Oculomotor Nerve in Scyllium canicula. 555 



nerve was so unexpected that I was led to examine other elasmobranch brains 

 of which I had sections, with a view to determining whether the condition in 

 S. canicula was peculiar to that species or whether the ganglion was of normal 

 occurrence but had escaped observation. 



The examination was without result, however, for in a number of specimens 

 of Acanthias, Raia, and Rhina I found no trace of ganglion cells upon the 

 third nerve root, but it cannot be asserted that in these species the ganglia 

 are absent, for in all of these brains the third nerve had been severed, it was 

 found, comparatively close to its origin. A search for these cells upon the 

 third nerve of Amphibia was somewhat more fruitful. In Ranct esculenta a 

 cluster of half a dozen large ganglion cells was found upon the nerve on one 

 side of my single specimen. In R. temporaria and 31olge sp. an odd cell or 

 two appeared in the sections of the nerve. In each case the cells were 

 found at or very near to the severed end, and it is highly probable that still 

 other cells had existed in the distal part of the nerve. 



The existence of these ganglia in S. canicula has not, I believe, been 

 recorded hitherto. This is, perhaps, not surprising, for the head of the adult 

 animal is far too large to be readily cut in its entirety, and, as my own 

 experience illustrates, where a study of the central nervous system is the 

 prime object, brains are likely to be removed from the head by the cutting of 

 the third nerves fairly closely to their superficial origin. And, in sections of 

 the entire heads of embryos of this species, of several different sizes, winch I 

 examined, I was unable to distinguish the future ganglion cells among the 

 numerous cellular elements present in the developing third nerve, although 



r 



some of my specimens were sufficiently advanced to show the elements of 

 the trigeminal ganglion already fairly well differentiated. 



Although I have seen these ganglia in but three specimens of S. canicula 

 I believe that they will prove to be actually of invariable occurrence in this 

 species. 



That " degenerated " ganglion cells occur upon the oculomotorius in man 

 has long been known, having been recorded by Thomsen in 1887.* They were 

 said to occur in the adult human subject as patches of altered tissue which, 

 Thomsen stated, were the remains of the encapsulated ganglion cells observed 

 by him in the root of the oculomotor in the new-born child. G-askell ( 89), 

 two years later, confirmed the discovery of these degenerated cells in the 

 third nerve of man and recorded the finding of similar cell masses in the root 

 of the fourth and sixth nerves also. He seems to have assumed that these 

 cells were always merely vestigial structures, but he nevertheless attached 

 considerable importance to the discovery. On the strength of the transient 



* Vide Tozer, 12. 



