ICHTHYOLOGY. 



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most fluid. In structure, however, it is nearly the same, 

 although characterised by a few constant marks. The 

 subdivisions of the brain and cerebellum, or their tu- 

 bercles and lobes, are more numerous than in the mam- 

 malia and birds. In one genus of fishes, the Gadus, 

 Dr Monro ( Structure and Pht/iiology of t'ithes, Edin. 

 p. 4K) found spheroidal bodies between the dura 

 and pia mater, and covering the greater part of their 

 nerves, like a coat of mail, in their course towards the 

 organ* to which they are destined. He was unable to 

 aamliin their use. 



The tpatal matron in fishes, instead of being convey- 

 ed in a vertebral canal, as in the mammalia, pane*) 

 through the space formed by the superior processes of 

 the vertebra. Like the other nerves of fishes, the size 

 of the spinal marrow is in proportion to the size of the 

 body, not to the brain from which it proceeds. 



Having offered these few observations on the brain, 

 we now proceed to consider the organs of the five 

 MOM*, adverting to their structure, their functions, and 

 their importance in the classification of fiiher 



1. Organi of melt. The external organs of smell 

 present several remarkable differences, according to the 

 species, varying in number, shape, and position. In 

 i.iany fishes the nostrils are single, while in others they 

 are divided at the surface by a transverse membrane, 

 and thus exhibit the appearance of being double on 

 each tide. They likewise vary in shape, being round 

 in the cod-fish, oval in the conger-eel, and oblong in 

 others. They are placed in the snout in many fishes, 

 near the eye* in some, and between the eyes and the 

 snout in other*. Where the openings are double on 

 each side, these are either placed contiguous to each 

 other, a* in the carp, or at a little distance, as in the 

 perch. The nostrils in some instances appear like short 

 tubes. 



The nasal opening* are furnished with a few muscu- 

 lar fibres, which are capable of executing a limited con- 

 tractile motion. This motion, however, in living fishes, 

 can seldom be perceived. 



Proceeding to the examination of the inside of the 

 nostril, we may observe, that in the sharks and skates 

 the nasal lamina- are placed parallel to each other on 

 both side* of a large lamina, which extends from one 

 end of the fossa to the other, and consist of folds of the 

 pituitary membrane. In the other fishes, whether car- 

 tilaginous or easeoo*, the lamina; proceed like radii from 

 an elevated and round tubercle. The pituitary mem- 

 brane in some fishes, a* in the pike, is furnished with 

 reticular ramifications of black vessels, but in the great- 

 er number of fisbe* these vessels are red. Between 

 these are situated some small papillx, which pour out 

 a thick mucilage. 



The olfactory rteroei, at their origin, form swellings 

 or knots, so large as frequently to have been mistaken 

 for the real brain. These tubercles in skates and sharks 

 are united into one homogeneous medullary mass, from 

 each of the lateral part* of which the olfactory nerves 

 arise. In the species of the genera I'letironectes, Clu- 

 pea, Esox, I'erca, and Salmo, there are two pair of tu- 

 bercle*, the anterior of which is smaller than the other. 



In the cartilaginous fishes, as the skate and shark, 

 the olfactory nerve is very soft. It is, in them, a bulb 

 which pane* obliquely forward toward* the nares, 

 which are at a greater or less distance from the brain 

 according to the specie*. The spinous fishes have the 

 olfactory nerve very long and slender. In those which 

 have the snout elongated, this nerve is received into a 

 tube. In those with short snouts, the 



nerve is surrounded by a fine membrane only, which Structure 

 appears to be the same as that which contains the tkt or , an< ? 

 oily humour that covers the brain. In the haddock, f"^'^"* 

 anil some otiier fishes, the olfactory nerve, in its course _r- " _J- 

 1 rum the brain to the nose, passes through a cineritious 

 ball, which resembles the cineritious matter connected 

 in our body to the olfactory nerve within the cranium. 



When the olfactory nerve arrives behind the folded, 

 membrane which we liave described, it is dilated to be 

 applied to the whole of its internal and convex surface. 

 In some fishes no previous enlargement takes place, 

 while in others the nerve swells into a real ganglion. 

 When expanded, it has been compared to the retina, but 

 the filaments of which it is composed are more dis- 

 tinct. 



The sense of smell in fishes is supposed by many to 

 furnish them with the most delicate tests, for searching 

 after and distinguishing their food. Dr Shaw (General 

 Zoology, vol. iii. p. 9.) states, that " if you throw a fresh 

 worm into the water, a fish shall distinguish it at a con- 

 siderable distance; and that this is not done by the eye- 

 is plain from observing, that after the same worm has 

 been a considerable time in die water, and lost its smell, 

 no fishes will come near it ; but if you take out the 

 bait, and make several little incisions into it, so as to 

 let out more of the odoriferous effluvia, it shall have 

 the same effect as formerly. Now it is certain, that had 

 the animals discovered this bait witli their eyes, they 

 would have come equally to it in both cases. In con- 

 sequence of their smell being the principal means they 

 have of discovering their food, we may frequently ob- 

 serve them allowing themselves to be carried down 

 with the stream, that they may ascend again leisurely 

 against the current of the water ; thus the odoriferous 

 particles swimming in that medium, being applied 

 more forcibly to their organs of smell, produce a strong- 

 er sensation.'' We do not presume to dispute the ac- 

 curacy of these observations, but we may observe, 

 that the well known voraciousness of fishes, the ea- 

 gerness with which they will seize a metal button, > 

 or any glittering object, the whole art of artificial 

 bait and fly-fishing, all seem to point out the or- 

 gan of sight as the principal instrument by which they 

 discover their food. Besides, the organs m smelling are 

 by no means favourably situated for receiving quickly 

 the impressions new objects are calculated to produce. 

 In the chondropterygit the nares communicate by ;i 

 groove with the angles of the mouth, but in general 

 the organs of smell nave no communication with tho-r 

 of mastication or respiration ; and as the external open- 

 ings are narrow, and but ill supplied with muscles, we 

 are at a loss to-conceive in what manner the water im- 

 pregnated with odoriferous particles is thus rapidly 

 applied to the extremities of the olfactory nerve. Al- 

 ternate absorption and ejection of the water have never 

 been observed. The same water we know must pass 

 through the mouth, and be spread over the extended 

 surface of the gills ; so that we may presume, until far- 

 ther light be thrown on the subject, that these latter or- 

 gan* may likewise contribute to warn the fish of the 

 presence or absence of salutary or noxious impregna- 

 tions. 



The organs of smell furnish the ichthyologist with 

 ome important characters in the description of the 

 species. These have hitherto been too much neglected, 

 as they have the advantage of being permanent. 



2. Organi of tight. The eyes of fishes, like all other 

 red-blooded animals, are two in number. They vary 

 greatly in position, both being, in some species, on the 



