LYMPHATIC SYSTEM. 



67 



in the Crocodilia, and in these animals the right aortic arch arises 

 from the left ventricle. But the separation of the arterial and 

 venous blood is even now not quite complete, for at the point where 

 the two aortic arches cross one another there is a passage (foramen 

 Panizza?) leading from one into the other, and through which a 

 communication may take place. 



It is only in Birds and Mammals, in which, as in the Crocodilia, 

 the right and left ventricle are completely separated, that a separation 

 between the two kinds of 

 blood is completely effected 

 (fig. 61). In Birds the right 

 aortic arch persists, and the 

 left entirely disappears ; while 

 in Mammalia the opposite 

 obtains, the left arch per- 

 sisting and giving rise to the 

 dorsal aorta. In these animals 

 the blood is essentially diffe- 

 rent from the chyle both in 

 colour and composition, and 

 there is present a special 

 system of chyle and lymph 

 vessels. This system origi- 

 nates in simple tissue spaces, 

 which are without walls, and 

 its main trunks open into the 

 vascular system. The con- 

 tents are derived from the 

 nutrient material absorbed 

 from the intestine (chyle), 

 and from the fluids which 

 have transuded into the 

 tissues from the capillaries (lymph), and they serve to renovate 

 the blood. In the actual course of the lymph and chyle, i.e., in the 

 lymphatic vessels themselves, are placed peculiar glandular organs, 

 known as lymphatic glands (blood glands), in which the lymph receives 

 its form elements (lymph corpuscles = white blood corpuscles). 



Organs of Respiration. The blood needs for the retention of its 

 properties not only this continued renovation by the addition of 

 nutrient fluids, but also the constant introduction of oxygen, with 

 the reception of which is clofely connected the excretion of cai'bonic 



A 



Fir:. GO. Heart and great vessels of a Cheloniim. 

 Ad, right auricle ; As, left auricle; Ao.it. right 

 aortic arch ; An.s, left aortic arch ; Ao, aorni ; 

 C, carotids ; Ap, pulmonary arteries. 



