536 THE BRAIN 



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

 for it. 



Isthmus Rhombencephali. If the dissectors examine the 

 rhombencephalon from the side they 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 this region 

 that the term isthmus rhombencephali is applied. It contains 

 the upper part of the fourth ventricle. 



Ventriculus Quartus. This cavity 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 aquceductus cerebri of the mid -brain. 

 The anterior wall is termed the floor, and is formed by the 

 dorsal surface of the ventral part of the upper portion of 

 the medulla oblongata and by the dorsal surface of the pons. 

 The posterior wall is called the roof. On either side a 

 narrow pointed prolongation of the ventricular cavity is 

 carried laterally, from its widest part, round the upper part 

 of the corresponding restiform body. This is termed the 

 lateral recess (Fig. 222, p. 525). It is 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 funiculus 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 cerebellum. 



Dissection. On one side cut through the brachium conjunctivum, the 

 brachium pontis and the restiform body, and so separate one lateral half of 

 the cerebellum, which must be laid aside for the present but must be preserved 

 for future use. 



When the dissection is completed the dissectors will be 

 able to recognise that the anterior part of the cavity of the 

 fourth ventricle is rhomboidal in form. It constitutes the so- 

 called rhomboidal fossa, which is surrounded by the lateral 

 boundaries of the ventricle and closed anteriorly by the pons 

 and the posterior surface of the ventral part of the upper 

 portion of the medulla oblongata. Only the lower part of the 



