152 ORGANIZATION OF THE HEAD v. 8- 



9. The cranial nerves of elasmobranchs 



These nerves are more easily studied in elasmobranchs than in any 

 other vertebrates, because of the relatively soft and transparent 

 cartilage through which they run. We may therefore take this oppor- 

 tunity to examine the whole series of cranial nerves in some detail in 

 the dogfish, beginning with the oculomotor nerve, the first ventral 

 root. Examination after removal of the brain will show clearly in any 

 vertebrate, including man, that this nerve arises from the ventral 

 surface, at the level of the hind end of the midbrain (optic lobes). 

 This is not true of the trochlear or pathetic nerve, which emerges 

 from the dorso-lateral surface of the brain but nevertheless is the 

 ventral root of the second segment. Its cells of origin lie close behind 

 those of the oculomotor nerve, in the ventral part of the brain. The 

 reason for the dorsal emergence is that the muscle lies dorsally and the 

 nerve has been modified so as to reach its muscle by running partly 

 within the tissues of the brain. The third ventral root (abducens), 

 which is very short, is clearly ventral. 



In looking for the dorsal roots that correspond to these three 

 segments we have to examine the trigeminal, facial, and auditory 

 nerves. The trigeminal of the dogfish, like that of man, has ophthalmic, 

 maxillary, and mandibular branches (Fig. ioo), but can be shown to 

 represent the dorsal roots of the two first segments. The ophthalmic 

 branch is a sensory nerve carrying fibres for skin sensation from the 

 snout. The maxillary branch supplies sensory fibres to the upper jaw, 

 whereas the mandibular is a mixed nerve to the skin and muscles of 

 the lower jaw. Besides these main branches there is also a small but 

 important sensory branch from the trigeminal to the eyeball (Fig. 

 ioo). This joins a motor root from the oculomotor nerve where the 

 latter swells slightly to form a ciliary ganglion. Two ciliary nerves 

 then carry motor and sensory fibres to the eyeball. In some specimens 

 a branch of the more anterior ciliary nerve leaves the eyeball anteriorly, 

 runs between the oblique muscles, and out of the orbit again to end 

 in the skin of the snout (Fig. ioo). Though this branch is small and 

 inconstant in the dogfish, its course corresponds exactly with that 

 of a much larger nerve in the related shark Mustelus and in skates and 

 rays. In these animals there are two ophthalmic branches of the 

 trigeminal nerve; one, having a course similar to that of the main nerve 

 in the dogfish, is the ramus ophthalmicus superficialis; the second, 

 the ramus ophthalmicus profundus, runs across within the orbit, gives 

 off the long ciliary nerve to the eyeball, passes between the oblique 



