212 CIRCULATORY SYSTEM. 



it cannot be referred, with strict propriety, to any determined 

 origin. When it comes from the axillary, it is very tortuous, 

 and has to ascend to its destination, which removes it entirely 

 from any interference with the course of the subclavian over 

 the first rib, and over the upper head of the serratus major 

 muscle. But in the other cases, it goes transversely backwards 

 and outwards, somewhat below the posterior cervical, and along 

 the posterior inferior margin of the clavicle, being covered by 

 the sterno-mastoideus, the platysma myodes, and the trapezius; 

 consequently, it is just in the way of the incisions which are 

 made for reaching the subclavian artery, from above the cla- 

 vicle. 



It reaches the superior costa of the scapula near the root of 

 the coracoid process, and passing through the notch there, is 

 distributed, by one large branch, upon the supra-spinatus mus- 

 cle; and by another, which goes across the anterior margin of 

 the spine of the scapula, to the infra-spinatus muscle. In its 

 course, it sends off several small ramifications to contiguous 

 parts. 



2. The External Mammary Arteries (Art. Mammarice sen Tho- 

 racicffi External) arise from the axillary, between the subclavius 

 and the pectoralis minor muscle. They are four principal 

 trunks, which go uniformly to certain parts, but vary consi- 

 derably in their origin ; for sometimes the latter is distinct in 

 the case of each artery, but frequently otherwise. Their dis- 

 tribution is as follows : 



a. The Thoracica Superior is distributed to the upper part of 

 the pectoralis major muscle, and to the pectoralis minor. Some 

 of its branches reach the mamma in the female, and anastomose 

 with the internal mammary and with the intercostals. 



b. The Thoracica Longa descends along the posterior face of 

 the pectoralis major, between it and the serratus magnus. It 

 gives many branches to the lower part of the pectoralis major, 

 to the integuments, and, in the female, to the mamma; anasto- 

 mosing likewise with the internal mammary and with the inter- 

 costals. 



c. The Thoracica Acromialis, immediately after -its origin, 

 makes for the fissure between the deltoid and the great pectoral 

 muscle, and divides there into an ascending and a descending 



