258 DISSECTION OF THE ARM. 



Separate the middle from the inner and outer heads of the muscle, and 

 trace the musculo-spiral nerve and vessels beneath it. Define the outer 

 head which reaches down to the spot at which the musculo-spiral nerve 

 appears on the outer side. 



The TRICEPS MUSCLE (fig. 79), is divided superiorly into three parts or 

 heads of origin, inner, outer, and middle : two .of these are attached to 

 the humerus, and one to the scapula. 



The middle piece, or head, A, is the longest, and has a tendinous origin, 

 about an inch wide, from the inferior costa of the scapula close to the 

 glenoid cavity, where it is united with the capsule of the shoulder joint. 

 The outer head, B, is narrow and arises from the back of the humerus 

 above the spiral groove, extending from the root of the large tuberosity 

 to that groove. The inner head, c, fleshy and wide, arises from the 

 posterior surface of the humerus below the spiral groove, reaching laterally 

 to the intermuscular septa, and gradually tapering upwards as far as the 

 insertion of the teres major (Theile). From the different heads the fibres 

 are directed with varying degrees of inclination to a common tendon at 

 the lower part. Interiorly the muscle is inserted into the end of the ole- 

 cranon process of the ulna, and gives an expansion to the aponeurosis of 

 the forearm. Between the tip of the olecranon and the tendon is a small 

 bursa. 



The triceps is superficial, except at the upper part where it is overlapped 

 by the deltoid muscle. It lies on the humerus, and conceals the musculo- 

 spiral nerve, the superior profunda vessels, and the articulation of the 

 elbow. On the sides the muscle is united to the intermuscular septa ; and 

 the lower fibres are continuous externally with the anconeus a muscle of 

 the forearm. 



Action. All the parts of the triceps combining in their action will bring 

 the ulna into a line with the humerus, and extend the elbow-joint. As 

 the long head passes the shoulder it can depress the raised humerus, and 

 adduct the bone to the thorax. 



The intermuscular septa are fibrous processes continuous with the in- 

 vesting aponeurosis of the arm, which are fixed to the ridges leading to 

 the condyles of the humerus : they intervene between the muscles on the 

 front and back of the limb, and give attachment to fleshy fibres. 



The internal is the strongest, and reaches as high as the coraco-brachialis 

 muscle, from which it receives some tendinous fibres. The brachialis an- 

 ticus is attached to it in front, and the triceps behind ; and the ulnar 

 nerve, and the inferior profunda and anastomotic vessels pierce it. 



The external septum is thinner, and ceases at the deltoid muscle. Be- 

 hind it is the triceps ; and in front are the brachialis anticus, and the 

 muscles of the forearm (supinator longus and extensor carpi radialis 

 longus) arising above the condyle of the humerus : it is pierced by the 

 musculo-spiral nerve and the accompanying vessels. 



Dissection. To follow the superior profunda vessels and the musculo- 

 spiral nerve, the middle head of the triceps should be cut across over them, 

 and the fatty tissue should be removed. The trunks of the artery and 

 nerve are to be afterwards followed below the outer head of the triceps to 

 the front of the humerus. 



To trace out the branches of the nerve and artery, which descend to the 

 olecranon and the anconeus muscle, the triceps is to be divided along the 

 line of union of the outer with the middle head. 



The superior profunda branch of the brachial artery (p. 254) turns to 



