490 THE BRAIN 



stretches laterally to the flocculus, thereby bringing those 

 two small portions of the cerebellum into association with 

 each other. Where it issues from the white matter of the 

 cerebellum it might almost be said to be in contact with the 

 anterior medullary velum, but as the two laminae are traced 

 forwards they diverge from each other : the anterior velum is 

 carried upwards between the brachia conjunctiva of the cere- 

 bellum, whilst the posterior medullary velum turns downwards, 

 round the nodule, and ends in a slightly thickened free 

 crescentic edge. The cavity of the fourth ventricle passes 

 backwards between the two vela, which form a tent-like roof 

 for it. 



Isthmus Ehombencephali. If the dissector examines the 

 rhombencephalon from the side he will recognise that there 

 is a region below the lamina quadrigemina and above the 

 cerebellum which is bounded dorsally by the anterior 

 medullary velum, laterally by the brachia conjunctiva, and 

 ventrally by the upper part of the pons : it is to that region 

 that the term isthmus rhombencephali is applied. It contains 

 the upper part of the fourth ventricle. 



Ventriculus Quartus. The fourth ventricle is somewhat 

 rhomboidal in form. Below, it tapers to a point and becomes 

 continuous with the central canal of the lower part of the 

 medulla oblongata ; above, it narrows, in a similar manner, 

 and is continued into the aquaeductus cerebri of the mid- 

 brain. The anterior wall is termed the floor^ and is formed 

 by the dorsal surface of the pons and the ventral part of the 

 upper portion of the medulla oblongata. The posterior wall 

 is called the roof. On each side a narrow pointed prolonga- 

 tion of the widest part of the ventricular cavity passes laterally 

 round the upper part of the corresponding restiform body. 

 The prolongations are termed the lateral recesses and are seen 

 to the greatest advantage when the cerebellum is divided 

 in the median plane and the halves are turned aside. 



The lateral boundary of the fourth ventricle, on each side, 

 is formed, from below upwards, by the clava, the upper part 

 of the fasciculus cuneatus, the restiform body or inferior 

 peduncle of the cerebellum, the brachium pontis or middle 

 peduncle of the cerebellum, and the brachium conjunctivum 

 or superior peduncle of the cerebellum. 



Dissection. On one side cut through the brachium con- 

 junctivum, the brachium pontis and the restiform body, and so 



