THE NERVES. 369 



Openings : 



1. Foramen Monroi, in front and above, leads into the right 

 and left lateral ventricles. 



2. Aditus ad aqu&ductum Sylvii, behind. 



3. Aditus ad infundibulum, below, leads into the infundibulum. 

 Commissures : 



1. C. anterior, a white cylindrical cord, passing before the an- 

 terior crura of thefornix from one hemisphere to the other. 

 From it the lamina terminalis passes vertically downwards 

 to the chiasma. 



2. C. Posterior, smaller than the last, in other respects like it, 

 uniting the two thalami, lying before the corpp. quadrigemina, 

 under the pineal gland with which it is united. 



3. (7. mollis, a gray band stretched like a bridge between the 

 anterior extremities of the thalami, passes transversely through 

 the centre, nearer to the roof than to the floor of the cerebral 

 ventricles. 



3. The Aqueduct of Sylvius \iter a tertio ad quartum ventri- 

 culum], a three-sided canal, passes from the posterior extremity 

 of the third ventricle, backwards and downwards, under the pineal 

 gland [its commissure and the posterior commissure] and the corpp. 

 quadrigemina, above the centre of the crura cerebri to the fourth 

 ventricle. 



4. The fourth ventricle, ventriculus quartus s. cerebelli, a rhom- 

 boidal space between the cerebellum and the isthmus, the former 

 of which forms the posterior wall (roof), the latter (the floor). It 

 is broad in the centre, in front narrow, and it passes backwards 

 into a point. The floor, that is, above, the posterior surface of the 

 pons Varolii, below, that of the medulla oblong., is rhomboidal, 

 and bounded laterally, below, by corpp. restiformia. The roof 

 consists above of the crura cerebelli superiora and valvula magna, 

 in the centre of the vermis inferior ; below, of a fibrous plate 

 from the neurilemma of the spinal marrow. The superior angle 

 receives, below, the valvula magna of aquceductus Sylvii ; the 

 lateral angle reaches to the corpus rhomboid, of the cerebellum ; 

 the inferior angle presents a fissure between medulla oblongata 

 and cerebrum, through which the fluid in the fourth ventricle com- 

 municates with that beneath the arachnoidea of the spinal cord. 

 The interior is smooth, covered with a serous membrane, and 

 contains, like the other cerebral ventricles, a. plexus choroideus. 



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