116 MESENCEPHALON. 



An isolated bundle of one of the anterior pillars of the fornix is sometimes visible at the 

 base of the brain passing to the corpus albicans (stria alba tubcris. Lenhossek). 



The tuber cinereum (fig. 86 ; fig. 82, i.e.} is a lamina of grey matter extending 

 forwards from the corpora albicantia to the optic commissure, to which it is attached. 

 It forms part of the floor of the third ventricle. In the middle it is prolonged 

 forwards and downwards into a hollow conical process, the infundibulum 

 (fig. 85, *'), to the extremity of which is fixed the pituitary body. On its outer 

 side close to the optic tract is a tract of grey matter with nerve-cells, termed by 

 Meynert the basal optic ganglion (see p. 119). According to Lenhossek this is 

 distinctly subdivided into three successive groups of nerve-cells, the most anterior 

 being just above the chiasma, the most posterior near the corpus albicans. 



The pituitary body or hypophysis cerebri (figs. 32, 39, h, and fig. 85), formerly 

 called pituitary gland, from its being erroneously supposed to discharge pituita into 

 the nostrils, is a small reddish grey mass, of a somewhat flattened oval shape, widest 

 in the transverse direction, and occupying the sella turcica of the sphenoid bone. 

 The pituitary body has a special prolongation of the dura mater completely 

 enclosing it, except above where there is a small aperture for the passage of the 

 infundibulum (see p. 182). The body consists of two lobes, of which the anterior 

 is the larger, and is concave behind, where it embraces the smaller posterior lobe. 

 The two lobes are entirely different, both in their structure and development ; and 

 it is only in mammals that they come into close connexion with one another. 



The posterior lobe is developed as a hollow downgrowth of the part of that cavity 

 of the embryonic brain, which afterwards becomes the third ventricle. In the 

 lower vertebrates, and especially in fishes, the cells which compose its walls become 

 converted into nerve-cells and -fibres, and as the lobus infundibuli it becomes an 

 integral part of the brain. But in the higher vertebrates it remains small and 

 almost undeveloped ; its cavity is obliterated, and all nervous structure becomes 

 obscured by the ingrowth of vessels and of connective tissue into the now solid 

 organ. The connective tissue forms reticulating bundles, between which occur 



Fig. 85. SAGITTAL SECTION OF THE PITUITARY BODY AND INFUNDI- 

 BULUM WITH THE ADJOINING PART OF THE THIRD VENTRICLE 



(Schwalbe). 



a, anterior lobe ; ', a projection from it towards the front of the 

 infundibulum, i ; It, posterior lobe connected by a solid stalk with the 

 infundibulum ; I.e. , lamina cinerea ; o, right optic nerve ; ch, section 

 of chiasma; r.o., recess of the ventricle above the chiasma; c.rn., 

 corpus mamillare. 



numerous spindle-shaped and branched cells, as well as a few larger corpuscles 

 containing pigment-granules in their protoplasm. Sometimes remains of the 

 original hollow are seen in the form of a cavity lined by columnar ciliated 

 cells. 



The anterior lobe, darker in colour than the posterior, is developed as a tubular 

 prolongation from the epiblast of the buccal cavity, with which it is therefore 

 originally in connection, although it soon becomes separated by the growth of 

 intervening tissue. In the adult it is constituted by a large number of slightly 

 convoluted tubules or alveoli, similar to those of a secreting gland, and in like 

 manner lined by epithelium, which in some cases fills up the tubule. The tubules 

 are united by connective tissue, which is especially abundant in the neighbourhood 

 of the larger blood-vessels, and also forms a sort of capsule to the organ. Moreover, 

 portions of the tubules are frequently cut off by the connective tissue so as to form 

 isolated vesicles. The outer layer of epithelium is columnar ; and in some of the 

 larger tubes, especially those next to the posterior lobe, cilia may be detected on the 

 cells. The blood-vessels are numerous, and the capillaries form a close network 



