494 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



In the vertebrata, amphioxus, the lowest form of fish, has a system 

 of blood-vessels with contractile walls, but no distinct heart, while in all 

 the other vertebrates there is a heart with, at fewest, three chambers 

 (sinus venosus, atrium, ventricle), arteries, capillaries, and veins, and a 

 . system of lymphatics connected with the 



veins. In many of the lower forms of verte- 

 brates we still find large venous sinuses, but 

 in the higher forms these are for the most 

 part replaced by definite vessels with mus- 

 cular walls. Important peculiarities, how- 

 ever, exist in the vascular systems of the 

 vertebrata dependent upon the character of 

 their respiration, whether pulmonated or 

 air-breathing, or branehiated or water- 

 breathing ; and further, as to whether their 

 blood is warm or cold. Mammalia and birds 

 are included in the group of warm-blooded 

 pulmonated vertebrata ; reptilia and amphibia 

 in the group of cold-blooded pulmonated animals ; and fish constitute 

 the group of cold-blooded branehiated vertebrata. 



In all of these animals the character of the circulatory apparatus 

 depends upon the manner in which the blood is oxy- 

 genated ; therefore, in those animals (certain of the 

 amphibia) which commence life as branehiated ani- 

 mals and subsequently become pulmonated, we find 

 that their circulatory apparatus becomes modified 

 accordingly, and presents two different styles corre- 

 sponding to the stage of their existence. In all forms 

 of vertebrata a portal system is present — that is, 

 the liver receives a special supply of venous blood 

 derived from the systemic capillaries of the abdominal 

 organs. 



Fig. 182. — Diagram of the 

 Circulation in the Fish. 

 (Beclard.) 



A, dorsal artery, or aorta; C, sys- 

 temic organs ; O, auricle ; V, ventricle ; 

 is, branchiae. 



Fig. 184.— Plan of 

 Circulation in 

 the Fish. {Car- 

 penter.) 



A, auricle ; B, ventricle ; 

 C, branchial artery; D, 

 _ „ _ gills ; E E, branchial veins 



Fig. 183.— Diagram of the Arterial Circulation in Fishes, uniting in f, the aorta; 

 after Wiedersheim. {Jeffrey Bell.) G, vena cava. 



In fishes (see Fig. 181, D),the lowest forms of vertebrates, the heart 

 consists of a single auricle with the sinus venosus, which is always present, 

 and a single ventricle, the former receiving the dark venous blood from the 



