2o6 Journal of Comparative Neurology and Psychology. 



The sensory nerve of this segment is the ophthalmicus 

 profundus. In selachians (119, 124, 58, 59, 96, 97) the neural 

 crest which gives rise to the profundus and trigeminus extends 

 over the mid-brain and cerebellum. As the ganglionic anlages 

 are being formed, those of the profundus and trigeminus are 

 separate, the profundus anlage being connected with the dorsal 

 wall of the mesencephalon and the trigeminus anlage with the 

 cerebellum. At a later time the profundus ganglion has lost 

 its connection with the mesencephalon and its root fibers pass 

 into the brain with the root of the trigeminus. When the pro- 

 fundus and trigeminus trunks are formed the profundus runs 

 forward along the inner side of somite 2 and the outer side of 

 somite i, the trigeminus runs over the outer side of somite 2. 

 The profundus and trigeminus are therefore one segment apart 

 and belong respectively to neuromeres v and vi. 



Essentially the same mode of development is seen in 

 Petromyzon (79, 76), and Koltzoff states that the segmental 

 arrangement is the same as in selachians. 



The separation of the profundus from the trigeminus as a 

 distinct segmental nerve is of course not new; it is in agreement 

 with the opinion long held by v. Wijhe, Wiedersheim 

 FuRBRiNGER and other comparative anatomists. 



The profundus is the general cutaneous nerve of the segment 

 to which somite i belongs. Its distribuion to the dorsal sur- 

 face of the head and snout, above and in front of the eye, seems 

 to indicate that it is only a dorsal ramus and also that in spread- 

 ing over the snout it has annexed an innervation territory not 

 primitively belonging to it. For the bearings of the latter 

 point see the treatment of segments anterior to segment 5, be- 

 low. When the head flexure is considered it is seen that the 

 area of distribution of the profundus is dorsal; and the almost 

 total absence of ventral structures in front of the mouth, i. e. 

 between the innervation territory of the maxillaris and the eye 

 and olfactory organ, explains the absence of a ventral ramus. 

 The N. ophthalmicus profundus is therefore to be considered as 

 the sole representative of the general cutaneous component in 

 segment 5, to which neuromere v, somite i, and N. Ill belong. 



