XXIV LIFE AND WORKS OF NICOLAUS STENO 



he was led into a discussion of the morphology of fishes and mam* 



mals. 



A few years after his description of the anatomy of the ray Steno 

 published an account of some discoveries, which he had made while 

 dissecting the head of a big shark. Again he first dwelt on the system 

 of the mucous canals ; then he described the lateral line system of canals, 

 which he had discovered in the eel and refound in all the fishes he had 

 afterwards examined; but as to the purpose of these canals he de* 

 clared himself to be ignorant. He next mentioned the organs of secre* 

 tion in the skin of the fish and the structure of the skin in general, 

 and here his generalizations, for once, carried him beyond the actual 

 facts. The structure of the eye was also touched upon by Steno, and 

 he was the first to correctly describe the stem which in the plagio* 

 stomes fixes the eye*ball in the orbit, as well as the optic nerve 

 which earlier zoographers had failed to detect. At the same time he 

 demonstrated the existence of a chiasma, which is lacking in other 

 fishes, and called attention to various facts concerning the central 

 nervous system, among others, as has already been mentioned, to the 

 small size of the brain. In connection with this Treatise Steno published 

 a brief description of another smaller shark (see above). He first men* 

 tioned, how through this dissection he found the proofs of the cor* 

 rectness of some of his earlier observations; he described the olfactory 

 organ, where his attention had especially been attracted by the exi* 

 stence of numerous folds of the skin, which he rightly regarded as 

 being of the same importance to the function as the folded and spon* 

 geous structure of the osseous tissue of the higher vertebrates. In the 

 same place Steno furthermore maintained that the testes of the female 

 mammals correspond to the ovaries of the oviparous animals; but this 

 has already been mentioned in a previous section. 



One part of Steno s description of the head of a big shark is, how* 

 ever, well worth dwelling upon, viz. the one which deals with the 

 shark's teeth. Steno gave figures of the shark's head with its mouth 

 open, as well as of single teeth, the figures being borrowed from 

 the MS. of Mercati's as yet unpublished work Metallotheca Vati= 

 cana; he described, how the several rows of teeth differed in con* 

 sistence, and admitted that he was unable to understand the use of 

 those teeth, which are quite soft and lie beneath the mucous mem* 

 brane. But as early as in the following Treatise he set forth as his 

 supposition that these soft teeth were meant to succeed the hard teeth, 

 according as the latter were falling out, and in his work De Solido In= 

 tta Solidum dxc. he finally arrived at a clear conception of their impor* 

 tance. He also made comparisons between sharks' teeth and the bodies, 

 Geology, which at his time were called glossopetrx. As to the formation of the 

 latter the then current views were very much at variance; but the 



