304 FASCL# AND MUSCLES OF THE HORSE 
Relations.—Laterally, the cutaneus, deltoid, infraspinatus, teres minor, and 
the lateral head; medially, the tensor fascie antibrachii, teres major, latissimus dorsi 
and posterior deep pectoral muscles, and the subscapular vessels; in front, the 
brachialis and the medial head, the deep brachial and posterior circumflex vessels, 
and the axillary and radial nerves; behind, the skin and fascia. 
Blood-supply.—Subscapular and deep brachial arteries. 
Nerve-supply.—Radial nerve. 
(b) Lateral head (Caput laterale tricipitis).—This is a strong, quadrilateral 
muscle, which lies on the lateral surface of the arm. Its proximal third is covered 
by the deltoid and teres minor muscles, the remainder only by the thin cutaneous 
muscle and the skin. 
Origin.—(1) The deltoid tuberosity and the curved rough line which extends 
from it to the neck of the humerus; (2) the strong fascia which extends from the 
deltoid tuberosity to the lateral surface of the elbow joint. 
Insertion.—(1) A small prominent area on the lateral surface of the olecranon; 
(2) the tendon of the long head. 
Action.—To extend the elbow joint. 
Structure.—The origin consists of short tendinous fibers. The belly is thick, 
and is composed of parallel bundles which are directed obliquely downward and 
backward. They are inserted partly into the tendon of the long head and partly 
into the olecranon below and in front of that tendon. 
Relations.—Laterally, the deltoid, teres minor, and cutaneus muscles; medially, 
the long and medial heads and the brachialis muscle. Branches of the circumflex 
vessels and axillary nerve emerge between the posterior edge of the muscle and the 
long head. The deep face of the muscle is related to the branches of the deep bra- 
chial artery and of the radial nerve. 
Blood-supply.— Posterior circumflex and deep brachial arteries. 
Nerve-supply.— Radial nerve. 
(c) Medial head (Caput mediale tricipitis) (Fig. 277).2—This is much the small- 
est of the three heads. It is situated on the medial surface of the arm, and extends 
from the middle third of the humerus to the olecranon. 
Origin.—The middle third of the medial surface of the shaft of the humerus, 
behind and below the teres tubercle. 
Insertion —The medial and anterior part of the summit of the olecranon, be- 
tween the insertion of the long head and the origin of the ulnar head of the deep 
digital flexor. 
Action.—To extend the elbow joint. 
Structure.—The muscle is fleshy except at its insertion, where it has a flat ten- 
don, under which a small bursa may be found. 
Relations.—Laterally, the humerus, brachialis, anconeus, and the lateral head; 
medially, the posterior deep pectoral, coraco-brachialis, teres major, latissimus dorsi, 
and tensor fascize antibrachii muscles, the brachial and deep brachial vessels, and 
the median and ulnar nerves; behind, the long head, branches of the deep brachial 
vessels, and the radial nerve. 
Blood-supply.— Deep brachial and ulnar arteries. 
Nerve-supply.—Radial nerve. 
5. Anconeus.—This is a small muscle which covers the olecranon fossa and 
is covered by the triceps. It is somewhat difficult to separate from the lateral head, 
Origin.—The distal third of the posterior surface of the humerus. 
Insertion.—The lateral surface of the olecranon. 
Action.—To extend the elbow joint, and to raise the capsule of the joint and 
prevent its being pinched during extension. 
' Also known as the anconzeus lateralis s. externus or caput medium. 
* Also known as the anconzeus medialis s. internus or caput parvum. 
