3S2 ZOOLOGY FOR MEDICAL STUDENTS chap. 



carried by the arterial trunks to the tissues and from these it passes 

 back through the veins to the atrial portion of the heart (A). 



In the typical double circulation the heart has become divided into 

 right and left halves, isolated from one another by a longitudinal septum 

 — the atrium being divided into a right and left auricle (Fig. 165, B, 

 R.A and L.A), the ventricle into a right and left ventricle (R.Vand L.V) 

 and the conus being split into two tubes which twist round one another 

 in spiral fashion. 



The blood starting from the left ventricle passes by the systemic 

 aorta {s.a) and the arteries which branch o£E from it to the tissues ; it 

 then passes back towards the heart through the veins {m.v) and eventually 

 arrives via the sinus venosus in the right auricle. From this it passes 

 by the right auriculo-ventricular opening into the right ventricle and 

 then onwards by way of the pulmonary artery (p.a) to the lung. There 

 it is oxygenated and it then passes back to the heart by the pulmonary 

 vein {p.v), arriving in the left auricle. Passing from this through the 

 left auriculo-ventricular opening into the left ventricle it starts again 

 on its journey towards the tissues. 



We thus have two distinct circuits — a systemic and a pulmonary — 

 the one concerned with the circulation through the general tissues, the 

 other with the circuit through the lung, the two circuits crossing one 

 another in the region of the conus. The pumping the blood through the 

 two circuits is effected by the ventricles and as might be expected the 

 left ventricle, having to force the blood through the longer and more 

 complicated circuit involved in the blood supply of the general tissues, 

 is provided with thicker and more muscular walls than the right ventricle 

 which has only to force the blood through the shorter and simpler pul- 

 monary circuit. 



The special interest of the Lung-fish heart lies in the facts (i) that 

 it shows us for the first time in the series of vertebrates— though not as 

 yet in absolutely completed form — ^the modifications characteristic of the 

 double circulation and (2) that it affords valuable evidence as to how 

 these modifications came about in evolution. Taking Lepidosiren as 

 our type we find that the atrium is divided into a right and left auricle 

 by a more or less spongy partition (Fig. 166, .r.A), incomplete at its 

 ventral edge next the atrio-ventricular opening, so that in this neigh- 

 bourhood the cavities of the two auricles are still in continuity. The 

 partition lies to the left of the opening from the sinus venosus {s.v) so 

 that the blood from this chamber of the heart, i.e. the blood which has 

 returned by way of the veins from the general tissues of the body, flows 

 into the right auricle. The blood from the lung on the other hand 



