The Lateral Sensory Canals, the Eye-Muscles, and the Peripheral Distribution of certain of the Cranial Nerves of Mustelus lævis

Abstract

ABSTRACT I have recently had occasion to examine three series of sections of the head of embryos of Mustelus lævis, one of them being an embryo 12·2 cm. long. It was not my intention, when these sections were prepared, to make any extended study either of the lateral canals or of the cranial nerves of the fish ; the investigation I had proposed relating entirely to the innervation of the sensory organs of the ampullae. I have long had a very decided impression, opposed to that of most workers on the subject, that these ampullary organs must be genetically related to the terminal buds of ganoids and teleosts rather than to the pit organs of those fishes; and I thought that I should easily be able to get some positive evidence of this in the general course and position of the nerves that innervate them in advanced selachian embryos. This positive evidence I have wholly failed to get, for the very simple reason that, in the main nerve trunks, I could not distinguish in my sections the ampullary fibres from the lateral canal ones. Disappointed in this at the very beginning of the investigation, I nevertheless decided to quite carefully trace the lateral canals and the nerves that innervate them and the ampullæ, as far back as my sections went, that is, nearly to the level of the first gill slit. Careful consideration of these observations has fully convinced me, though indirectly, that the ampullary organs do represent the terminal buds of ganoids and teleosts, and not the pit organs. As, in this research, I was also led to trace the other cranial nerves of the region under consideration, and as my observations differ in certain respects from, and complete in others, the results of earlier writers on the subject, I have thought best to fully describe, not only the lateral canals of the head and the ampullary canals, but also the facial, trigeminal, and eye-muscle nerves, notwithstanding the fact that there will necessarily be, in these descriptions, a certain amount of repetition of well-known facts.