NERVES OF THE DOGFISH 327 



2. The radix mesencephalica V 



The radix mesencephalica V is formed in Squalus from the 

 anterior rootlet of the portio minor of the trigeminus roots. 

 This anterior rootlet passes in through the brain substance al- 

 most exactly at a right angle to the longitudinal axis of the med- 

 ulla (fig. 25, pmn.Vl). In it can be distinguished two constitu- 

 ents, a dorsal and a ventral (fig. 13, pmn.Vl, mes.V). On reach- 

 ing the anterior continuation of the tractus spinalis trigemini, the 

 dorsal fibers turn abruptly within the latter as the radix mesen- 

 cephalica V, and the ventral fibers pass directly mesially into 

 the lateral motor column. The mes. V continues anteriorly and 

 dorsally, ventrolateral to the lateral angle of the fourth ventricle. 

 Before the posterior peduncle of the cerebellum is reached, the 

 fibers of the mes. V- have separated from those of the spinal V 

 tract, and curving around the lateral angle of the ventricle ascend 

 rapidly in the base of the cerebellum, in their passage losing their 

 compact arrangement and taking the form of a diffuse fibrous 

 band as they pass along the lateral wall of the ventricle (fig. 12). 

 Reaching the transverse level of the posterior border of the mid- 

 brain, the radix curves around the lateral border of a fissure-like 

 at eral extension of the fourth ventricle in the lateral wall of the 

 cerebellar segment (fig. 19), dividing into two limbs, of which a 

 lateral one sends out branches into the lateral substance of the 

 cerebellum. The mesial limb of the radix meets the root of the 

 trochlearis rising from its nidulus situated anteriorly and ven- 

 trally (figs. 12 to 14, 18 and 19). The two tracts pass through 

 each other, interlacing in a manner difficult of analysis, the 

 trochlearis passing posteriorly and dorsally from the place of 

 crossing to its decussation in the posterodorsal wall of the mid- 

 brain. The radix after passing through the trochlearis divides 

 diffusely and is distributed to the mesencephalic tectum, presum- 

 ably to the nidulus magnocellaris (figs. 14 and 18). 



According to Neal and others, the radix mesencephalica V in 

 Squalus is a motor tract originating in the nidulus magnocellaris 

 and passing out of the brain peripherally through the maxillo- 

 mandibular division of the trigeminus. The writers agree with 



