776 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



3. The Posterior or Cerebellar Ventricle (Fig. 429, 5). 



This ventricle^ (or sinus rhomboidalis), situated beneath the cerebuUum, 

 between its peduncles, and above the medulla oblongata and pons Varolii, is a 

 cavity elongated from before to behind, and ahnost entirely occupied by the 

 vermiform processes. 



Its superior ivall is formed by these two processes, the valve of Vieussens, 

 and that of Renault. The inferior, or floor of the cavity, is represented by the 

 depression on the upper face of the medulla oblongata, and which is prolonged 

 in front, above the pons Varolii, to near the testes.^ 



The anterior extremity communicates with the aqueduct of Sylvius. The 

 posterior occupies the summit of the calamus scriptorius. 



Structure of the Isthmus. 



The isthmus, being only a prolongation of the spinal cord, ought to resemble 



it in its structure ; and this is, in 



Fig. 428. 



fact, what is observed, particularly 

 in its posterior part, the common 

 features in their organization, how- 

 ever, disappearing as we approach its 

 anterior extremity. 



We will follow the distribution of 

 the white and grey substance in the 

 isthmus. 



The white substance includes the 

 longitudinal tvhite fibres, as well as the 

 transverse white fibres. Among the 

 first may be mentioned the infero- 

 lateral columns. These decussate at 

 the neck of the medulla oblongata, in 



DISSECTION OF THE MEDULLA OBLONGATA, SHOW- 

 ING THE CONNECTION OF ITS SEVERAL FASCI- 

 CULI, OR STRANDS. 



A, Corpus striatum ; B, thalamus opticus ; C, D, 

 corpora quadrigemina ; E, commissure connecting 

 them with the cerebellum ; F, corpora resti- 

 formia ; P, P, pons Varolii ; st, st, sensory tract ; 

 mt, mt, motor tract ; g, olivary tract ; p, pyra- 

 midal tract ; og, olivary ganglion ; op, optic 

 nerve ; 3m, root of third pair (motor) ; 5s, sen- 

 sory root of the fifth pair. 



successive layers ; afterwards they 

 pass along the inferior middle fissure, 

 where they constitute the motor por- 

 tion of the pyrafnids of the medulla 

 oblongata, and, traversing the pons 

 Varolii, they then form the upper 

 stage of the crura cerebri, finally 

 entering the corpora striata. 



The posterior columns also decus- 

 sate a little in front of the antero-lateral columns. When this intercrossing of 

 fibres has terminated, they form the deep or sensitive portion of the pyramids, pass 

 beyond the pons Varolii, constitute the upper stage of the crura cerebri, and pass 

 into the substance of the thalami optici. 



With regard to the inferior columns of the spinal cord, they do not decussate 

 on entering the isthmus, but they are displaced by the infero-lateral. We then 

 find them in the centre of the medulla, beneath the grey substance in the floor of 



' As the cerebellum concurs in the formation of this cavity, it would perhaps be better to 

 defer its study until that organ has been described. 



* For the features of this region, see the description of the upper face of tiie medulla 

 oblongata. 



