TEE ISTHMUS. 771 



the tubercula testes, forms a well-defined triangular space, designated the band of 

 Eeil, lateral triangular fasciculus, and lateral oblique fasciculus of the isthmus. 



4. The Crura Cerebelli. 



The cerebellum is attached to the upper face of the isthmus, by two short and 

 thick lateral funiculi of white substance, between which is comprised the posterior 

 ventricle ; these constitute the crura cerebelli. 



Three distinct fasciculi enter into the composition of each of these cords — an 

 anterior, a posterior, and a middle. 



The latter, or middle cerebellar peduncle (crus cerebelli ad pontem), is the 

 largest of the three. It is formed by the prolongation of the extremities of the 

 pons Varolii (Figs. 425, 2 ; 426, 6). 



The posterior cerebellar peduncle (crus ad medullam oblongatum) — the most 

 slender— is formed by the restiform body, one portion of which is reflected below 

 the posterior root of the auditory nerve to reach the substance of the cerebellum. 

 It is closely united to the preceding, from which it is with difficulty distinguished 

 (Fig. 425, 3). 



The anterior cerebellar peduncle {processus e cerebello ad testes) (Fig. 425, 4) is 

 a fasciculus very distinct from the other two, related by its inner border to 

 the middle peduncle, which it obliquely crosses, loses itself in the cerebellum by 

 its supero-posterior extremity, arriving behind the testes, and passing beneath 

 these small organs by its antero-inferior extremity, along with the band of Reil 

 or supero-lateral fasciculus of the cerebral peduncles. 



In studying the structure of the cerebellum, we will see how these peduncles 

 comport themselves in its interior. 



5. Valve of Vieussens (Fig. 425, 6). 



This designation is given to a very thin, white layer which unites, on each 

 side, the two anterior cerebellar peduncles. In shape it is nearly a parallelogram. 

 Its superior face is covered by the cerebellum ; the inferior concurs in forming 

 the roof of the cerebellar {fourth) ventricle. The two lateral borders are joined 

 to the peduncles which this valve unites ; the anterior is attached behind the testes ; 

 while the posterior adheres to the anterior vermiform process {linguetta laminosa) 

 of the cerebellum. 



Gall has considered this layer as a commissure of the anterior cerebellar 

 peduncles, and we think rightly ; for we see it formed almost exclusively of 

 transverse fibres which run from one of these peduncles to the other. These 

 fibres are most apparent in front, where the membrane is much thicker ; behind, 

 they are mixed with some longitudinal fasciculi. 



6. Corpora Quadrigemina or Bigemina (Figs. 425, 7, 8 ; 426, 8, 9). 



These are four round eminences, placed in pairs, which surmount the cerebral 

 peduncles behind. The two posterior, the smallest, are also named the tubercula 

 testes, and the anterior pair the tubercida nates. 



The posterior corpora quadrigemina, or tubercula testes, are related, in front, 

 to the anterior eminences ; behind, to the anterior cerebellar peduncles and the 

 valve of Vieussens, from which they are separated by a transverse groove, at 

 the bottom of which arise the pathetici nerves. An oblique band unites them, 



