THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



The anterior border — also convex, but indented in its middle — largely overhangs 

 the crura cerebri, which are limited on this side by a well-marked fissure. 



The extremities are bent upwards to enter the substance of the cerebellum, in 

 the form of tAVo thick cords, which constitute the middle crura cerebelli (Fig. 

 425, 6). They exhibit the apparent origin of the trifacial nerves. 



The pons Varolii does not exist in Birds. 



Kig. 425. 



3. The Crura Cerebri (Figs. 425, 

 426). 



These are two very large white fasci- 

 culi, visible at the inferior surface and 

 sides of the isthmus, covered superiorly 

 by the corpora quadrigemina and thalami 

 optici, and continuous — above the pons 

 Varolii — with the fibres of the medulla 

 oblongata ; while their anterior extremi- 

 ties enter the cerebral hemispheres. 



These peduncles are separated from 

 each other by a middle fissure — the inter- 

 pedunndar — which bifurcates in front to 

 circumscribe the fisiform tubei'de {corpus 

 albicans, hidhi fornicis) — a small, single, 

 and rounded elevation of a white colour 

 like the peduncles, covered by the 

 pituitary gland (Fig. 423, 4), the root 

 of which is represented by the tuber 

 cinereum, and which is situated in front 

 of this body (see Description of the pitui- 

 tary gland on p. 773).^ 



Behind, the crura cerebri are limited 

 by the anterior border of the pons 

 Varolii. In front, they are circumscribed 

 by the optic nerves, which pass ob- 

 liquely around their anterior extremity 

 and join on the middle line before the 

 tuber cireneum, to form a commissure 

 called the chiasma (or commissure) of the 

 optic nerves (Fig. 424, 8). On the sides, 

 their tissue is confounded with that of 

 the corpora quadrigemina and thalami 

 optici, which are superposed on the crura 

 cerebri. It may be remarked that the 

 part of their lateral face situated below 



• The inferior fHce of the crura cerebri offer some more interesting details. Tlius, behind 

 the corpus albicans is a triangular space— ^/se interpedunmlar perforated layer (locust perforatus 

 posticus, pons Tarini) pierced by a large number of openings for the posterior group of arterioles 

 given off from the circle of Willis. On each crus is seen, near the middle line, a longitudinal 

 fissure from which issues the nerve of the third pair ; outside tliis is another groove that 

 separates a fasciculus of white fibres from a mammilated surface of a grey colour. There are 

 also observed some fasciculi of transverse fibres which proceed from the band of Reil. The 

 crura cerebri represent two stages, separated by a mass of grey substance named the locus niger. 



SUPERIOR VIEW OF THE ENCEPHALIC ISTHMUS. 



1, 1, Corpora restiformia ; 2, section of the 

 middle cerebellar peduncle ; 3, section of the 

 posterior cerebellar peduncle ; 4, anterior 

 cerebellar peduncle ; 5, floor of the postt^rior 

 ventricle ; 6, valve of Vieussens ; 7, 7, tuber- 

 cula testes ; 8, 8, tubercula nates ; 9, 9, 

 thalami optici; 10, corpus geniculatuin in- 

 ternum; il, corpus geniculatum externum; 

 12, corpus striatum ; 13, taenia semicircularis; 

 14, pineal gland; 15, its peduncle; 16, com- 

 mon anterior opening ; 17, 17, anterior pillars 

 of the trigonum or fornix ; 18, trifacial 

 nerve; 19, facial nerve; 20, auditory nerve; 

 21, glosso-pharyngeal nerve ; 22, pneumo- 

 gastric nerve ; 23, spinal nerve. 



