ANATOMY OF THE NERVOUS SYSTEM. 171 



right, having the figure 3 placed on it, has its median side 

 exposed in the section. The third ventricle is crossed about 

 its middle by the middle commissure, t, and from its anterior 

 end the foramina of Monro, of which the right, m, is partly 

 exposed in the section, lead to the lateral ventricles. From 

 the fore % part of the third ventricle two conical extensions pass 

 downward, one directed to z, the optic commissure, from which 

 the optic nerves pass, and the other, named the infundibulum, 

 to the pituitary body, fit. The latter consists of an anterior 

 and posterior lobe, and in the human brain contains no ner- 

 vous elements. The anterior lobe, indeed, is an outgrowth 

 from the pharynx of the embryo, and only secondarily be- 

 comes attached to the brain. It is not known to have any 

 function in existing vertebrates. From the posterior part of 

 the floor of the third ventricle the iter leads as a narrow pas- 

 sage dorsal to the crura cerebri, Cr, and ventral to the corpora 

 quadrigemina, o, d, to the fourth ventricle, 4. Projecting 

 from the posterior wall of the third ventricle is a small coni- 

 cal non-nervous mass, the pineal body, which, though of no 

 functional importance, is of interest, in the first place be- 

 cause the philosopher Descartes considered it the special seat 

 of the soul, and in the second because embryology and com- 

 parative anatomy show that it is the remnant of a third 

 median eye, which primitive vertebrates possessed on the 

 dorsal side of the head. In some existing reptiles its original 

 structure is more complete than in man, but in none is it 

 functional. Just beneath the attachment of the pineal body 

 is a slight thickening of the posterior wall of the third ven- 

 tricle containing transverse fibres, and named the posterior 

 commissure. The third ventricle and the parts immediately 

 surrounding it constitute the inter-brain or thalamencephalon, 

 which with the two cerebral hemispheres and the corpus cal- 

 losum and fornix makes up the fore brain. 



The mid-brain, consisting mainly of the crura cerebri, Cr, 

 and the corpora quadrigemina, o, d,and traversed by the nar- 

 row iter, is continuous posteriorly with the hind brain, con- 

 sisting of pons Varfilii, P; cerebellum, Cb; and medulla oblon- 

 gata, Mo. The 'thin-roofed cavity of the fourth ventricle, 4, 

 lies near its dorsal side. Where cut in making the section 

 the cerebellum shows a curious branching core of white nerve 

 matter, surrounded by gray, named arbor vitce by the old 

 anatomists. The pons consists mainly of transverse fibres 



