180 CIRCULATORY SYSTEiM. 



Anterior Aorta. 



The anterior, smaller, and shorter division of the main trunk, 

 whose length falls somewhat short of an inch, in ascending, 

 bends forward, and divides below the body of the third dorsal 

 vertebra into the right and left avteriee innoniinatse vel com- 

 munes. The course of this vessel is below the windpipe and 

 rather to the left of it, within the space of the mediastinum. 

 The trunk itself furnishes no branches; but its bifurcations give 

 origin to those large arteries which are distributed over the breast, 

 neck, head, brain, and anterior extremities. 



Arterice Linominata vcl Communes. 



The right artery is considerably longer than the left, and mea- 

 sures nearly as much again in circumference, in consequence of its 

 having to form the common carotid artery (the trunk from which 

 the two carotids spring) after it has detached branches to the 

 right side correspondent in size, number, and distribution to those 

 into which the left division resolves itself. These vessels form a 

 very acute angle as they leave the anterior aorta, and pursue 

 their course horizontally forward, diverging very gradually as 

 they advance: the right is placed lower than the left, and lies 

 more immediately under the windpipe, having the vena cava an- 

 terior upon the other side of it; the left describes a curve upward 

 in passing forward, inclines outward, and gets under the esopha- 

 gus : both branch out about the middle of the space between the 

 two first ribs, where the right ends in the two carotid arteries, 

 the left in the vessels now to be described, which, as was ob- 

 served before, are nearly the same on one side as on the other. 



1. The dorsal artery generally comes off' in one common 

 root with the next vessel. From its origin, it curves upward and 

 backward to arrive at the second intercostal space, which it per- 

 forates very obliquely close to the posterior edge of the first rib, 

 a little below its head. Out of the chest, it still pursues the 

 same oblique course, crossing over the transverse process of the 

 second dorsal vertebra, and then ascending upon the spines of 

 the withers, among the muscles attached to which its ramifica- 

 tions are expended. Its branches worthy of names are — a. Su- 

 perior Mediastinal, a twig detached near its origin to the me- 

 diastinum, fj. Anterior intercostal, a branch sometimes nearly 

 equal in size to the dorsal itself, given off' just before the vessel 

 penetrates the intercostal muscle, by which the second, third, 

 fourth, and fifth intercostal arteries are furnished ; the reason for 

 which is, that the posterior aorta (the vessel furnishing the majo- 



