4 THE ANATOMY OF THE HORSE. 



sufficient extent, it may be necessary to cut the mastoido-humeralis in 

 front of the shoulder. On looking into the space the dissector will 

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

 tissue, which envelopes its contents, and facilitates 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 corresponds to the arm-pit of the human subject, and is 

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

 supply of the fore 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 outer side are the 

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

 inner 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 

 it is most extensive. Superiorly the outer and inner boundaries meet 

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

 limit of the space may be taken as formed 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 first 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-humeralis and the subscapulo-hyoideus, while 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 



