522 THE ARTERIES. 



the appendages of the auricles and the cardiac vessels. About the middle 

 of its course, it is united to the posterior aorta by means of a yellow elastic 

 fibrous cord (the ligamentum arteriosum), the remains of the ductus arteriosus 

 which, in the foetus, establishes a large communication between these two 

 vessels (Fig. 258, e). 



The walls of the pulmonary artery are much thinner than those of the 

 aorta, and are yellow and elastic, as in the other canals of the same order. 

 We remember, however, having seen them in an Ass, formed almost entirely 

 of red muscular fibres, analogous to the fasciculi of the heart. 



It may be repeated that the pulmonary artery conveys into the lungs 

 the dark blood carried to the right heart by the veins of the general 

 circulation. 



CHAPTER III. 



THE AORTA. 



IF we take a general survey of the aortic trunk, we will find that it arises 

 from the base of the left ventricle, ascends to beneath the dorso-lumbar 

 column, curving backwards and downwards, and reaches the entrance to the 



rlvis, where it terminates by four branches. It furnishes, besides, about 

 to 2J inches from its origin, a secondary trunk, which soon divides into 

 two new arteries, the right and largest of which gives olf a particular 

 trunk, the common origin of the two long vessels destined for the head. 



This disposition permits us to recognise in the aorta seven principal 

 sections : 



1. The aortic trunk or common aorta: the source of all the arteries 

 belonging to the red-blood system, and giving rise to the anterior and 

 posterior aorta. It only furnishes blood directly to the heart itself. 



2. The posterior aorta : the veritable continuation of the common aorta, 

 is distributed to the posterior moiety of the trunk and to the abdominal 

 limbs ; it terminates by a double bifurcation. 



3. The internal and, 4, external iliac arteries : branches of this bifurcation 

 which are almost entirely expended in the posterior limbs. 



5. The anterior aorta: the smallest of the two trunks furnished by the 

 common aorta, is chiefly destined to the anterior moiety of the trunk and 

 the thoracic limbs. 



6. The axillary arteries, or brachial trunks: these come from the bifurca- 

 tion of the preceding artery, and are continued by their terminal extremity 

 into the fore-limbs. 



7. The carotid arteries, or arteries of the head : these emanate by a 

 common trunk from the right brachial bifurcation. 



ARTICLE I. AORTIC TRUNK OR COMMON AORTA. 



The point of departure for all the arteries carrying red blood, the aortic 

 trunk proceeds from the left ventricle by becoming continuous with the 

 festooned fibrous zone which circumscribes the arterial orifice of that 

 cavity. It passes upwards and a little forwards, bifurcating, after a 

 course of 2 or 2 J inches, into the anterior and posterior aortse. 



Its volume, inferior to that of its two terminal branches, is not uniform ; 

 as at its origin, and opposite the sigmoid valves, it presents (an enlargement 

 the bulbua aortse caused by) three dilatations described as the sinus of 

 the aorta (sinus aortici, sinus Valsalvse). 



