74 PROFESSOR J. C. EWART ON THE 



It might be asserted that in the case of Lsemargus, the sensory fibres of the five 

 dorsal branches of the branchial divisions of the vagus have united together to form the 

 lateralis nerve. A careful examination, however, of the lateralis seems to point to a 

 different conclusion. The fibres which form the lateralis nerve spring from the side of 

 the medulla nearly in a line with the middle roots of the facial nerve ; and the anterior 

 fibres lie in front of, and on a higher level than, the roots of the glossopharyngeus. 



Further, although the lateralis accompanies the other divisions of the vagus through 

 the long vagus canal behind the auditory capsule, it is only intimately related to the 

 nerve of the first vagus cleft. In connection with the lateralis and the first division of 

 the vagus (vagus I.) there is a ganglionic swelling crowded with large ganglionic cells. 

 Taking these and other facts into consideration, it may be inferred that the lateral canal 

 has been mainly developed from a sense organ on a level with the first vagus cleft, and 

 in relation with a special group of sensory fibres (similar to those of the facial which 

 innervate the three cranial cauals already described), which afterwards gave rise to the 

 lateralis nerve. It may be further inferred that by growing in different directions, this 

 embryonic sense organ has given rise to the pre-commissural, commissural, and trunk 

 portions of the lateral canal, with or without involving the branchial sense organs lying 

 above the second, third, and fourth vagus clefts. 



IV. The Dorsal Branches of the Cranial Nerves. 



Having succeeded in making out the innervation of the cranial canals in an Elasmo- 

 branch, in showing that the canals instead of being mostly supplied by the trigeminal, 

 as has been hitherto supposed, are intimately related to certain well-marked divisions or 

 branches of the facial and vagus nerves ; and having in previous papers dealt with the 

 cranial nerves of Lsemargus, Raia, and Torpedo, it is now possible to construct a plan 

 indicating the arrangement of the dorsal branches in an adult Selachian. 



Leaving out of consideration the olfactory and optic nerves, the first nerve with a 

 dorsal branch is the ophthalmicus profundus (o.n., fig. 2). Notwithstanding the fact 

 that this nerve has a distinct root in embryo Elasmobranchs, that it sometimes remains 

 separate from the trigeminal in the adult, and that in Laemargus, Raia, Torpedo, and 

 others, it has a large ganglion lying sometimes a considerable distance in front of the 

 Gasserian ganglion, notwithstanding all these facts, this nerve has sometimes been 

 considered a part of the trigeminal, sometimes as belonging to the oculo-motor, or as 

 " a communicating nerve between the third and fifth" (36, p. 153). 



A discussion of the views held as to the oculo-motor and profundus nerves would be 

 out of place in this paper. It is, however, desirable to say a word as to the composition 

 of a dorsal branch. In the case of the dorsal branch of a typical spinal nerve, the fibres 

 seem to be entirely distributed to the skin, and hence may be spoken of as somatic 

 sensory fibres. In the case of a typical cranial nerve, on the other hand, the dorsal 

 branch not only contains ordinary sensory fibres for the skin, but also special (in at least 



