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V. — The Lateral Sense Organs of Masmobranchs. I. The Sensory Canals of 

 Leemargus. By J. C. Ewaet, M.D., Regius Professor of Natural History, 

 University of Edinburgh. (Plates I. and II.) 



(Read 6th July 1891.) 



Introductory. 



Some years ago, when studying the electrical organs of' the torpedo, I was forced to 

 the conclusion that the nerves supplying the batteries had not been accurately described, 

 and that notwithstanding the statements in the most recent works, the first electric nerve 

 is not derived from the trigeminus. Finding some difficulty in making out the arrange- 

 ment of the cranial nerves in the greatly specialised torpedo, I directed my attention, 

 first to the skate, and later to certain sharks, more especially to the Greenland shark 

 (Leemargus). I had not proceeded far before I was convinced that we had still much to 

 learn as to the anatomy of the cranial nerves of both the higher and lower vertebrates. 



Up to a certain point I made satisfactory progress, and early in March 1889 was in 

 a position to communicate to the Eoyal Society a preliminary paper " On the Cranial 

 Nerves of Elasmobranch Fishes " (1) ; and, in the following year, papers on the cranial nerves 

 of the torpedo (2) and on the development of the ciliary ganglion (3). When, however, I 

 endeavoured to interpret my results, more especially when I endeavoured to compare the 

 cranial nerves in Selachians with those of the higher vertebrates, innumerable difficulties 

 presented themselves. After full consideration, there seemed only two possible lines 

 along which further progress was possible. The one was to study anew the development 

 of the cranial nerves in two or more vertebrate groups ; the other to make a special 

 study of the innervation of the more important organs peculiar to fishes. 



Many able investigators having already directed their attention to the development 

 of the nervous system, without, it must be confessed, arriving at any very generally 

 accepted conclusions, I ventured to think that, with the help of the embryological facts 

 already established, I would best succeed by trusting mainly to the old methods of the 

 comparative anatomist. I believed that, by a careful study of the conditions in the adult 

 Selachians, certain morphological questions would be settled, and that a new base of 

 operations might be opened up for the embryologist. Hence, instead of publishing the 

 views I entertained as to the relation of, e.g., the complex facial of the Selachian, with its 

 comparatively simple homologue in the higher vertebrates, I decided to first thoroughly 

 work up the lateral sense organs — structures which reach a remarkable development in 

 fishes, but are entirely absent in the higher vertebrates. In this way I hoped to 

 determine which of the many large and well-marked nerves of the fish we should expect 

 to find absent in Sauropsida and Mammalia. 



VOL. XXXVII. PART I. (NO. 5). L 



