48 



NATURAL HISTORY OF VERTEBRATES. 



flG. 42.— Heart of Amla 

 from below; ac, anterior, 

 pc,_ posterior cardinal 

 veins; rfc, ductus Cuvieri; 

 h, hepatic vein ; c, conus 

 arteriosifs ; r, ri^lit, I, left 

 auricle; v, ventricle. 



bei-, both of which communicate through the single large atrio-ventricular opening with 

 the much thicker-walled ventricle. This opening is provided with valves, so that when 

 the sponge-like ventricle is dilated with blood, and contracts upon its contents, the blood 

 is prevented from passing backwards into the atrium, and is 

 forced on into the slenderer part of the ventricle known as tlie 

 conus arteriosus. This also contains valves disposed iu three 

 rows, which allow the blood to flow only in one direction, namely, 

 forwards into the arterial trunk that lies on the ventral aspect of 

 the gill-arches. Ordinary Teleosts have this arterial trunk dilated 

 into a bulb next the heart, which looks like the conus of the Gan- 

 oids and sharks, but is not a part of the ventricle like the latter, 

 and is separated from that chamber by a single row of valves. 

 The blood, driven out of the ventricle, flows forwards along the 

 arterial trunk, whence it is distributed to the gills by a series of 

 afierent branchial arteries (a). Fig. 43, which ascend the gill- 

 arches, and break up into the capillary network of the gill-flla- 

 ments. There the blood is aerated, and is then collected into an 

 efferent artery (e), which begins low down on the arch, but gi-adu- 

 ally widens as it ascends, and receives more and more of the 

 aerated blood. Having arrived at the top of the arches, the 

 efferent arteries generally unite in a great arterial trunk, the dorsal aorta, which lies 

 immediately beneath the vertebral column, and gives off arteries at intervals to the 

 various parts of the body. Much difference is to be observed 

 throughout the fishes, both in the mode of the origin of the 

 afferent arteries and in the union of the efferent arteries into 

 the dorsal aorta. A closer inspection of the condition in 

 Amia will enable us to proceed towards that in the higher 

 vertebrates. Four pairs of afferent and efferent arteries are 

 present. From the foremost of the latter are continued for- 

 wards carotid trunks for the supply of the head ; the rest of 

 the blood of the first pair, with the whole of that of the second, 

 flowing into the dorsal aorta. Little of the blood from the 

 third pair of efferent arteries can reach the aorta, for they 

 unite into a vessel (cceliaco-mesenteric) which supplies the 

 viscera. Finally the blood in the fourth pair is distributed 

 entirely to the air bladder. 



The aerated blood distributed in this way is soon converted 

 into venous blood by contact with the tissues, and is then 

 carried towards the heart through the veins. That from the 

 cm, cceliaco-mesenteric ; q, head is poured into the venous sinus through the anterior car- 

 IrcUeT'The'miaerated b'lood dinal Or jugular veins ; that from the trunk, in part through 

 ^'ratedTby 'white v^'sseis.''^^ the posterior cardinal veins. Part of the blood from the trunk, 

 however, passes through the kidneys, and is collected in a sin- 

 gle median stem, the inferior vena cava, which joins the hepatic vein from the 

 liver before entering the venous sinus. Just as some of the venous blood from 

 the trunk passes through the kidneys, so the venous blood from the intestines passes 

 through the liver on its way to the heart. It is conveyed into the latter organ by the 

 portal vein, which breaks up into a capillary net-work, allowing the blood to be acted 



*lr= 



Pig. 43. — Diagram of circula- 

 tion in Amla seen from be- 

 Jo-vv; a, atferent, and e, etfe- 

 rent gill arteries ; aa, ab- 

 dominal aorta; ab, artery 

 to air-bladder ; c, carotid"; 



