LEPUS CUNICULUS 317 



ventricular region shows a noticeable difference between the two 

 sides. The cavity of the left ventricle appears almost circular in 

 outline, and its walls are noticeably thicker than the right. The 

 cavity of the right ventricle appears crescent-shaped and partly 

 wrapped around the left, so that the latter ventricle with the septum 

 ventriculorum occupies by far the larger part of the heart. The 

 walls of both ventricles are seen to be thrown up internally into a 

 series of prominent rounded muscular ridges, the trabeculae carneee. 

 In order to strengthen them the edges of the atrio- ventricular valves 

 are each provided with a number of tough ligamentous cords, the 

 chordae tendinese, which take their origin from raised pyramidal 

 muscular projections, the musculi papillares, on the walls of the 

 ventricles. 



From the anterior median end of the left ventricle comes off the 

 large aorta, the main artery of the body, and at its base lie a series 

 of three pocket-shaped semilunar valves, which can close it com- 

 pletely. Behind two of the valves are small apertures leading to 

 the two coronary arteries, which supply the actual tissue of the heart 

 itself. The large pulmonary artery originates from the anterior 

 median end of the right ventricle, and this, too, is guarded at its base 

 by three semilunar valves similar to those in the aorta. The 

 pulmonary artery soon divides into two main branches, one going to 

 each lung. The conus arteriosus no longer exists a separate element 

 in the mammalian heart, it has been absorbed into the ventricle. 



We may now consider quite briefly the action of the heart. 

 Blood from the body is brought back by the Venae cavae into the 

 right atrium, which, therefore, contains non-aerated blood. This 

 is then passed into the right ventricle, and it leaves this through the 

 pulmonary artery, so that, as there is no communication between 

 one side of the heart and the other, the right side of the heart and 

 all the vessels connected with it contain nothing but non-aerated 

 blood. In the lungs the blood is aerated and brought back by the 

 pulmonary veins to the left atrium, whence it passes to the left 

 ventricle, and thence via the aorta to all parts of the body. The 

 left side of the heart and its vessels, therefore, contain only aerated 

 blood. We see then that in the vertebrate series we have three 

 possible arrangements in the blood circulatory system. Firstly, 

 in the fish, as exemplified by Scyllium, the blood goes from the 

 single ventricle to the gills to be oxygenated, thence to the whole 

 of the body, and finally back to the single atrium, by the ductus 

 Cuvieri ; no other course is possible. THis is then fitly termed a 

 single or a branchial circulation. Secondly, in the mammal, as we 

 have seen, blood from the body is brought to the right side of the 



