CHAP. X.] THE FOURTH VENTRICLE. 273 



nary elements, of which, however, the caudate vesicles constitute a 

 principal portion (p. 213). These are so disposed that their processes 

 pass off chiefly towards the circumference, their obtuse extremities 

 being directed towards the laminae. Besides these, there is in each 

 layer of vesicular matter a thin lamina composed of round clear 

 nucleus-like particles, which cohere to each other without the in- 

 tervention of any matrix or other connecting substance. Pine 

 nerve-tubes and blood-vessels pass through it. This lamina is in- 

 termediate to two which contain nerve-vesicles one of which is 

 in immediate connexion with the fibrous matter of the cerebellum, 

 the other with the pia mater. 



Of the fourth Ventricle. The divergence of the restiform bodies 

 in their ascent to the hemispheres of the cerebellum leaves a con- 

 siderable space, which is of a lozenge shape, having its superior 

 angle towards the brain, its lateral angles towards the cerebellar 

 hemispheres, and its inferior angle at the point of separation of 

 the restiform bodies. Along its floor are seen the central or olivary 

 columns of the medulla oblongata, extending upwards to the optic 

 thalami. A fissure, continuous with the posterior median fissure, 

 separates these columns. Some bundles of white fibres, which may 

 be traced to the soft portion of the seventh pair of nerves, cross 

 these bundles nearly at right angles to them and to the fissure 

 (p. 266), and form with the latter the calamus scriptorius, the white 

 fibres constituting the barbs of the pen. The roof of this ventricle 

 is formed in front by the anterior laminae of the superior vermiform 

 process, which constitute the valve of Vieussens; and behind by 

 the inferior vermiform process. A process of pia mater enters 

 it at its inferior angle, just as the choroid plexus penetrates the 

 inferior cornu of the lateral ventricles of the brain. The reflexion 

 of the lining membrane on the process of pia mater seems to close 

 up the ventricle below, and cut off its direct communication with 

 the subarachnoid space. A canal, which passes through the meso- 

 cephale, establishes the communication of this with the third 

 ventricle, iter a tertio ad quartum ventriculum. 



The fourth ventricle properly belongs to the medulla oblongata. 

 It is, therefore, present in all the vertebrate classes, and is, in point 

 of size, directly proportionate to the medulla itself. 



Of the Mesoccphale. This term, suggested by Chaussier, denotes 

 that this portion of encephalon is the bond of union to the rest, 

 the cerebrum above, the medulla oblongata below, and the cere- 

 bellum behind. 



The inferior surface of the mcsocephale, the pons Varolii, con- 



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