30& PHILOSOPHY OF ZOOLOGY. 



Tlie surface of the skin of fishes is almost always covered 

 with a slimy fluid, to protect them from the penetrating in- 

 fluence of the surrounding element. This mucus is poured 

 out from small pores, situated under the scales in every 

 part of the body of some fishes, while in others, these ex- 

 cretory ducts are arranged in a determinate order. The 

 openings of the ducts, in some species, have correspond- 

 ing apertures in the incumbent scales. These ducts were 

 first observed and described by Steno, in his works, " De 

 MuscuUs et Glandulis^ p. 42. and " Elementorum Myolo- 

 g'ice Specimen,'''' 1669, 8vo. p. 72. The subject was after- 

 wards investigated by Peuradlt, Loeenzini, and Rivi- 

 Nus, and more recently by Monro. To this last author we 

 are indebted for many excellent observations and sketches *. 



• " In the skate, numerous orifices, placed pretty regularly over the 

 surface, have been observed by Steno to discharge this slimy matter. With 

 respect to these last, I have remarked some memorable circumstances. 

 First, I have discovered one very elegant serpentine canal between the skin 

 and muscles, at the sides of the five apertures into the gills. Farther for ■ 

 wards it surrounds the nostrils ; then it passes from the under to the upper 

 part of the upper jaw, where it runs backwards as far as the eyes. From 

 the principal part of this duct, in the under side or belly of the fish, there 

 are not above six or eight outlets ; but from the upper part near the eyes 

 there are upwards of thirty small ducts sent off, which open upon the sur- 

 face of the skin. The liquor discharged from these has nearly the same 

 degree of viscidity as the synovia in man. But besides the very picturesque 

 duct I have been describing, I have remarked on each side of the fish, a lit- 

 tle farther forwards than the five breathing holes, a central part, from which 

 a prodigious number of ducts issues, to terminate on almost the whole sur- 

 face of the skin, excepting only the snout or upper jaw. At these centres 

 all the ducts are shut ; and in their course ihey have no communication with 

 each other. In these two central parts, or on the beginning of the mucous 

 ducts, a pair of nerves, nearly as large as the optic, terminate ; and, which 

 is a curious circumstance with respect to them, they are white and opake in 

 their course, between the brain and their ducts ; but when they divide, they 

 become suddenly so pellucid, that it is impossible to trace them farther, or 

 to distinguish them from the coats of the ducts," Struct, and Phys. p. 21. 



