184 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



groups of vertebrates, while other vessels, both veins and arte- 

 ries, are developed to supply the various organs. The alterations 

 from these outlines are now to be traced. 



Heart. The heart always retains its primitive twist, and 

 the atrium and its derivatives lie dorsal to or even in front of 

 the ventricular portion. In the lower fish-like forms the heart 

 of the adult can be reduced to the condition outlined above, but 

 from the dipnoi and amphibia upwards (that is, with the appear- 

 ance of lungs) this organ is more or less completely divided into 

 right and left halves by a vertical septum which grows from 

 behind forwards. In the groups just mentioned this septum 

 divides the atrium into right and left halves, the auricles of the 

 heart. The sinus venosus retains its connection with the right 

 auricle, while the pulmonary vein, bringing blood from the lungs, 

 empties into the other. Thus the two auricles receive different 

 kinds of blood. That which enters by way of the sinus comes 

 from all parts of the body, and is consequently poor in oxygen 

 and contains much carbon dioxide ; while that coming from the 

 lungs is rich in oxygen and lacking in carbon dioxide. These 

 two kinds of blood are known respectively as venous and 

 arterial. 



In the contraction of the auricles the blood from the left 

 auricle is first forced into the ventricle, while the venous blood 

 follows later, and thus comes to occupy the posterior portion of 

 the ventricle. These two kinds of blood are now forced through 

 different portions of the aortic arches. In the truncus arteriosus 

 a spiral fold or valve occurs, extending as far forward as the pos- 

 terior aortic arch. When the ventricle contracts, this blood at 

 first flows forward in the ventral aorta into the anterior aortic 

 arches, which consequently receive arterial blood. As the aorta 

 fills, the spiral valve moves in such a way that the venous blood 

 flows into the posterior arches. In the reptiles the cardiac 

 septum extends into the ventricle, dividing it partially or com- 

 pletely (crocodiles) into right and left halves. Even in those 

 cases where the septum between the two halves is incomplete, 

 there is a physiological division, for at the time of contraction 

 the walls and the septum come together so as to separate the 

 two sides. In the birds and mammals the separation is com- 



