometimes contain a colloid material similar 

 to that found in the thyroid body, and their walls are surrounded by a close net- 

 work of lymphatics and capillary blood-vessels. The posterior lobe is developed 

 by a hollow outgrowth from the embryonic brain, and during foetal life contains a 

 cavity which communicates through the infundibulum with the cavity of the third 

 ventricle. In the adult it becomes firmer and more solid, owing to the growing 

 in of a sponge-like connective tissue arranged in the form of reticulating bundles, 

 between which are branched cells, some of them containing pigment. 



The lamina cinerea begins at the posterior border of the optic commissure, in 

 continuity with the tuber cinereum. It passes forward and downward over the 

 commissure, to which it is adherent, and then turns upward, forms the anterior 

 boundary of the third ventricle, and terminates, as the lamina terminalis, by 

 blending with the middle portion of the anterior extremity of the corpus cal- 

 losum. It is continuous on each side with the gray matter of the anterior per- 

 forated space. The angle made by the upward bend of the lamina is known as 

 the optic recess. 



The anterior boundary of the third ventricle is the lamina cinerea below ; 

 above this, for a very short distance, the anterior boundary is the layer of epithe- 

 lium covering portions of the posterior aspects of the anterior commissure and 

 anterior pillars of the fornix, as already explained. 



The SIDES of the ventricle are the internal surfaces of the thalami. Each is 

 slightly convex, and just in front of the middle point of each is attached the cor- 

 responding extremity of the middle commissure, a band of gray matter which 

 passes right across the ventricle. It is frequently broken in examining the brain, 

 and might then be supposed to be wanting. A little more anteriorly is seen a 

 somewhat curved, from above downward and backward, elevation (anterior pillar 

 of fornix. already explained). As these pillars, traced upward, become free, they 

 bend sharply upward and backward, thus forming a completed curve, each enclos- 

 ing in front and above the foramen of Monro, which has for its posterior boundary 

 a part of the anterior extremity of the optic thalamus. 



Antero-inferiorly to the curved elevations is still, on each side, a small portion 

 left of the internal surface of the thalamus, connected to the similar opposite por- 

 tion, below and in front, by the tuber cinereum and lamina cinerea. It is thus 

 seen that all these structures really form the anterior extremity of the third ven- 

 tricle (see page 706), which is the prosencephalon, or, in the foetal brain, the first 

 secondary cerebral vesicle. Hence the "curved elevations" may be regarded as 

 indicating, approximately, the line of division or constriction between the first 

 and second secondary cerebral vesicles (prosencephalon and thalmencephalon 

 proper), while the foramina of Monro are to be regarded as opening from the 

 prosencephalon, and thus represent the foetal foramina caused by the bulging out 

 of the hemisphere vesicles (see Figs. 412, 413, 414). 



The choroid plexuses of the third ventricle, formed like those of the fourth, 

 lie along the roof, projecting downward, one on each side of the middle line. 



