Mammalia: Visceral Systems 621 



birds and mammals are not known. It is quite probable that they 

 existed in Archaeopteryx and somewhere along the line of theromorph 

 reptiles and the earliest mammals. 



In some lizards, and especially well developed in the chameleon, 

 the posterior end of the lung is prolonged into several elongated 

 pouches whose very thin and simple walls are nonrespiratory. These 

 air-sacs extend back into the posterior abdominal region. Their 

 extreme inflation produces a marked swelling of the body — possibly 

 useful in enabling the animal to intimidate its enemies. These reptilian 

 air-sacs are of interest in connection with the existence of air-sacs in 

 birds. 



II. Circulatory Organs 



The statement that the adult mammalian skull is simpler than that 

 of a reptile and much simpler than that of a teleost fish finds its parallel 

 with reference to the circulatory system. In the early mammalian 

 embryo the heart and main blood-vessels are laid down along the lines 

 which they follow in fishes. In the later course of development there is 

 much modification, the general tendency being to concentrate the 

 blood channels into fewer and centralized vessels, as, e.g., by reduction 

 of the several aortic arches of the embryo to such an extent that only 

 a lateral half of one arch remains in its original relations, and by re- 

 placement of the pair of postcardinal veins by a single median postcava 

 as the main venous channel of the trunk. The relative simplicity of the 

 adult system is therefore secondary, as is true of the adult skull. 

 Viewed in its entirety, the mammalian circulatory system is more 

 complex than that of a fish because the adult pattern is achieved only 

 by an elaborate metamorphosis of the originally complex system of the 

 early embryo. 



Heart and Aortic Arches 



In the early mammalian embryo the heart is a simple tube having 

 a posterior auricular (or atrial) enlargement and an anterior ventric- 

 ular enlargement whose outlet is a single undivided arterial trunk which 

 leads forward into the several aortic arches (Fig. 474) . Later the heart 

 and its outlet become completely divided longitudinally so that the 

 aerated blood from the lungs is entirely separated from the blood which 

 is returned to the heart from the general systemic circulation. The 

 systemic venous blood is received by the right auricle (atrium) and 

 passes thence to the right ventricle, which pumps it to the lungs 

 (Fig. 475). Returning from the lungs, the blood enters the left auricle 

 (atrium) and passes thence to the left ventricle, which pumps it into 

 the general arterial channels. 



At the base of the aorta and of the pulmonary artery are semilunar 



