THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 557 



soon comes into intimate contact with the trabeculae, and envelops 

 each one of them with a special covering (His). Thus there arise 

 in the spongy wall of the ventricle numerous spaces lined with 

 endothelium, which toward the surface of the heart end blindly, but 

 which communicate with the central cavity and like this receive into 

 them the stream of blood. 



The embryonic heart of Man and Mammals resembles in its first 

 condition that which has been described up to this point the heart 

 of the lowest Vertebrates, the Fishes. In the former as in the 

 latter it consists of a region, the atrium, which receives the venous 

 blood from the body, and of another, the ventricle, which drives the 

 blood into the arterial vessels. Corresponding to this condition of 

 the heart, the whole circulation in embryos of this stage and in Fishes 

 is still a simple and a single one. This becomes changed in the 

 evolution of Vertebrates, as in the embryonic life of the individual, 

 with the development "of the lungs, upon the appearance of which a 

 doubling of the heart and of the blood-circulation is introduced. 



The cause of such a change is clear, from the topographical relation 

 of the two lungs to the heart, the former arising in the immediate 

 vicinity of the heart by evagination of the fore-gut (fig. 314 Ig). 

 The lungs therefore receive their blood from an arterial stem lying 

 very near the heart, from the fifth [sixth] pair of aortic arches that 

 arise from the truncus arteriosus. Similarly they give back again the 

 venous pulmonary blood directly to the heart through short stems, 

 the pulmonary veins, which, originally united into a single collecting 

 trunk (BoRN, ROSE), open into the atrium at the left of the great 

 venous trunks. Therefore the blood that flows directly out of the heart 

 into the lungs also floivs directly back again to the heart. Herein is 

 furnished the prerequisite for a double circulation. This comes into 

 existence when the pulmonary and the body currents are separated from 

 each other l)y means of partitions throughout the short course of the 

 vascular system which both traverse in common (viz., atrium, ventricle, 

 and truncus arteriosus). 



The process of separation begins in the vertebrate phylum with the 

 Dipnoi and Amphibia, in which pulmonary respiration appears for 

 the first time and supplants bronchial respiration. In the amniotic 

 Vertebrates it is accomplished during their embryonic development. 

 Therefore we now have to follow out further the manner in which, 

 in the case of Mammals and especially of Man, according to the 

 recent investigations of His, BORN, and ROSE, the partitions are 

 formed how atrium and ventricle are each divided into right and 



