MUSTELUS LJiYIS. 145 



it arises from the brain could not be satisfactoi-ily deterniined 

 in my 12"2 cm. embryo, because of the crowding together of 

 the parts here concerned, and the different bundles of fibres 

 that compose the several roots could not be separately traced 

 through the ganglion, my sections not having been prepared 

 for this especial reseai'ch. In my 55 mm. embryo the 

 apparent roots could, on the contrary, be easily distinguished, 

 and the different components of the several peripheral nerves 

 could be determined in a general, but certainly not in a 

 complete or absolutely accurate way. From a consideration 

 of the two embryos the following approximately correct 

 results were arrived at. 



The so-called anterior root of the trigeminus is formed by 

 two somewhat distinct rootlets — a dorsal one, having a deeper 

 origin in the medulla, and a ventral one, having a more 

 superficial origin. The dorsal rootlet is certainly largely 

 formed by a bundle of fibres that have a course corre- 

 sponding exactly to that ascribed by Haller (28) to the 

 inner motor trigeminus root of his descriptions of Scyllium. 

 The ventral rootlet certainly arises largely from the " dorso- 

 laterale Laugsbahn " of Haller's descriptions, that is, from 

 the ascending fiftli tract of English authors. Haller says 

 (p. 437) that in Scyllium all the sensory fibres of Trigeminus I 

 are derived from this latter tract, and this, if true of Scyllium, 

 must doubtless also be true of Mustelus. Both rootlets in 

 Mustelus enter the anterior portion of a large intracranial 

 ganglion. Different regions, I'ather than separate and in- 

 dependent parts of this ganglion, can be distinguished, and a 

 considerable part of the fibres of the ventral rootlet are seen 

 to enter a ventro-lateral part of the ganglion, which, opposite 

 the antei-ior half of the ganglion, is separate and independent 

 from the rest of the ganglion, but posteriorly is completely 

 fused with it. From this lobe or process of the ganglion the 

 ramus ophthalmicus profundus arises, that nerve hence, very 

 probably, being composed entirely of general cutaneous 

 fibres derived from the ascending fifth tract. The i-amus 

 profundus of Mustelus can thus in no way be the homologue 



