ICHTHYOLOGY. 



157 



pelled forwards by the rapid flexure of the extremity 

 acting laterally upon the resistance offered by the water. 

 Generally speaking, neither the pectoral nor the ventral 

 fins are of any material use during swift progressive mo- 

 tion ; they rather serve to balance the body, or to aid its 

 gentler movements while in a state of comparative re- 

 pose. In fli/ing fishes, as they are called, the pectoral 

 fins are of such great length and expansion as to support 

 the animal in the air; and the strength of muscular ac- 

 tion might probably suffice even for a longer flight, but 

 for the necessity of constant moisture for the purposes of 

 respiration. The drying of the gills in an individual of 

 this class is attended by results analogous to those pro- 

 duced by submersion in the case of a land animal ; — and 

 a flying fish is obliged to descend to respire, in like man- 

 ner as a swimming quadruped, or disguised mammiferous 

 animal (as we may term a whale), is under the necessity 

 of ascending for the same purpose. 



The head of fishes exercises but a slight movement 

 independent of the rest of the body ; but the jaws, hyoid 

 bone, palato-temporal and branchial arches, and pliaryn- 

 gial and opercular bones, are very free in their motions. 

 The muscles of fishes, like those of other vertebrated 

 animals, are composed of fleshy fibres more or less colour- 

 ed, and of tendinous fibres of a white or silvery colour. 

 With the exception, however, of certain special muscles 

 which are sometimes of a deep red, the flesh of fishes is 

 much paler than that of quadrupeds, and still more so than 

 that of birds. In some species it is even entirely white. 



SECT. V THE NERVOUS SYSTEM AND SENSES OF FISHES. 



The sensitive system of fishes, like that of the higher 

 classes, is composed of the external senses, of a central 

 medullary apparatus, and of nerves of communication. 

 As in the classes alluded to, the central portion of the ner- 

 vous system, that is, the brain and spinal marrow, occu- 

 pies the cavity of the cranium and vertebral column. 



As fishes respire through the intervention of water alone, 

 that is, as they can scarcely avail themselves, in rendering 

 their blood arterial, of any tiling more than the small por- 

 tion of oxygen contained in the air which is suspended in 

 the water, their blood is necessarily cold, and their gene- 

 ral energy, and the activity of their senses and movements, 

 are less than among Mammalia and birds. Their brain 

 also, though of similar composition, is proportionally much 

 smaller ; and the external organs of the senses do not seem 

 of such a nature as to be capable of impressing or convey- 

 ing towards it any vivid excitement. Indeed the most 

 striking characteristic of the brain of fishes is its extreme 

 smallness, when compared either with the total size of the 

 body, with the mass of nerves which proceed from it, or 

 with the cavity of the cranium in which it is contained. 

 In the burbot, or Gadus lota, the weight of the brain to 



that of the spinal marrow is estimated by Carus to be as 8 Introduc 

 to 12, and to that of the whole body as I to 720. It was t'o"- 

 previously known that the brain of the pike weighed in ^'"V*^ 

 proportion to that of the whole body as 1 to 1303. Now, 

 in many small birds, the brain, viewed in relation to the rest 

 of the body, is equal to a twentieth part. In the genera- 

 lity of fishes, the spinal marrow extends along the whole 

 of the caudal vertebra; ; and it is thus that it preponderates 

 over the brain. The Lop/iius piscatorius, however, and a 

 few other species, form remarkable exceptions to this rule, 

 as in them the sjiinal marrow disappears before it reaches 

 the eighth vertebra ; but in the greater proportion of 

 cases it may be said that the spinal cord in this class 

 terminates by a single thread in the last caudal vertebra.' 

 The brain of fishes by no means fills up the cavity of the 

 cranium ; and the interval between the pia-mater which en- 

 velopes the brain itself, and the dura-mater, which lines 

 the interior of the skull, is occupied only by a loose cellu- 

 losity, frequently impregnated by an oil, or sometimes, as 

 in the sturgeon and thunny, by a rather compact grease. 

 It has also been remarked, that this void between the cra- 

 nium and the brain is much less in young subjects than in 

 adults ; from which it may be inferred, that the brain does 

 not increase in an equal proportion with the rest of the 

 body. Cuvier, in fact, has fovmd its dimensions nearly 

 the same in different individuals, of which the general size 

 of the one was double that of the other. 



When compared with that of quadrupeds, the brain of 

 fishes has been said to possess an embryonic character, 

 and to have its greatest development in the cerebellum, 

 the seat of the appetites. Of all vertebrated animals, fish 

 in fact exhibit the smallest apparent signs of sensibility. 

 Having no elastic air to act upon, they are necessarily 

 mute, or nearly so ; and all the sensations which the de- 

 lightful faculty of voice has called into being among the 

 higher tribes, are to them unknown. Their glazed im- 

 moveable eyes, their fixed and bony faces, their bodies and 

 members moving altogether, if they move at all, admit of 

 little play in their physiognomy, and of scarcely any ex- 

 pression to their emotions. Their ears, surrounded on 

 every side by the bones of the cranium, destitute of exter- 

 nal conch, without any internal cochlea, and composed 

 merely of some sacks and membranous canals, scarcely 

 suffice for the perception of the loudest sounds. Even 

 their sight may be supposed to find but little exercise in 

 those profound depths where so many of the inhabitants of 

 ocean dwell, although the largeness of the visual organs in 

 many species probably in some measure makes amends for 

 this deficiency of light. But even in those species the eye 

 cannot change its direction ; still less can it alter its focus, 

 so as to accommodate the vision to a varying distance ; for 

 the iris neither dilates nor contracts, and the pupil remains 

 for ever the same in all degrees of light. No tear moistens 

 its glazed surface, no eyelid clears or protects it, and it 



bone of the cincture, which supports the two last named, will then necessarily represent the humerus, and the first and second (46-7) 

 the shoulder blade. There still remains to be mentioned a species of style, almost always composed of two pieces, 49 and 50. 



Carpal bones. At the outer edge of the radial and cubital bones adhere the small flat bones, 53, compared to the carpus. Their 

 function is to support the rays of' the pectoral fin, 53, a, however numerous these may be, with the exception of the first, which arti- 

 culates directly with the radius or upper bone, 62. 



B07ies of the hinder cxtremitij. The os innominata, the thigh, the tibia, and the tarsus, are represented in fishes by a single bone, 

 80, usually of a triangular form, but more or less complicated by processes and projecting plates. Its posterior side affords attach- 

 ment to the rays of the ventral -fins. In eels and others, in which the ventral fins are wanting, the bone is also absent. 



The rays of the extremities. These rays, that is, those of the pectoral and ventral fins, 82 and 53, o, without being as symmetrical as 

 those of the vertical fins, are equally divisilde into halves. Except the external ray of the ventral in the Acanthopterygii, 81, they 

 are almost always articulated, but their bases become solid, and there tlie articulation is scarcely if at all perceptible. 



* In regard to the shortness of the spinal cord in Lophius, the fact, as above referred to, is taken from the dissertation of Apostolo- 

 Arsaki, a Greek doctor, who published De. piscium ccrcbro et medulla spinali, Halle, 1813 ; but in a note to the Hist. Nat. dcs Poissmis 

 of Cuvier (vol. i. p. 437), we find the following correction of that statement ; — " Sa moelle regne presque tout le long de IMpine ; 

 mais elle est enveloppde et cache'e par les nert's, qui naissent beaucoup plus haul qu'ils ne sortent." It is certain, however, from Cu- 

 vier's recent statement, that the supposed character is truly exhibited by the moon-fish (Lainpris guttatus, Retz ; opah of Pennant), 

 " Ou la moelle dpiniere est tellement raccourcie qu'elle ne semble qu'une petite proiminence conique de I'enc^phale, de laqueUe les 

 diffi^rentes paires de nerfs partent comme une queue de cheval." {Ibid.) 



