272 A MANUAL OF ANATOMY. 



and become continuous with the axillary fascia. The 

 attachment of the lesser pectoral (clavipectoral) fascia to 

 the axillary fascia draws upward the latter, and this ex- 

 plains why it is that the floor of the axilla is so hollowed 

 out or concave externally (the pit of the arm). This con- 

 cavity is greatest when the arm is midway between the side 

 of the chest and right-angle abduction. 



The branches of the acromiothoracic artery will be 

 found in the interval between the pectoralis major, deltoid, 

 and clavicle. They are the humeral, acromial, clavicular, 

 and thoracic. Figs. 60, 61, 62. 



The humeral (descending) is found along with the ceph- 

 alic vein in the groove between the deltoid and pectoralis 

 major muscles, supplying both. 



The acromial may be a branch directly from the main 

 trunk or often from the humeral. It supplies the region 

 about the shoulder. 



The clavicular branch is a constant branch to the sub- 

 clavius, but may arise from the thoracic branch. 



The thoracic distribution is usually, by two or more 

 branches, to the pectoral muscles and side of the chest. 



The External Anterior Thoracic Nerve. This will be 

 found coming through the costocoracoid membrane and 

 passing to the pectoralis major. Fig. 62. 



DISSECTION. 



Remove the costocoracoid membrane and expose the subclavius muscle, 

 first portion of the axillary artery and its two branches, the accompanying 

 vein, and cords of the brachial plexus. 



Clean the pectoralis minor muscle and the floor of the axilla external to 

 the muscle, saving the nerves and arteries found. 



Subclavius. Fig, 62. 



Origin. From the cartilage and first rib at their point 

 of junction. 



