ICHTHYOLOGY. 



161 



Inlrmluc- and which is merely the anterior portion of the peritoneum 

 timi. strengthened by aponeurotic fibres. This sinus is extend- 

 "'"^'^ ed transversely, and receives by several different trunks 

 the veins of the liver, of the generative organs, of the kid- 

 neys, of the fins, branchiiv, and throat, and finally those of 

 the head, which tlicmselves partly pass by a sinus at the 

 back of the cranium. The first-mentioned sinus sends the 

 whole of this blood by a single orifice of its anterior con- 

 vexity into the auricle, which receives it through the open- 

 ing of its anterior portion. Two thin membranous valvules 

 protect this comnumication, and are turned towards the 

 auricle. The latter organ is ])laced in the pericardium, in 

 front of the great sinus, and above the ventricle, that is, on 

 its dorsal asjjcct. It presents very various and often re- 

 markable configurations. In osseous fishes it is usually of 

 a tetrahedral form, — in the cartilaginous kinds more i're- 

 quently rounded and depressed. It is situate beneath the 

 auricle, the cavity being so turned as to be almost vertical 

 next that organ, and horizontal towards the bulb. Its coats 

 are extremely robust, and furnished internally « ith power- 

 ful fleshy columns, its substance being composed of two 

 different layers. But it is in the bulb of the branchial ar- 

 tery that we find the most vigorous fibres, usually disposed 

 in a circular form. The prolongation of this bulb issues 

 from the pericardium, and becomes the branchial artery, 

 advancing forward beneath the single chain of small bones 

 which unites the arches of tlie branchiae. The branchial 

 artery soon divides, and in such a manner as to send a 

 branch to each brancliia. These branches pass along a 

 hollow groove on the convexity of each branchial arch, 

 and more external than the vein which follows the same 

 track, but in an opposite direction. To the arch are attach- 

 ed a great number of leaflets, parallel to each other, usu- 

 ally terminated in a forked point, and sometimes deeply 

 divided. The principal branch which passes along the 

 groove of the arch gives a smaller branch to each of the 

 leaflets ; and this branch, after being twice bifurcated, fur- 

 nishes an infinity of lesser branchlets, which meander over 

 the surface of each leaflet, till they are finally converted 

 into extremely minute veins. These little vessels meet on 

 each side in a branchial vein, which proceeds along the in- 

 ternal margin of the lateral lobe of the leaflet, and the two 

 veins open into the trunk of the great vein of the bran- 

 chia. 



On passing out of the dorsal side of the branchia, the 

 branchial veins assume the structure and functions of arte- 

 ries ; even before their arrival at this point, the anterior 

 have already sent several branches to different portions of 

 the head ; and it is necessary to remark, that the heart and 

 several parts situate in the chest receive their blood from 

 a branchial vein, by means of an offset issuing from near 

 its source, and consequently anterior to its exit from the 

 branchia;. Nevertheless, it is only by the re-union of the 

 trunks proceeding from the four branchiae that the great 

 artery is formed which carries the blood to the viscera and 

 all the parts of the trunk, and which is by consequence 

 the representative of the aorta of the Mammalia, — but of 

 m aorta which possesses neither auricle nor ventricle at 

 its base. Thus, according to Cuvier's views, the left cavi- 

 ties of the heart of quadrupeds do not exist in fishes, but 

 are replaced by a simple vascular apparatus, situate above 

 the branchise, in like manner as the right cavities are placed 

 beneath them. 



SECT. via. THE RESPIRATION OF FISHES. 



It is thus by an almost infinite subdivision of the vessels 

 over the surface of the branchiae or gills, that the blood of 



fishes becomes subjected to the influence of an ambient Introduc- 

 fluid. This fluid is of course water, which is made to flow tion. 

 incessantly between the branchiae by the movement of the '^■"''Y'*^ 

 jaws, and of the opercular and hyoidean apparatus. This 

 mode of respiration is equally necessary to fishes, as the di- 

 rect respiration of air is to other animals ; but the action 

 of water on the blood is much more feeble than that of air. 

 It appears that it is neither the water itself, nor the oxygen 

 contained in it, which effects the respiration, but the small 

 portion of air which is held in solution or mingled with the 

 water. If this is expelled by ebullition, fishes cannot live ; 

 and many species are obliged to rise fre<iuently to the sur- 

 iiice for the purpose of breathing atmospheric air. It is 

 easy to suffocate various kinds, by keeping them beneath 

 the surface, enclosed in a gauze net. In the respiration of 

 fishes, as in that of other animals, both the atmospheric air 

 and that contained in the water give out their oxygen. 

 The absorption of the latter, however, is very trifling 

 among these aquatics, for it has been calculated that a man 

 consumes fifty thousand times more than is required by a 

 tench. When fishes are deprived of water, they perish not 

 so much for want of oxygen, as because their branchiae be- 

 come dry, and their blood can no longer circulate with 

 freedom. Hence the species of which the branchial orifice 

 is small, as the eel, or those which possess receptacles for 

 moisture, like Aiiabas and Ophicephalus, long survive ex- 

 posure ; while such as have their gills greatly cleft and open, 

 as the herring, expire almost instantly when withdrawn 

 from their moist abode. 



SECT. I.K. THE SWIMMING BLADDER OF FISHES. 



One of the most remarkable and characteristic organs of 

 fishes is the swimming bladder, commonly so called. In 

 many genera it has no opening or external communication, 

 and the air which it contains must therefore be the result 

 of secretion. It is composed of an extremely fine internal 

 tunic, and of another of a thicker texture and peculiar 

 fibrous structure, remarkable for producing the finest kind 

 of isinglass. It is enclosed within the general coating with 

 which the peritoneum invests the other viscera. It is 

 sometimes simple, as in perch, sometimes furnished with 

 more or less numerous appendages, as in some of the had- 

 dock tribe, or branched, as in certain SciantB} Occasion- 

 ally we find it divided, as it were, into two parts, by a re- 

 striction, as in the genus Cyprinus, many of the Salmonida;, 

 and others. The Catastomce have it even divided into 

 three. It is chiefly among the abdominal fishes that we 

 find it communicating by a tube or tunnel with the intes- 

 tinal canal, and either directly with the oesophagus, as in 

 Cyprmus, or with the base of the stomach, as in the her- 

 ring. That of the sturgeon opens into the former portion 

 by means of a large orifice. The contents of the swim- 

 ming bladder are usually found to be azote, mingled with 

 some fractional parts of oxygen or carbonic acid. A dif- 

 ference of opinion, however, seems to exist regarding 

 the proportion of oxygen, which is estimated as much 

 greater both by Configliachi and Biot. Some physiologists 

 appear to have regarded the swimming bladder as a true 

 lung, which both admitted and returned the external air ; 

 but in many species the air-duct which connects the bladder 

 with the gullet is entirely wanting ; and in many others 

 which remain constantly at prodigious depths, the quantity 

 of oxygen gas in the swimming bladder is greater than in 

 those the abode of which is near the surface. Indeed the 

 oxygen is said to increase in quantity in proportion to the 

 depth at which the species dwells. Carus considers it pro- 

 bable that the vessel in question performs a part analogous 



' For representations of various forms of the swimming bladder of fishes, see Plate CCXCVII. figs. 3, 6, 7, a 

 .YII. 



