THE ENCEPHALON OE BKAIN. 541 



; ce strands, the pedunculi cerebri, may be seen issuing from the inferior surface 

 the cerebral hemispheres. As they pass downwards these peduncles are inclined 

 diquely towards the median plane, so that when they plunge into the pons they 

 e situated in close apposition the one to the other (Fig. 478). Turning round 

 .e lateral side of each peduncle, where it emerges from the cerebrum, a flattened 

 ,nd termed the optic tract may be observed. These bands come from the anterior 

 irt of the hollow, where they are joined together by a short connecting piece 

 rmed the optic chiasma. The optic nerve is inserted, on each side, into the 

 itero-lateral angle of the chiasma. 



The pedunculi cerebri, the optic tracts, and the optic chiasma enclose a deep 

 lomboidal or lozenge-shaped interval on the base of the brain, which is termed 

 >e fossa interpeduncularis. Within the limits of this area the following parts may 

 j seen as we pass from behind forwards : (1) the substantia perforata posterior ; 

 !) the corpora mamillaria : (3) the tuber cinereum and the stalk of the hypo- 

 lysis cerebri (O.T. pituitary body). 



At its posterior angle, immediately in front of the pons, the interpeduncular 

 ssa is very deep and is floored by a layer of gray matter, in which are 

 nmerous small apertures. This is the substantia perforata posterior. Through 

 le apertures which are dotted over its surface the small postero-medial basal 

 ranches of the posterior cerebral artery enter the brain. 



The corpora mamillaria are two small white pea-like eminences placed side by 

 de in front of the substantia perforata posterior. . 



The tuber cinereum is a slightly-raised field of gray matter, which occupies the 

 iterval between the anterior portions of the optic tracts in front of the corpora 

 tamillaria. Springing from the anterior part .of the tuber cinereum, immediately 

 ihind the optic chiasma, is the mfundibulum, or the stalk which connects the 

 ypophysis cerebri with the base of the brain (Fig. 478). 



Lateral to the limits of the anterior part of the interpeduncular space there is, on 

 ich side, a small depressed triangular field of gray matter, which leads laterally 

 ito the lateral cerebral fissure. It is perforated by the antero-medial and the 

 atero-lateral groups of basal arteries, and receives the name of the substantia 

 erforata anterior. 



General Connexions of the Several Parts of the Brain. The medulla 

 blongata, the pons, and the cerebellum occupy the posterior cranial fossa, and 

 ley are separated from the cerebral hemispheres, which lie above them, by a 

 artition of dura mater, termed the tentorium cerebelli. Further, they surround 

 cavity, a portion of the primitive cavity of the early neural tube, which is termed 

 ne fourth ventricle of the brain, and they all stand in intimate connexion, one 

 "ith the other. The medulla oblongata is for the most part carried upwards into 

 he pons ; but at the same time two large strands from its dorsal- aspect, termed 

 he restiform bodies, are prolonged into the cerebellum, and constitute its 

 iferior peduncles, or the chief bonds of union between the medulla (oblongata and 

 pinalis) and the cerebellum. The pons has large numbers of transverse fibres 

 ntering into its composition, and the great majority of these are gathered together 

 n each side in the form of a large rope -like strand. This plunges into the 

 orresponding hemisphere of the cerebellum, and constitutes its middle peduncle, 

 iriiich is known as the brachium pontis. 



The cerebrum, which forms the great mass of the brain, occupies the anterior 



-nd middle cranial fossae, and extends backwards into the occipital region above 



i he tentorium and the cerebellum. The greater part of the cerebrum is formed by 



he cerebral hemispheres, which are separated from each other in the median 



)lane by the longitudinal fissure. At the bottom of this fissure is the corpus 



;allosum, a broad commissural band which connects the two hemispheres with 



>ach other. Each hemisphere is hollow, the cavity in its interior being termed 



he lateral ventricle of the brain. Between and below the cerebral hemispheres, 



md almost completely concealed by them, is the inter-brain or diencephalon. The 



rincipal parts forming this portion of the brain are two large masses of gray 



i-tter, termed the thalami. Between these is the third ventricle of the brain a 

 leep narrow cavity occupying the median plane. The third ventricle communicates 



