THE ISTHMUS. 679 



obliquely crosses, loses itself in the cerebellum by its supero-posterior 

 extremity, arriving bebmd the testes, and passing beneath these small organs 

 by Its antero-mferior extremity, along with the band of Eeil or supero-lateral 

 tasciculus 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. 323, 6.) 

 This designation is given to a very thin, white lamella 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 floor of the cerebellar (fourth) ventricle. The two 

 lateral horders are joined to the peduncles this valve unites ; the anterior is 

 attached behind the testes; while the posterior adheres to the anterior 

 vermiform eminence (linguetta laminosa) of the cerebellum. 



Gall has considered this lamella as a commissure of the anterior cere- 

 bellar 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. (Fig. 823, 7, 8.) 



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

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

 the tuhercula testes, and the anterior pair the tuhercula nates. 



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

 ■with the anterior eminences : behind, with the anterior cerebellar peduncles 

 and the valve of Vieussens, from which they are separated by a transverse 

 groove, from the bottom of which arise the pathetici nerves. An oblique 

 band unites them, outwardly, to that portion of the optic layer designated 

 the corpus geniculatiom internum. 



The anterior corpora quadrigemina, or tuhercula nates, are distinguished 

 from the preceding not only by their larger volume, but by their colour, 

 which is grey, that of the testes being white. They are also rounder, nearer 

 each other, and covered by the cerebral hemispheres ; while the posterior 

 rather lie beneath the cerebellum. A curved groove isolates them, in front, 

 from the thalami optici. 



7. Thalami Optici. (Fig. 323, 9.) 



This name is given to that part of the upper face of the isthmus which 

 is situated in front of the corpora quadrigemina. These thalami are 

 therefore placed above the anterior part of the cerebral peduncles. 



Larger altogether than the corpora quadrigemina, and more so before 

 than behind, each exhibits a grey, slightly convex, and very irregularly- 

 quadrilateral surface, covered by the velum interpositum, which separates it 

 from the cornu Ammonis (pes hippocampi), and from the posterior pillars of 

 the cerebral trigonal (fornix). 



Inwardly, they incline towards each other in forming on the median 

 line a somewhat deep fissure, in which runs, from before to behind, two 

 white longitudinal bands that will be noticed hereafter as the anterior 

 peduncles of the pineal gland. This fissure enters, behind, into the common 



