ICHTHYOLOGY. 



219 



Introduc- and among fishes the Lancelet offers the simplest nervous The fourth and last division of the brain, protected by Introduc- 



tion. 

 Nervous 

 System. 



system of the whole series of vertebrated animals, 



The spinal chord, or the nervous columns, for which, in- 

 dependent of its protecting investments, the term myelon 

 has been well devised by Professor Owen, is extremely 

 short in those fishes whose lumpish shape originates in the 

 abreviation and coalescence of the posterior vertebrae. In 

 Orthagoriscus and Lampris, the whole myelon appears to 

 be a minute ganglionic process of the brain. In Telrodon 

 and Diodon it is also very short, or about a twentieth of 

 the length of the fish ; but the rest of the neural canal is in 

 them occupied by a bundle of caudal nerves, usually named, 

 fi-om their resemblance to the tail of a horse, cauda equina. 

 Lophius has a myelon which reaches the twelfth vertebra ; 

 but its pointed end is concealed by a long cauda equina. 

 In most fishes there is no cauda equina, and the myelon 

 runs to the end of the neural canal. 



In fishes the brain consists of a few ganglionic masses, 

 which vary in number in different groups of species, and, 

 as has been mentioned, are wholly wanting in the lowest 

 form of the class. It is proportionally larger in the young 

 fish, and Professor Owen states that, on an average, its 

 weight is only a three-thousandth part of that of the whole 

 fish. Cams estimated the brain of the Bin-bot {Lola) as 

 forming the seven hundred and twentieth part of the en- 

 tire weight ; and that of the Pike has been found to be the 

 thirteen hundred and fifth part. 



We mean to mention only the more constant or impor- 

 tant encephalic masses, and even these in a very cursory 

 way. The myelon, on expanding into the brain, assumes 

 the name of " epencephalon," which is protected by 

 the occipital neural arch of the skull. In proceeding 

 from the spinal column onwards towards the nose, the 

 first rounded masses that we observe in the epencephalon 

 are not constantly present in fishes, nor do they exist in 

 any other class of vertebrals. They are called " vagal 

 lobes," and are largely developed in the Cypriiiidce, and 

 enormously in the Torpedo, Narcine, and other electric 

 Rays. In these they supply the large electric nerves. 

 They are also large in Gyninotus, which, however, does not 

 receive its electric nerves from these organs. The medulla 

 oblongata, and cerebellum, are more constant parts of the 

 occipital division of the brain. Fishes that possess much 

 muscular energy, and take their prey by the velocity of 

 their movements, have the largest cerebellum ; and the 

 other extreme of its size is found in the feeble Myxine or 

 Glutinous Hag which passes its parasitic existence buried in 

 the flesh of other fishes. It is also a mere fold in Poli/p- 

 terus, and is very small in the sedentary Discoboli, whose 

 ventral fins are employed as suckers to attach the fish to 

 the rocks. A mong osseous fishes the cerebellum attains the 

 largest size in the warmer-blooded, impetuous Tunny ; and 

 in the cartilaginous Sharks it is still more largely developed. 

 The next division of the brain, the " mesencephalon," pro- 

 tected by the neural arch of the parietal vertebra, is usually 

 larger than the other divisions in osseous fishes. To it be- 

 long the two optic lobes, whose function is denoted by their 

 name. They are hollow in most fishes, and are often 

 larger than the cerebellum, as in Polypterus and Lepido- 

 siren, of equal size with that organ in Eels, and smaller in 

 the Sharks and in Amblyopsis. Their existence at all in 

 this fish, which is blind, and in the Myxine, which is equally 

 destitute of sight, shows that they have some other function 

 than merely communicating with the optic nerves. The 

 " prosencephalon," inclosed in the neural arch of the frontal 

 vertebra, corresponds to the cerebral hemis])heres of the 

 higher vertebrals. It is smallest in the Myxines, equal to 

 the cerebellum in the Perch, and exceeds in size that of 

 the Stickleback, Gurnard, Lepidosteu^, Lucioperca, Am- 

 hlyopsis, and Raia. In Poli/p/erus and Lepidosiren it sur- 

 passes all the rest of tlie brain. 



the nasal neural arch, consists of two lobes, named " rhinen- 

 cephala" or " olfactory lobes," as they send forwards the 

 nerves that supply the organ of smell. 



Organ of Vision. — The position, direction, and dimen- 

 sions of the eyes of fishes vary greatly. In some they have 

 an upward aspect, and are of\en very close upon each other; 

 in others they are lateral, and so wide apart as to be even 

 directed slightly downwards. But of all anomalies, one of 

 the most extraordinary which their position presents, is that 

 of the Pleuronectes (such as Turbot, Floimders, Soles, &c.), 

 in which the two eyes are placed, as it were, the one above 

 the other, and both upon the same side of the head. In 

 certain species of the Eels and Siluri, they are so small as 

 to be scarcely visible ; while in other groups, such as Pria- 

 canthus, Pomalomus, and Orthagoriscus, they surpass in 

 proportional diameter whatever is known of the same organs 

 in the higher classes. Some ichthyologists have remarked 

 that fishes which habitually descend to great depths in the 

 ocean have large eyes ; and in the internal parasites and in 

 fishes which live in subterranean waters, we have examples 

 of the most rudimentary form of the organ of vision. In 

 the Glutinous Hag, and in the Amhh/opsis spelteus, the eye 

 is merely a minute black fold of skin on which a cerebral nerve 

 terminates. In Apterichtliys emeus, the eye-ball, still rudi- 

 mentary, is covered by the skin. It may be said in general 

 that the eye of fishes is large, and that its pupil especially 

 is broad and open ; a character probably connected with 

 the necessity of collecting whatever devious rays of light 

 may penetrate the obscure depth of waters. Fishes have 

 no true eyelids. The skin always passes over the eye, to 

 which it is slightly adherent ; and it is for the most part 

 sufficiently transparent for the passage of the solar rays. 

 In some species, such as Eels, it passes over without the 

 slightest fold or duplication. In others, as the well-known 

 Mackerel and Herring, it forms an adipose fold both before 

 and behind ; but these folds are fixed, and being unprovided 

 with muscles, have no mobility. Sharks have one, some- 

 what more moveable, on the inferior margin of the orbit. 

 The globe of the eye itself is very slightly moveable, 

 although, like that of man, it is furnished with six muscles. 

 Perhaps the most singular eye presented by the class of 

 fishes is that of Annbleps, which has two corneae, separated 

 by an opaque line, and two pupils pierced in the same iris, 

 so that one might deem it double ; but there is only one 

 retina, and a single vitreous and crystalline humour. 



Fls. 4G. 



Uranoscopus macropygus, an Acantbopterygian fipb in whieli tlie only spi- 

 nous ray is one in front of the ventral tin. The points of the pubic bones 

 are also spinous, and the undeveloped interspinous bones of the first 

 dorsal lie beneath the skin. The eye-ball is retractile to beneath the edge 

 of the orbit. 



In Uranoscopus macropygits and others of that genus, 

 the eyes can be elevated and dejiressed at the will of the fish, 

 on a cylinchical stem, above or below the plane of the flat 

 dorsal aspect of the head on wliich it is situated ; and in 

 the ferocious Zygcena, the eyes are stilted out fi-om the 

 sides of the head on cartilaginous expansions of the skull. 

 Fishes, in the range of their vision and acuteness of sight, 

 are very far inferior to birds, and inferior generally to 

 reptiles and beasts, yet, at the same time, it is evident that 

 they perceive their prey fi-oni a considerable distance ; and 

 the angler, who knows either how rapidly they seize, or how 



tion. 



Nervous 



System. 



