164 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



second set about the middle of its length. Each series consists 

 of three pouch-like flaps (pocket-valves) attached to the wall 

 by their posterior edges, their anterior edges being free. All 

 the valves of the heart are so arranged as to allow blood to flow 

 freely in the direction described, but not in the opposite direction. 



The arteries are well-defined tubes with largely muscular 

 walls. The blood leaving the heart passes through a mid- 

 ventral cardiac aorta to be distributed to the gills, where the 

 blood receives a supply of oxygen from the water by which 

 the gills are bathed, and gets rid of its C0 2 . It is then 

 collected by a series of efferent branchial vessels into a median 

 dorsal aorta, from which arise arteries distributing blood to all 

 parts of the body with the exception of the head. 



The head is supplied with blood by arteries arising direct 

 from the foremost of the efferent branchial vessels. 



The blood-vessels of the gills are: (1) the afferent branchial 

 arteries bringing the blood from the cardiac aorta, of which one 

 supplies the gills of each gill-arch, the three hinder pairs arising 

 separately from the cardiac aorta, while those supplying the two 

 foremost pair of arches arise by one pair of vessels, each of which 

 then divides into two. (2) The efferent branchial arteries, carrying 

 pure blood into the dorsal aorta, and arising from loops which 

 surround each of the gill-clefts except the last. The blood from 

 the gill of the last (or fifth) gill-cleft is received by an efferent 

 vessel which opens into the efferent loop of the fourth cleft. 

 Each efferent loop communicates also by means of a short vessel 

 with the loops in front of and behind it. 



The dorsal aorta, formed by the union of the efferent branchial 

 arteries, runs back below the vertebral column to the posterior 

 end of the body, becoming the caudal artery in the tail. It also 

 has a forward continuation which divides into two branches that 

 are connected with the carotid arteries. 



Branches of the Dorsal Aorta. (1) A pair of snbclavian arteries to pectoral 

 fins. (2) A cceliac artery, supplying liver, anterior end of stomach, begin- 

 ning of intestine, liver, and pancreas. (3) An anterior mesenteric artery to 

 intestine and reproductive organs. (4) A lieno-gaztric artery to stomach, 

 spleen, and pancreas. (5) A posterior mesenteric artery to rectal gland. 

 (6) Numerous pairs of small parietal arteries to body walls. (7) Numerous 

 pairs of small renal arteries to kidneys. 



Each half of the head is supplied with pure blood by : (1) a carotid 

 artery, running forwards from the top of the first efferent loop and dividing 

 into (a) external carotid to upper jaw and snout; (b) internal carotid to brain. 



