CIRCULATION OF FISHES. 85 



26. Digestion seems to be carried on very rapidly, and the 

 chyle is absorbed by numerous lymphatic vessels, which empty, 

 by many trunks, into the venous system, near the heart. 



27. The blood of fishes is red; the globules are elliptical in 

 form, and of considerable size. 



28. The heart (Fig. 45, r.) is placed under the throat, in a cavity, 

 separated from the abdomen by a sort of diaphragm, (.<?,) (as we 

 have just said,) and protected by the pharyngeal bones above, by 

 the arches of the branchiae on the sides, and generally by the 

 humeral cincture behind. It is composed of an auricle, which 

 receives the venous blood collected in a large sinus (a kind of 

 large vein) situated near it, and of a ventricle placed below, and 

 giving origin, at its anterior extremity, to a pulmonary artery, 

 the base of which is inflated, and constitutes a contractile bulb. 

 This vessel soon divides into lateral branches, which are dis- 

 tributed to the gills, and the blood, after traversing these organs, 

 goes to the head through another vessel, which also runs along 

 the arches of the branchiae. There these canals send some 

 branches to the neighbouring parts, and unite to form a great 

 dorsal artery, which is directed backwards, beneath the spinal 

 column, and sends branches to all other parts of the body. But all 

 the venous blood does not go directly into the sinus mentioned 

 above ; that of the intestines, and some, other parts, before 

 returning to the heart, is carried through the liver by the vena 

 porta. 



29. We see now that the blood, in passing through the circu- 

 latory circle, entirely traverses the respiratory apparatus as in 

 mammals and birds, but it only passes o/tce tkrouqh the heart, 

 which must render its progress slower. The heart itself corres- 

 ponds in its functions to the right half of the same organ in the 

 superior vertebrate animals. 



30. Respiration is effected by means of the air which is always 

 found dissolved in the water, and takes place on the surface of a 

 multitude of very vascular and projecting lamellae, attached to 

 the external edge of the branchial arches. Generally, there are, 

 on each side, four branchiae each composed of two rows of 

 elongated lamellae. In most of the cartilaginous fishes there are 

 five, and, in the lamprey, we find seven. In almost all the bony 



2fi. Is the digestion of fishes very rapid ? 

 27. What is the character of the blood in fishes 1 



23. What is the situation of the heart ? What are the peculiarities of 

 the circulatory apparatus in fishes 1 



29. In what particulars does the circulation of fishes differ from 4hat of 

 mammals and birds ? 



30. How is respiration effected in fishes ? 



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