

CHAPTER VII. 



THE CIRCULATORY SYSTEM. 



THE circulatory apparatus comprises the channels for the con- 

 veyance of the blood-stream, the vessels, and the dilated and special- 

 ized portion of the vascular tube, constituting the heart, for the pro- 

 pulsion of the current. In development and structure the several 

 parts of the vascular system possess much in common, although 

 variations in the details of the walls of the blood-channels suffice to 

 distinguish the different portions. 





HE BLOOD-VESSELS. 



The blood-vessels occur in three forms, as arteries, veins, and 

 capillaries, the latter constituting an expanded system of thin- 

 walled tubules, intimately related to the organs, and especially de- 

 ^v signed to facilitate the interchanges be- 

 FIG. it tween the nutritive current which they 



and the tissues through which they 



The arteries possess three coats the 

 inner, or intima, the middle, or media, 

 amd the external, or adventitia. Since 

 these coats vary in relative thickness and 



FIG. 116. 



Section of human artery of medium 

 size : /, the intima, consisting of the 

 endothelium (e), the sub-endothelial 

 tissue (j), and the internal elastic 

 membrane (_r) ; M, the media, com- 

 posed of the involuntary muscle and 

 the bundles of elastic tissue (y) ; A , 

 the adventitia, containing irregular 

 elastic trabeculae (2). 



Endothelium of artery of frog : the vessel has been treated 

 with silver, hence the boundaries of the endothelial plates are 

 indicated by the dark lines of stained cement-substance. Sev- 

 eral pseudo-stomata appear as minute dark areas between the 

 cells. 



in details of structure with the size of the vessel, it is usual to classify 



