4 THE ANATOMY OP THE HORSE. 



front of the shoulder. On looking into the space, the dissector Avill now 

 see it occupied by a large amount of loose, areolar connective-tissue, 

 which envelops its contents, and fiicilitates the play of the shoulder on 

 the wall of the thorax. This areolar tissue must be cleaned away from 

 the axillary vessels and the brachial plexus of nerves, but most of the 

 branches of these will be more conveniently followed after separation of 

 the limb from the trunk. 



The Axilla coiTesponds to the arm-pit of the hiiman subject, and is 

 the important space across which the large vessels and nerves for the 

 supply of the foi-e limb are transmitted. 



Boundaries of the space. — In the natural movements of the limb, and 

 before dissection, the space can hardly be said to have any existence 

 except at its lower part ; but in the dissected condition it may be 

 observed to have the following boundaries. On its oute?- side are the 

 subscapularis, teres major, and (in part) latissimus dorsi muscles. The 

 {nne7' side of the space is formed by the anterior part of the chest-wall 

 covered by the serratus magnus, lateralis sterni, and intercostal muscles. 

 Inferiorly the space is enclosed by the deep pectoral muscles, and there 

 the space is most extensive. Superiorly the outer and inner boundaries 

 meet at the insertion of the serratus magnus into the scapula. The 

 antei^or limit of the space may be taken as foi*med by the mastoido- 

 humeralis and the reflected portion of the anterior deep pectoral ; while 

 posteriorly the space is closed by the panniculus carnosus and skin where 

 these are carried from the wall of the thorax over the outer aspect of 

 the shoulder. 



The Axillary Artery (Plates 3 and 5) begins within the thorax. On the 

 left side it arises as one of the terminal branches of the anterior aorta; while 

 on the right it is a branch of the arteria innominata. It leaves the chest 

 and reaches the axilla by turning round the anterior border of the fii-st 

 rib, below the inferior insertion of the scalenus. It crosses the axillary 

 space, inclining downwards and backwards ; and at the anterior border 

 of the teres major tendon it is directly continued as the brachial artery. 

 In this course it gives off" four vessels, viz., inferior cervical, external 

 thoracic, suprascapular, and subscapular ; but only the first two are to 

 be followed at present. 



The Inferior Cervical Artery (Plates 1 and 2) arises from the front 

 of the axillary where that vessel turns round the first rib. After a course 

 of about two inches it bifurcates, its superior branch passing between 

 the mastoido-lnuneralis and the subscapulo-hyoideus, Avhile the inferior 

 division passes into the groove between the mastoido-humeralis and the 

 anterior superficial pectoral, where it has already been seen in company 

 with the cephalic vein. 



The External Thoracic Artery (Plate 3) arises about the same point 

 as the preceding, but from the opposite side of the parent vessel ; and 



