CIRCULATORY APPARATUS. 85 



ing with the corresponding veins the pulmonary circulation. The 

 remainder of the arteries constitute the general or systemic circu- 

 lation. 



Structure of arteries. 



They are more or less rigid and elastic, with three coats — (i) 

 an external fibrous or adventitia, (2) a middle muscular or media, 

 (3) an internal serous or intima, continuous with the endocar- 

 dium. 



The external tunic is nourished by vasa vasorum, and the mus- 

 cular coat receives branches from the sympathetic system, called 

 vasomotor nerves. 



The capillaries are small vessels between the arterial and venous 

 systems. They are composed of a basement membrane, covered 

 by a layer of delicate cells, continuous with those of the arteries. 



THE AORTA. 



It is the beginning of the arterial system, given off from the 

 left ventricle, and guarded by the semilunar valves. Just above 

 the valves it shows three dilatations, the sinuses of Valsalva. It 

 passes upward and forward for about two inches, when it divides 

 into the anterior and posterior aortas. 



Branches. 



Right and left coronary to heart (see above). 



ANTERIOR AORTA. 



The smaller of the two passes forward and upward for two or 

 two and a half inches, where it divides into the two axillary 

 arteries or brachial trunks. 



The right is much the larger, and gives off the carotid arteries. 



Course. 



They curve forward and outward over the anterior border of 

 the first rib below the scaleni, accompanied by the brachial 

 nerves to the space between the sub-scapularis and the adductor 

 of the arm, where it becomes the humeral artery. 



Branches. — From thoracic portion. 



I. Dorsal or transverse cervical. 



To muscles and integument of the withers, neck and shoulders. 



