224 



ICHTHYOLOGY. 



Introduc- 

 tion. 

 Digestive 

 System. 



Fig. 81. 

 Labial teeth of Petromyson mordax. 



In the majciity of osseous fishes, besides the Hps, which, 

 liaving no i^cculiar 

 muscles, can exert 

 hut Httle strength in 

 retaining the ah-, 

 nients, there is gene- 

 rally in the inside of 

 each jaw, behind the 

 anterior teeth, a kind 

 of membranous fold 

 or valve, formed by a 

 replication of the in- 

 terior skin, and di- 

 rected backwards, of 

 which the effect is to 

 hinder thealimentary 

 substances, and espe- 

 cially the water gulp- 

 ed during respiration, 

 from esca])ing again 

 by the mouth. This 

 structure does not, 

 as formerly supposed, p. j, 



constitute a character Pharyngeal teeth of Petromyzm mordax. 



restricted to the genus Zens, but exists in an infinity of 

 fishes. The food seized by the teeth of the jaws, and 

 detained by the valve just mentioned, is carried still farther 

 back by the teeth of the palate and tongue when these 

 exist, and is at the same time prevented by the rakers of the 

 branchial arches fi-om penetrating between the intervals of 

 the branchiae, where it might injure the delicate organs of 

 respiration. The movements of the jaws and tongue can thus 

 send the food only in the direction of tlie pharynx, where it 

 undergoes additional action on the part of the teeth of the 

 pharyngeal bones, which triturate or carry it backwards 

 into the oesophagus. The last-named part is clothed by a 

 layer of strong, close-set, muscular fibres, sometimes form- 

 ing various bundles, the contractions of which push the 

 alimentary matter into the stomach, thus completing the 

 act of deglutition.' 



SECTION VII.— THE CIRCULATION OF FISHES, 



Fishes, in common with warm-blooded animals, are pro- 

 vided with a complete circulation for the body, and with 

 another equally comjilete for the organs of respiration, and 

 with a particular abdominal circulation terminating at the 

 liver by means of the vena porta ; b\it their peeuiiar cha- 

 racter consists in this, that the branchial circulation alone 

 is provided at its base with a muscular apparatus or heart, 

 corresponding to the right auricle and ventricle of the higher 

 classes, while nothing of the kind exists at the base of 

 the circulating system of the body ; in other words, the 

 let\ auricle and ventricle are entirely wanting — the bran- 

 chial veins changing into arteries without any muscular 

 envelope. 



The muscular apparatus of their circulation is composed 

 of the auricle, the ventricle, and the bulb of the pulmonary 

 artery, and the auricle itself is preceded by a large sijnis, in 

 which all the veins of the body terminate; thus there 

 are in a single series four cavities separated by constric- 

 tions, into which the blood must flow in its progress fi-om 

 the body to the branchiae. Their size is small in propor- 



tion to the dimensions of the body, and does not in- Introduc- 

 crease in the same ratio with the growth of the indi- tion. 

 vidual. Three of these receptacles, the auricle, the heart. Vascular 

 and the bulb, are lodged in a pericardiimi, which is itself /^ "^^Ij 

 placed beneath the pharyngeal bones, between the in- ^ ^ " 

 ferior parts of the branchial arches, and for the most part 

 protected externally by the scapular arch. Except in 

 the Plagiostomes, the great venous sinus is not j)laced in 

 the pericardium, but between the posterior partition of 

 that cavity and the membrane which represents the dia- 

 phragm, and which is merely the anterior portion of the 

 peritoneum strengthened by aponeurotic fibres. 'I'his sinus 

 is extended transversely, and receives by several different 

 trunks the veins of the liver, of the generative organs, of 

 the kidneys, of the fins, branchiae, and throat ; and finally 

 those of the head, which themselves partly i)ass 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 an- 

 terior convexity into the auricle, which receives it through 

 the opening of its anterior portion. Two thin membranous 

 valvules protect this communication, and are turned towards 

 the auricle. The latter organ is placed in the pericardium, in 

 fi-ont of the great sinus, and above the ventricle, that is, on its 

 dorsal aspect. Tiie ventricle presents very various and 

 often remarkable configurations. In osseous fishes it is 

 usually of a tetrahedral form, — in the cartilaginous kinds more 

 fiequently 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 with power- 

 ful fleshy cohimns, 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 imites the arches of the branchiae. The branchial 

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

 branch to each branchia. 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 at- 

 tached a great mmiber of leaflets, parallel to each other, 

 usually 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, 

 furnishes an infinity of lesser branchlets, which meander 

 over the surface of each leaflet, till they finally termir 

 nate in extremely minute veins. These little vessels 

 meet on each side in a branchial vein, w hich proceeds along 

 the internal margin of the lateral lobe of the leaflet, and 

 the two veins open into the trunk of the great vein of the 

 branchia. 



On passing out of the dorsal side of the branchiae, 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 fi'om 

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

 its source, and consequently anterior to its exit fi-om the 

 branchiae, Nevertheless, it is only by the re-union of the 



The various notices (as already intimated) of the .internal structure of fishes contained in the article Comparative Anatomy of 

 this work (vol. iii.) absolve us from the necessity of presenting any details regarding the form and constitution of the intestinal canal, 

 and of certain other important interior organs of tlie class. The forms of the stomach, whether '• siphonal'' or " ccpcal," the pancreatic 

 apparatw passing from slender ccecal appendages to the intestinal canal, which secrete a proper fluid but do not admit the food, gra- 

 dually increasing in complexity to the pancreatic conglomerate gland of the sturgeon, are parts of structure which the ichthyologist is 

 required always to pay attention to ; and also to the extension of the inner coats of the intestine by valvular folds, producing a more 

 extensive secreting surface, and delaying the passage of the food. The two forms of spiral valves in the lower gut of sharks, and in 

 some of the osseous apodals, require special notice, . ' ■ 



