POSTERIOR HUMERAL REGION. 305 



receiving the attachment of the muscular fibres, they join together above the 

 elbow, and are inserted into the back part of the upper surface of the olecranon 

 process, a small bursa, occasionally multilocular, being interposed between the 

 tendon and the front of this surface. 



The long head of the Triceps descends between the Teres minor and Teres 

 major, dividing the triangular space between these two muscles and the humerus 

 into two smaller spaces, one triangular, the other quadrangular (fig. 171). The 

 triangular space transmits the dorsalis scapulae vessels ; it is bounded by the Teres 

 minor above, the Teres major below, and the scapular head of the Triceps, ex- 

 ternally : the quadrangular space transmits the posterior circumflex vessels and 

 nerve; it is bounded by the Teres minor above, the Teres major below, the 

 scapular head of the Triceps internally, and the humerus externally. 



Relations. Its posterior surface is overlapped by the Deltoid above, superficial 

 in the rest of its extent. By its anterior surface, with the humerus, musculo-spiral 

 nerve, superior profunda vessels, and back part of the elbow-joint. Its middle or 

 long head is in relation, behind, with the Deltoid and Teres minor ; in front, with 

 the Subscapularis, Latissimus dorsi, and Teres major. 



The Subanconeus is a small muscle, distinct from the Triceps, and analogous to 

 the Subcrureus in the lower limb. It may be exposed by removing the Triceps 

 from the lower part of the humerus. It consists of one or two slender fasciculi, 

 which arise from the humerus, immediately above the olecranon fossa, and are 

 inserted into the posterior ligament of the elbow-joint. 



Nerves. The Triceps and Subanconeus are supplied by the musculo-spiral 

 nerve. 



Actions. The Triceps is the great Extensor muscle of the forearm ; when the 

 forearm is flexed, it serves to draw it into a right line with the arm. It is the 

 direct antagonist of the Biceps and Brachialis anticus. When the arm is extended, 

 the long head of the muscle may assist the Teres major and Latissimus dorsi in 

 drawing the humerus backwards. The long head of the Triceps protects the 

 under part of the shoulder-joint, and prevents displacement of the head of the 

 humerus downwards and backwards. 



MUSCLES OF THE FOREARM. 



Dissection. To dissect the forearm, place the limb in the position indicated in fig. 168 ; make 

 a vertical incision along the middle line from the elbow to the wrist, and connect each extremity 

 with a transverse incision ; the r flaps of integument being removed, the fascia of the forearm is 

 exposed. 



The deep fascia of the forearm, continuous above with that inclosing the arm, 

 is a dense highly glistening aponeurotic investment, which forms a general sheath 

 inclosing the muscles in this region ; it is attached behind to the olecranon and 

 posterior border of the ulna, and gives off' from its inner surface numerous inter- 

 muscular septa, which inclose each muscle separately. It consists of circular and 

 oblique fibres, connected together at right angles by numerous vertical fibres. It 

 is much thicker on the dorsal than on the palmar surface, and at the lower than 

 at the upper part of the forearm, and is strengthened by tendinous fibres, derived 

 from the Brachialis anticus and Biceps in front, and from the Triceps behind. 

 Its inner surface gives origin to muscular fibres, especially at the upper part of 

 the inner and outer sides of the forearm, and forms the boundaries of a series of 

 conical-shaped fibrous cavities, in which the muscles in this region are contained. 

 Besides the vertical septa separating each muscle, transverse septa are given off 

 both on the anterior and posterior surfaces of the forearm, separating the deep 

 from the superficial layer of muscles. Numerous apertures exist in the fascia for 

 the passage of vessels and nerves ; one of these, of large size, situated at the front 

 of the elbow, serves for the passage of a communicating branch between the 

 puperficial and deep veins. 



The muscles of the forearm may be subdivided into groups corresponding to 

 20 



