Herkick, MorpJiology of Brain of Bony Pis/ics. 325 



half as long as the exposed part of the cerebellum, and is 

 thrust cephalad into the third ventricle nearly to the superior 

 commissure. 



Metoicephalon . — 1 he medulla is remarkable for its great 

 depth, as well as its great width. The width is due, in 

 large measure, to the presence of the wide, lateral lobes 

 connected with the descending tracts of the cerebellum. 

 The depth is, in part, due to the great development of the 

 two pairs of dorsal tuberosities. Of these the cephalic or 

 trigeminal tubers are much the larger. They rise in the floor 

 of the fourth ventricle to a height nearly as great as that of 

 the cerebellum. They are strictly intra-ventricular, and are 

 covered by the membranous roof of the ventricle. The 

 interior is occupied by fibres and cells pertaining chiefly to 

 the trigeminal nerves. The other pair of pi-ominences, the 

 vagal tubers, are, for the most part, extra-ventricular. The 

 fourth ventricle lies between them, and its membranous roof 

 extends from the mesal surface of the one to that of the other. 

 Most of the fibres of the dorsal root of the vagus take their 

 origin from this pair of tuberosities. Behind the vagal 

 tuberosities the fourth ventricle closes. Its caudal 

 limit is marked by an elevated ridge, or crest, of 

 transverse fibres. Farther caudad, at the exit of the so called 

 first spinal nerve, the medulla is thickened on both dorsal and 

 ventral surfaces, behind which it passes at once into the 

 spinal cord. On the ventral surface there is another con- 

 siderable thickening at the exit of the trigeminus. The 

 ventral surface otherwise presents very few features of note. 

 The ventral fissure, too, is not so deep as the dorsal. The 

 cranial nerves are discussed elsewhere in this article. 



Vetitricles. — In discussing the encephalic ventricles we 

 shall pass from the spinal cord cephalad. The canalis spinalis 

 is very small. It gradually enlarges in the medulla up to the 

 exit of the dorsal root of the vagus, where it is suddenly 

 extended dorsad to the surface, thus forming the fourth 

 ventricle. From this point the ventricle is very deep, but 



