THE BLOOD-VASCULAR SYSTEM 



109 



Fig. 60. The larger terminal rami- 

 fications of the left inferior bronchial 

 ramus, from the dorsal surface; metallic 

 cast of the interior. Cf. Figs. 57, 59, and 

 114, i. 



the other, introduces a peculiar 

 condition into the history of 

 vertebrates. This condition is 

 characterized by the appearance 

 of gill structures in the embryos 

 of all higher forms (Fig. 21), by 

 the somewhat later development 

 of air sacs in addition to these, 

 and by the gradual elaboration 

 of the latter in the vertebrate 

 series from a simple type, as 

 illustrated in the frog or better 

 in lower tailed amphibians, to 

 the greatly branched lung tubes 

 of mammals and the highly 

 complex and special arrangements 

 in birds. 



The Blood-Vascular System 



In the rabbit, as in all vertebrates, the blood- vascular system 

 (Fig. 61) embraces a central, muscular organ of propulsion, the heart, 

 and a series of branched tubes, the blood-vessels, the latter being of 

 three different kinds: (a) thick-walled, elastic, distributing vessels 

 — arteries; (b) microscopic terminal canals in the peripheral 

 organs — capillaries; and (c) thin-w^alled collecting vessels — veins. 



The chief mammalian feature in this system consists in the 

 division of the heart into two portions, respectively left and right, 

 each consisting of a receiving chamber, or atrium, and a driving 

 chamber, or ventricle, and the arrangement of their vascular 

 connections in such a way that two complete circulations are 

 established. One of these is the long, or systematic circulation. 

 It is concerned with the distribution of blood to the various 

 parts of the body, with the exception of the conveyance of blood 

 to the lungs for aeration (but incljjding the supply of the deeper 

 tissues of the lungs, through the bronchial arteries). It is estab- 

 lished by the left ventricle, the aorta, the carotid and subclavian 

 branches of its arch, and the parietal and visceral branches of its 

 thoracic and abdominal portions. The blood is collected from the 

 anterior portions of the body through paired internal and external 



