126 DISSECTION OF THE DOG 



commencement of its companion vein. The artery appears at the axillary 

 border of the scapula in the interval bounded by the teres major, infra- 

 spinous, and long head of triceps muscles. Crossing the infraspinous 

 muscle and the aponeurotic origin of the deltoid, it ends in the trapezius. 



M. supraspinattts. — The supraspinous muscle fills the whole of the supra- 

 spinous fossa of the scapula from which it takes origin. In addition, some of 

 its fibres arise from the subscapular muscle. 



Its insertion is into the greater tubercle of the humerus and into the insertion 

 of the deep pectoral muscle. 



M. deltoidetts. — Irregularly triangular in outline and placed in the angle 

 formed by the spine of the scapula and the humerus, the deltoid muscle can 

 be divided into two portions : (1) arising from the spine of the scapula by 

 aponeurosis ; (2) taking origin from the acromion. The two parts blend into 

 a common insertion to the deltoid tuberosity of the humerus. 



Veins of the arm. — The main veins of the arm begin as superficial veins 

 in the forearm. 



V. mediana cubiti. — The median vein of the forearm divides, on a level with 

 the tendon of insertion of the biceps, into a median basilic and a median 

 cephalic. The median basilic vein (v. mediana basilica) crosses the medial 

 face of the biceps and, receiving the ulnar vein or veins, becomes the basilic 

 vein (v. basilica). This follows the brachial artery to the distal border of the 

 teres major muscle, where it receives the deep humeral vein and becomes the 

 axillary (v. axillaris). 



V. mediana obphalica. — The median cephalic vein runs in a groove formed 

 by the brachio-cephalic and brachialis muscles, receives the radial vein, and 

 becomes the cephalic vein (v. cephalica). After a short single course the 

 cephalic vein becomes double. One of its parts, the original vein, travels 

 along the groove between the superficial pectoral and deltoid muscles and 

 finally joins the external jugular vein. The other part, generally the larger, 

 follows the border of the deltoid, and disappears between the lateral and long 

 heads of the triceps to join the subscapular vein. Before its disappearance a 

 small twig leaves it to cross the brachio-cephalic muscle superficially and enter 

 the external jugular vein. 



Dissection. — Reflect both parts of the deltoid muscle. This will expose the 

 infraspinous and teres minor muscles, the origin of the lateral head of the 

 triceps and the termination of the axillary nerve. The last named 

 appears, along with the posterior circumflex vessels, from a triangular 

 space bounded by the long and lateral heads of the triceps and the teres 

 minor. 



M. infraspinatus. — The infraspinous muscle occupies the fossa of the 

 same name. Its origin is from the whole extent of the fossa and in addition 



