Origin ([Fig. 83], g, g′) from a long V-shaped line two to four millimeters wide on the lateral surface of the humerus. The apex of the V is just distad of the teres minor tubercle (d). Its dorsal limb (g′) extends along the lateral supracondyloid ridge to a point opposite the proximal margin of the supracondyloid foramen; its ventral limb (g) extends in the direction of the deltoid ridge to the middle of the bone. No muscle-fibres take origin between the two limbs. The fibres converge and end in a flat tendon which joins the tendon of the clavobrachial ([Fig. 79], k).

Insertion ([Fig. 87], c).—The dorsal portion of the depressed rough area on the lateral surface of the ulna just distad of the semilunar notch.

Relations.—Outer surface with the acromiodeltoid ([Fig. 75], f), the caput laterale of the triceps ([Fig. 75], h), the brachioradialis ([Fig. 75], k), and the proximal end of the extensor carpi radialis longus ([Fig. 75], l). Inner surface with the humerus. Medial edge with the pectoralis major ([Fig. 65], l) and the biceps ([Fig. 77], g).

Action.—Flexor of the antibrachium.

M. triceps brachii.—The triceps muscle of the cat, like that of man, is divisible (as its name indicates) into three main portions. These are a lateral portion (caput laterale), an intermediate or long portion (caput longum), and a medial portion (caput mediale). The first two correspond to the similarly named heads in man; the medial portion falls into a number of subdivisions whose homologues are uncertain.

[Fig. 79].—Deep Muscles on the Medial Side of the Humerus.

a, M. supraspinatus; b, M. subscapularis; c, M. coracobrachialis; d, M. teres major (d′, its tendon); e, part of M. latissimus dorsi joining the teres major; f, cut end of M. pectoralis minor; g, long part of caput mediale of M. triceps brachii; h, intermediate part of caput mediale of M. triceps; i, M. brachialis (i′, its tendon); j, short part of caput mediale of M. triceps; k, cut end of M. clavobrachialis; l, l′, cut ends of M. biceps; m, M. brachioradialis; n, M. extensor carpi radialis longus; o, M. extensor carpi radialis brevis; p, fifth head of M. flexor profundus digitorum; q, first head of M. flexor profundus digitorum. 1, humerus; 2, olecranon; 3, medial epicondyle of humerus.

Caput laterale (anconeus lateralis) ([Fig. 75], h; [Fig. 68], l).—The lateral portion is a flat muscle (most of it subcutaneous) on the lateral side of the brachium, connecting the proximal end of the humerus with the olecranon process of the ulna.

Origin ([Fig. 81], h′).—By a flat tendon from the proximal portion of the deltoid ridge and the distal border of the teres minor tubercle.