Chapter IV: Muscles of the Thoracic Limbs

The muscles connecting the thoracic limbs with the rest of the body have been described.

1. Muscles of the Shoulder.

A. MUSCLES ON THE LATERAL SURFACE OF THE SHOULDER.

=M. deltoideus.=--The deltoid muscle of the cat is divided into two (or three) portions which are together equivalent to the human deltoid. These are the spinodeltoid, the acromiodeltoid, and possibly the clavobrachial, which is frequently called clavodeltoid.

=M. spinodeltoideus= (Fig. 75, _e_; Fig. 68, _i_).--A rather thick, flat muscle between the scapular spine and the deltoid ridge of the humerus. It forms a chord of the angle between the glenoid border of the scapula and the humerus.

_Origin_ by short tendon-fibres from the glenoid border of somewhat more than the middle third of the spine of the scapula (Fig. 76, _g_), and from a tendinous raphe between the spinotrapezius (Fig. 68, _j_), acromiotrapezius (Fig. 68, _h_), and infraspinatus (Fig. 75, _c_). The origin may pass toward the vertebral border of the scapula onto the infraspinatus muscle (Fig. 75, _c_).

_Insertion_ (Fig. 81, _h_) by a flat tendon upon the deltoid ridge of the humerus, nearly parallel to that of the pectoralis major.

_Relations._--Outer surface with the integument and at the insertion with the acromiodeltoideus (Fig. 75, _f_). Inner surface with the infraspinatus (_c_), teres minor (Fig. 80, _c_), caput laterale (Fig. 75, _h_), and caput longum (Fig. 75, _g_) of the triceps muscle.

_Action._--Flexes the humerus and rotates it outward.

=M. acromiodeltoideus= (Fig. 75, _f_; Fig. 68, _g_).--A flat muscle which overlies the distal end of the spinodeltoideus (Fig. 75, _c_). It connects the acromion with the humerus.

_Origin_ (Fig. 76, _d_).--From the glenoid border of the acromion, and sometimes the adjacent metacromion as far as the tip.

_Insertion._--Mostly upon the outer surface of the spinodeltoideus (Fig. 75, _c_). The outer fibres are continued to the bone, especially at the lateral border of the muscle, and are inserted along a line ventrad of the line of insertion of the spinodeltoideus, and extending farther distally (Fig. 81, _g_). Some of the outer fibres pass into the brachialis (Fig. 80, _h_).

_Relations._--Outer surface with the integument and the clavobrachial (Fig. 68, _e_). Inner surface with the infraspinatus (Fig. 75, _c_), teres minor (Fig. 80, _c_), spinodeltoid (Fig. 75, _e_), and caput laterale of the triceps (Fig. 75, _h_).

_Action._--Like that of the spinodeltoid.

=M. clavobrachialis= (Fig. 65, _k_; Fig. 68, _e_).--A flat, triangular muscle on the cranial surface of the shoulder, forming a direct continuation of the clavotrapezius (Fig. 68, _d_). These two are frequently described as constituting a single muscle, the cephalohumeral or cephalobrachial. (The clavobrachial is frequently given the name clavodeltoid; as its homology with the human clavodeltoid appears doubtful, it seems well to use the name clavobrachial, as proposed by Clasen.)

_a_, M. supraspinatus; _b_, part of insertion of M. rhomboideus; _c_,
M. infraspinatus; _d_, M. teres major; _e_, M. spinodeltoideus; _f_,
M. acromiodeltoideus; _g_, caput longum of M. triceps brachii; _h_,
caput laterale of M. triceps brachii; _i_, M. brachialis; _j_, M.
biceps; _k_, M. brachioradialis; _l_, M. extensor carpi radialis
longus; _m_, M. extensor communis digitorum (_m′_, tendon of same);
_n_, M. extensor lateralis digitorum; (_n′_, tendon of same); _o_, M.
extensor carpi ulnaris (_o′_, tendon of same); _p_, M. extensor
indicis proprius; _q_, fifth head of M. flexor profundus digitorum;
_r_, M. flexor carpi ulnaris. 1, vertebral border of the scapula; 2,
spine of the scapula; 3, greater tuberosity of the humerus; 4,
olecranon; 5, transverse ligament of the wrist.]

_Origin._--Its superficial fibres are continuations of the clavotrapezius (Fig. 68, _d_); other fibres have origin from the clavicle and from a raphe laterad of the clavicle which is common to this muscle and the clavotrapezius. It passes distad along the cranial surface of the arm, growing narrower as it approaches the convexity of the elbow.

_Insertion._--Eight to ten millimeters from the ulna it joins the brachialis muscle (Fig. 79, _i_) to be inserted with it by a flat tendon (Fig. 79, _i_; Fig. 87, _c_) upon a rough area on the medial surface of the ulna just distad of the semilunar notch, and about midway between the dorsal and ventral borders.

_Relations._--Outer surface with the integument. Inner surface with the pectoralis major (Fig. 65, _l_), biceps (Fig. 77, _g_), lateral head of the triceps (Fig. 75, _h_), acromiodeltoid (Fig. 75, _f_), and brachialis (Fig. 80, _h_). Medial border with the pectoantibrachialis (Fig. 65, _m_). Lateral border free except near the insertion, where it is in relation with the brachialis.

_Action._--Flexor of the antibrachium.

=M. supraspinatus= (Fig. 75, _a_) occupies the whole of the supraspinatus fossa of the scapula. It is covered by strong fascia which stretches from the free edge of the spine to the coracoid border of the scapula and to the coracoid half of its vertebral border.

_Origin_ (Fig. 76, _b_) by fleshy fibres from the whole surface of the supraspinatus fossa, from the above-mentioned fascia, and from the subscapularis (Fig. 77, _a_) craniad of the coracoid border of the scapula.

_Insertion_ (Fig. 83, _a_).--It passes over the capsule of the shoulder-joint, to which it is closely attached, and is inserted into the free border of the great tuberosity ventrad (or proximad) of the fossa for the infraspinatus (Fig. 83, _c_).

_Relations._--Outer surface with the spinotrapezius (Fig. 68, _j_), acromiotrapezius (Fig. 68, _h_), levator scapulæ ventralis (Fig. 68, _f_), clavotrapezius (Fig. 68, _d_), and cleidomastoid (Fig. 65, _h_). Inner surface with the scapula. The distal end of the coracoid border is closely related to the pectoralis minor (Fig. 79, _f_). The glenoid border is related to the origin of the deltoidei.

_Action._--Extends the humerus after it has been flexed on the scapula.

=M. infraspinatus= (Fig. 75, _c_).--This fills the infraspinatus fossa, its fibres converging to the insertion on the great tuberosity of the humerus.

_a_, M. infraspinatus; _b_, M. supraspinatus; _c_, origin of M.
biceps; _d_, M. acromiodeltoideus; _e_, M. teres minor; _f_, M.
levator scapulæ ventralis; _g_, M. spinodeltoideus; _h_, M.
acromiotrapezius; _i_, M. rhomboideus; _j_, M. teres major; _k_-_l_,
line of insertion of M. spinotrapezius.]

_Origin_ (Fig. 76, _a_).--By fleshy fibres from the whole infraspinatus fossa, and by a raphe between it and the teres major (Fig. 75, _d_), and sometimes by fibres from the teres minor (Fig. 80, _c_), triceps, spinodeltoideus (Fig. 75, _e_), spinotrapezius (Fig. 68, _j_), and subscapularis (Fig. 77, _a_).

_Insertion_ (Fig. 83, _c_).--By a flat tendon which passes over the capsule of the joint, into the ventral half of the infraspinatus fossa on the great tuberosity of the humerus. (The dorsal half of the fossa is covered by a synovial bursa.)

_Relations._--Outer surface with the spinotrapezius (Fig. 68, _j_), spinodeltoid (Fig. 68, _i_), acromiodeltoid (Fig. 68, _g_), teres major (Fig. 75, _d_), and levator scapulæ ventralis (Fig. 68, _f_). Inner surface with the scapula. Glenoid border with the teres minor (Fig. 80, _c_), the teres major (Fig. 75, _d_), and the long head of the triceps (Fig. 75, _g_).

_Action._--Rotates the humerus outward.

=M. teres minor= (Fig. 80, _c_).--A small muscle from the glenoid border of the scapula to the proximal end of the humerus.

_Origin_ (Fig. 76, _e_) by a sheet of tendinous fibres from the glenoid border of the scapula beginning about one-fifth the length of the border from the glenoid fossa and extending to its middle. It is often attached to the infraspinatus (Fig. 80, _b_) and the caput longum of the triceps (Fig. 75, _g_).

_Insertion_ (Fig. 83, _d_) by a short tendon into the tubercle just distad of the infraspinatus fossa on the great tuberosity of the humerus.

_Relations._--Outer surface with the spinodeltoideus (Fig. 75, _e_), acromiodeltoideus (Fig. 75, _f_), and the infraspinatus (Fig. 80, _b_). Inner surface with the lateral (Fig. 75, _h_) and long (Fig. 75, _g_) heads of the triceps and the capsule of the joint.

_Action._--Assists the infraspinatus to rotate the humerus outward.

B. MUSCLES ON THE MEDIAL SURFACE OF THE SHOULDER.

(Fig. 77.)

=M. subscapularis= (Fig. 77, _a_).--A triangular mass from the subscapular fossa to the lesser tuberosity of the humerus.

_Origin_ (Fig. 78, _a_).--From the whole subscapular fossa except along the fusiform area for the attachment of the levator scapulæ and the serratus anterior near the vertebral border, and except over a quadrangular area about one centimeter long near the glenoid angle. The origin is by fleshy fibres directly from the periosteum except along two or three lines marked by oblique ridges. To these lines are attached tendinous fibres. At the glenoid border the area of origin sometimes occupies part or the whole of the surface of the fossa for the origin of the teres major, so that the teres arises from the fascia on the surface of the subscapularis. At the coracoid border some of the fibres may take origin from the adjacent fascia of the supraspinatus (Fig. 77, _d_).

_Insertion_ (Fig. 82, _b_).--The fibres converge to the glenoid border, and the insertion is by a strong, flat tendon into the dorsal border of the lesser tuberosity of the humerus.

M. epitrochlearis, M. clavobrachialis, and the breast-muscles have
been removed. _a_, M. subscapularis; _b_, insertion of M. levator
scapulæ and of M. serratus anterior; _c_, M. teres major; _d_, M.
supraspinatus; _e_, M. latissimus dorsi (_e′_, cut edge of that part
which becomes united with the bicipital arch); _f_, M.
coracobrachialis; _g_, M. biceps brachii (the capsule of the joint has
been laid open to show its tendon); _h_, cut insertions of pectoralis
muscles; _i_, caput longum of M. triceps brachii; _j_, long portion of
caput mediale of M. triceps brachii; _k_, intermediate portion of
caput mediale of M. triceps brachii; _l_, short portion of caput
mediale of M. triceps brachii; _m_, cut end of M. clavobrachialis;
_n_, M. brachioradialis; _o_, M. extensor carpi radialis longus; _p_,
M. extensor carpi radialis brevis; _q_, M. pronator teres; _r_, M.
flexor carpi radialis; _s_, M. palmaris longus (_s′_, its tendons);
_t_, M. flexor carpi ulnaris (_t_, ulnar head; _t′_, humeral head);
_u_, third head of M. flexor profundus digitorum; _u′_, fifth head of
M. flexor profundus digitorum; _u″_, one of the tendons of M. flexor
profundus digitorum; _v_, M. pronator quadratus; _w_, M. abductor
brevis pollicis; _x_, ulnar part of M. flexor sublimis digitorum. 1,
transverse ligament of the wrist.]

_Relations._--Lateral surface with the scapula and the capsule of the shoulder-joint. Medial surface with the levator scapulæ (Fig. 73, _h_), serratus anterior (Fig. 73, _i_), part of the scalenus (Fig. 73, _f_), the transversus costarum (Fig. 73, _j_), and the coracobrachialis (Fig. 77, _f_). Cranial border with the supraspinatus (Fig. 77, _d_). Caudal border with the teres major (Fig. 77, _c_) and infraspinatus (Fig. 75, _c_).

_Action._--Pulls the humerus inward (mediad).

=M. teres major= (Fig. 75, _d_; Fig. 77, _c_).--A thick muscle, triangular in cross-section, lying parallel with the glenoid border of the scapula.

_Origin_ (Fig. 76, _j_; Fig. 78, _b_) from the vertebral one-third of the glenoid border of the scapula, and from fascia covering the subscapularis (Fig. 77, _a_) and the infraspinatus (Fig. 75, _c_) over a small area near the glenovertebral angle of the scapula. It may touch the insertion of the rhomboideus.

_Insertion_ (Fig. 82, _f_) by a tendon common to it and the latissimus dorsi (Fig. 79, _d′_), as already described.

_Relations._--Outer surface with the caput longum (Fig. 75, _g_) and the long portion (Fig. 80, _f_) of the caput mediale of the triceps, the latissimus dorsi (Fig. 77, _e_), and the cutaneus maximus. Inner surface with the serratus anterior (Fig. 73, _i_), the scalenus (Fig. 73, _f_), the transversus costarum (Fig. 73, _j_), and the biceps (Fig. 77, _g_). Dorsal border with the subscapularis (Fig. 77, _a_) and infraspinatus (Fig. 75, _c_).

_Action._--Rotates the humerus inward and flexes it in opposition to the infraspinatus, teres minor, and the deltoidei.

_a_, M. subscapularis; _b_, M. teres major; _c_, M. serratus anterior;
_d_, M. levator scapulæ; _e_, M. rhomboideus; _f_, M.
occipitoscapularis; _g_, M. biceps; _h_, M. coracobrachialis; _i_,
caput longum of M. triceps.]

2. =Muscles of the Brachium or Upper Arm.=

M. clavobrachialis and the pectoralis group (Fig. 65, page 109) lie partly in this region; they have been described.

=M. coracobrachialis= (Fig. 77, _f_; Fig. 79, _c_).--A very short muscle covering the inner (medial) surface of the capsule of the shoulder-joint. It extends from the coracoid process to the proximal end of the humerus.

_Origin_ (Fig. 78, _h_).--By a round tendon from the tip of the coracoid process.

_Insertion_ (Fig. 82, _e_) usually by fleshy fibres on an area five to eight millimeters long and about half as wide, which lies on the medial side of the humerus parallel with the ridge which runs from the dorsal end of the lesser tuberosity to the shaft and close to it. The proximal end of the area is about one centimeter from the proximal end of the head of the humerus.

_Relations._--Medial surface with the biceps (Fig. 77, _g_), pectoralis minor (Fig. 65, _o_), and teres major (Fig. 77, _c_). Lateral surface with the capsule of the shoulder-joint. Dorsal border with the subscapularis (Fig. 77, _a_) and the long portion of the caput mediale of the triceps (Fig. 77, _j_).

_Action._--Adducts the humerus.

The part of the coracobrachialis just described is known as the _short_ head. A _long_ head is sometimes found. It is a conical bundle of fibres of varying size, which arises from the tendon of origin of the short head. It passes distad into a long and extremely slender tendon, the insertion of which varies greatly in different individuals. It is commonly on the humerus in the region of the supracondyloid foramen.

=M. epitrochlearis= or extensor antibrachii longus (Fig. 65, _r_).--A thin, flat muscle on the inner or medial side of the brachium, from the lateral surface of the latissimus dorsi (Fig. 65, _q_) to the olecranon process of the ulna.

_Origin_ from the lateral or outer surface of the ventral border of the latissimus dorsi (Fig. 65, _q_) near the insertion of the cutaneus maximus. Fibres are often attached to the teres major and the pectoralis minor.

_Insertion_ by a flat tendon which is closely connected with that of the pectoantibrachialis (Fig. 65, _m_) and is continuous with the general antibrachial fascia, into the caudal border of the cutaneous (dorsal) surface of the olecranon process of the ulna.

_Relations._--Outer (medial) surface with the integument, the latissimus dorsi (Fig. 65, _q_), and the cutaneus maximus. Inner (lateral) surface with the biceps (Fig. 65, _t_), caput longum (Fig. 65, _s_), and caput mediale (Fig. 77, _j_, _k_) of the triceps.

_Action._--Extends the antibrachium and tends to supinate the hand by rotating the ulna.

=M. biceps brachii= (Fig. 77, _g_; Fig. 65, _t_).--A thick, fusiform muscle lying on the front (ventral) surface of the humerus.

_Origin_ (Fig. 76, _c_) by a strong, round tendon from the bicipital tubercle of the glenoid angle of the scapula, at its coracoid margin. (In man there is a second head--coracoid head--from the coracoid process.) The tendon passes through the capsule of the joint and then along the bicipital groove, which is converted into a canal by a strong ligament.

_Insertion_ by a rounded tendon (Fig. 87, _b_) on the bicipital tuberosity of the radius (Fig. 86, _k_).

_Relations._--The tendon of origin passes through the capsule of the joint, which is covered by a part of the pectoralis minor. The muscle is then spanned for the middle part of its course by the bicipital arch (Fig. 65, _t′_), a description of which is given below. Farther distad it is covered by the clavobrachial (Fig. 65, _k_) and epitrochlearis (Fig. 65, _r_). At its distal end (Fig. 79, _l′_) the muscle passes between the pronator teres on the radial side and the conjoined tendon of the brachialis and clavobrachial (Fig. 79, _i′_) on the other. The biceps lies throughout most of its length on the ventral surface of the humerus, and touches along the medial border of its inner surface the coracobrachialis (Fig. 77, _f_), the teres major, (Fig. 77, _c_), the intermediate portion of the caput mediale of the triceps (Fig. 77, _k_), and the short portion (Fig. 77, _l_) of the same. Its lateral border touches the pectoralis minor (Fig. 65, _o_) and the brachialis (Fig. 79, _i_).

_Action._--Flexes the forearm, and tends to supinate the hand.

=The bicipital arch= (Fig. 65, _t′_) is a tendinous arch formed over the biceps muscle. Its outer or lateral pillar is formed by the tendon of the pectoralis minor (_o_), to which may be added part of the latissimus (_q_) tendon. Its inner or medial pillar is formed by the conjoined tendon of the teres major and latissimus dorsi (Fig. 79, _d′_), while the xiphihumeralis (Fig. 65, _p_), cutaneus maximus (Fig. 62, _b_), and epitrochlearis (Fig. 65, _r_) are connected with one or the other pillar or with the muscles composing them. The caudal portion of the deep layer of the pectoralis major (Fig. 65, _l_) may be connected with the inner pillar.

=M. brachialis= (Fig. 79, _i_; Fig. 75, _i_).--From the lateral surface of the humerus to the ulna.

_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.

_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.

_Insertion_ (Fig. 86, _c_) by a thin, flat tendon into the lateral border of the dorsal surface of the ulna between a point opposite the distal margin of the semilunar notch and the proximal end of the olecranon.

_Relations._--Lateral surface with the integument, and at the proximal end with the spinodeltoid and acromiodeltoid (Fig. 75, _e_ and _f_). Medial surface with the brachialis (Fig. 75, _i_), the caput longum (_g_), the caput mediale, the brachioradialis (_k_), the anconeus (Fig. 80, _l_), and the origins of the extensor carpi radialis longus (Fig. 75, _l_), the extensor communis digitorum (_m_), and the extensor digitorum lateralis (_n_).

=Caput longum= (anconeus longus) (Fig. 75, _g_; Fig. 68, _k_; Fig. 77, _i_).--A prismatic, fusiform mass connecting the scapula with the olecranon.

_Origin_ (Fig. 78, _i_) by a thick, flat tendon from a triangular area one or two centimeters long at the glenoid end of the scapula on the glenoid border. The base of the area is about one millimeter from the margin of the glenoid fossa.

_Insertion_ (Fig. 86, _b_).--The muscle ends in a thick tendon which passes over the bifurcated ventral end of the olecranon and is inserted upon the rounded tuberosity which forms the dorsal angle of the olecranon.

_Relations._--Lateral surface with the caput laterale (Fig. 75, _h_), a portion of the caput mediale (Fig. 77, _j_ and _k_), the spinodeltoid (Fig. 75, _e_), the infraspinatus (Fig. 75, _c_), and the integument. Medial surface with the epitrochlearis (Fig. 65, _r_) and the conjoined portions of the latissimus dorsi and teres major (Fig. 79, _d′_). Ventral border with the caput mediale (Fig. 77, _j_ and _k_).

=Caput mediale.=--The medial head consists of three portions (Fig. 79, _g_, _h_, _j_).

(1) The long portion (anconeus posterior) (Fig. 77, _j_; Fig. 79, _g_; Fig. 80, _f_).

_Origin_ (Fig. 83, _e_) from a triangular area on the dorsal surface of the humeral shaft. The base of the area is against the articular head, and its apex about one-sixth the length of the humerus from the head. It is between the coracobrachialis and the lateral head of the triceps.

_a_, part of M. supraspinatus; _b_, part of M. infraspinatus; _c_, M.
teres minor; _d_, M. teres major; _e_, part of M. latissimus dorsi
joining M. teres major; _f_, long part of caput mediale of M. triceps
brachii; _g_, cut insertion of M. acromiodeltoideus; _h_, M.
brachialis; _i_, M. brachioradialis; _j_, intermediate part of caput
mediale of M. triceps brachii; _k_, M. extensor carpi radialis longus;
_l_, M. anconeus; _m_, M. extensor communis digitorum. 1, great
tuberosity of humerus; 2, 3, superficial radial and dorsal
interosseous branches, respectively, of the radial nerve.]

_Insertion_ (Fig. 86, _a_).--The muscle ends in a long, slender tendon which passes through the furrow on the ventral angle of the olecranon and is inserted into an oblique ridge which forms the dorsal limit of the furrow. The tendon is separated from the floor of the furrow by a synovial bursa.

(2) The intermediate portion (anconeus internus) (Fig. 77, _k_; Fig. 79, _h_; Fig. 80, _j_).

_Origin_ (Fig. 82, _g_) by fleshy fibres from a triangular area proximad of the middle of the dorsomedial surface of the humerus. The length of the area equals about one-fourth the length of the bone. Its apex points proximad and is almost continuous with the apex of the area of origin of the long portion.

_Insertion_ (Fig. 87, _i_) by short tendon-fibres into the medial border of the ventral and proximal surfaces of the olecranon. It is connected with the tendon of the long portion.

(3) Short portion (Fig. 79, _j_; Fig. 77, _l_).

_Origin_ (Fig. 82, _h_) from the outer surface of the bony bar which encloses the supracondyloid foramen of the humerus, as far as the medial epicondyle.

_Insertion_ (Fig. 87, _e_) by fleshy fibres into the medial border of the olecranon distad of the tuberosity for the insertion of the caput longum.

_Relations_ of the caput mediale.--Lateral surface with the caput laterale (Fig. 75, _h_), the humerus, and the anconeus (Fig. 80, _l_). Dorsal surface with the caput longum (Fig. 77, _i_), the epitrochlearis (Fig. 65, _r_), and the teres major (Fig. 79, _d_). Ventral surface with the biceps (Fig. 77, _g_).

_Action of the Triceps._--The entire triceps group forms a powerful extensor of the forearm. The short portion of the medial head tends also to rotate the arm outward, so as to supinate the hand.

=M. anconeus= (Fig. 80, _l_).--A triangular muscle on the outside of the elbow-joint from the humerus to the ulna.

_Origin_ (Fig. 83, _i_).--An irregularly triangular area at the distal end of the dorsal surface of the humerus. The area of origin is limited laterally by the sharp lateral supracondyloid ridge. It sometimes extends onto the lateral epicondyle.

FIG. 82.--MEDIAL SIDE OF HUMERUS, WITH THE AREAS OF ATTACHMENT OF
MUSCLES.

FIG. 83.--DORSAL SURFACE OF THE LEFT HUMERUS, WITH THE AREAS OF
ATTACHMENT OF MUSCLES.

Fig. 81.--_a_, M. subscapularis; _b_, M. supraspinatus; _c_, Mm. teres
major and latissimus dorsi; _d_, M. pectoralis minor; _e_, deep layer
of M. pectoralis major; _f_, superficial layer of M. pectoralis major;
_g_, M. acromiodeltoideus; _h_, M. spinodeltoideus; _h′_, caput
laterale of M. triceps; _i_, M. brachialis; _j_, second portion of M.
brachialis; _k_, M. extensor carpi radialis longus; _l_, M. extensor
carpi radialis brevis; _m._ M. extensor communis digitorum; _n_, M.
extensor lateralis digitorum; _o_, M. extensor carpi ulnaris; _p_,
short portion of caput mediale of M. triceps; _q_, M. pronator teres;
_r_, M. palmaris longus; _s_, third and fourth parts of M. flexor
profundus digitorum; _t_, M. flexor carpi radialis; _u_, second head
of M. flexor profundus digitorum.

Fig. 82.--_a_, M. supraspinatus; _b_, M. subscapularis; _c_, M.
pectoralis minor; _d_, long portion of caput mediale of M. triceps;
_e_, M. coracobrachialis; _f_, Mm. teres major and latissimus dorsi;
_g_, intermediate portion of caput mediale of M. triceps; _h_, deep
layer of M. pectoralis major; _i_, superficial layer of M. pectoralis
major; _j_, M. anconeus; _k_, short portion of caput mediale of M.
triceps; _l_, M. pronator teres; _m_, M. flexor carpi radialis; _n_,
second head of M. flexor profundus digitorum; _o_, third and fourth
heads of M. flexor profundus digitorum; _p_, M. palmaris longus; _q_,
M. flexor carpi ulnaris.

Fig. 83.--_a_, M. supraspinatus; _b_, M. subscapularis; _c_, M.
infraspinatus; _d_, M. teres minor; _e_, long portion of caput mediale
of M. triceps; _f_, M. coracobrachialis; _g_, _g′_, M. brachialis;
_h_, intermediate portion of caput mediale of M. triceps; _i_, M.
anconeus; _j_, short portion of medial head of M. triceps; _k_, M.
pronator teres; _l_, second and third heads of M. flexor profundus
digitorum; _m_, M. palmaris longus; _n_, M. extensor carpi ulnaris.]

_Insertion_ (Fig. 86, _d_).--The lateral surface of the ulna from the distal margin of the semilunar notch to the proximal end of the olecranon. The muscle covers the capsule of the joint and is closely adherent to it.

_Relations._--Outer (dorsal) surface with the caput laterale (Fig. 75, _h_) and the intermediate portion (Fig. 80, _j_) of the caput mediale. Inner surface with the bones and the capsule of the joint.

_Action._--Keeps the capsule tense and probably rotates the ulna slightly so as to pronate the hand.

3. =Muscles of the Antibrachium or Forearm.=

_Fascia of the Forearm._--The muscles of the forearm are not in contact with the skin, but are covered by the strong =antibrachial fascia=. This consists of two layers. The =superficial= one is a continuation of the general subcutaneous fascia of the arm; it covers the forearm as a continuous sheet which becomes thinner distad, and is finally lost near the wrist. The =deep= layer forms a dense, tendinous sheet which is closely applied to the muscles. It begins partly as a continuation of the general fascia of the arm, partly from the tendons of the triceps and epitrochlearis muscles. The sheet is attached to the dorsal surface of the ulna, between the extensor carpi ulnaris and the first head of the flexor profundus digitorum. It surrounds the forearm as a continuous sheath. On the lateral surface it dips between the extensor muscles, becoming closely attached to them and forming a partial sheath for each; it is attached with special firmness to their tendons. On the medial side of the arm the fascia passes smoothly over the flexors without dipping between them; it is continuous, however, with the border of the pronator teres, and distad of this muscle it is firmly attached to the radius.

At the wrist the fascia becomes attached to the longitudinal ridges on the dorsal surface of the head of the radius, bridging thus the intervening grooves and holding the tendons in place between the ridges. The fascia then continues to form the dorsal fascia of the hand. On the ventral side of the wrist the fascia becomes thickened to form a strong =transverse ligament= (Fig. 77, 1), which is attached at its radial and ulnar ends to the carpus, and which holds in place the tendons of the flexor muscles. Distad of the transverse ligament the fascia spreads out in the palm of the hand, and becomes united with the fibrous pad which is situated here. On the fingers the fascia becomes attached to the phalanges, forming tendinous sheaths, the so-called =vaginal ligaments=, for the flexor muscles. Near the base and the head of the first phalanx strong, narrow, ring-like bands, the =annular ligaments= (Fig. 88, 2), are formed in these sheaths; these surround and bind down the tendons of the flexor muscles.

A. MUSCLES ON THE ULNAR AND DORSAL SIDE OF THE FOREARM (extensors and supinators) (Fig. 75, p. 158).

=M. brachioradialis= or supinator longus (Fig. 75, _k_; Fig. 77, _n_).--This is a ribbon-like muscle lying on the lateral side of the brachium and the ventrolateral border of the antibrachium just beneath the superficial fascia. It connects the humerus and the radius.

_Origin._--By a thin tendon from about the middle fifth of the dorsal border of the humerus, or sometimes proximad of the middle. The muscle curves over the outer surface of the brachialis (Fig. 75, _i_), to which it is closely applied, and runs along the lateral border of the antibrachium to its insertion.

_Insertion_ by a short tendon into the ridge which forms the ventral limit of the groove on the outer surface of the styloid process of the radius and upon the adjacent ligaments.

_Relations._--Outer surface with the integument, and at the proximal end with the caput laterale of the triceps (Fig. 75, _h_). Inner surface with the brachialis (_i_), the extensor carpi radialis longus (_l_) and brevis, and the extensor communis digitorum (_m_).

_Action._--Supinator of the hand.

=M. extensor carpi radialis longus= (Fig. 75, _l_; Fig. 77, _o_).--This lies along the radial side of the forearm, and connects the humerus and the second metacarpal.

_Origin_ (Fig. 81, _k_) from the middle portion of the lateral supracondyloid ridge between the origin of the brachialis (_j_) and the anconeus. About the middle of the forearm the muscle ends in a slender tendon (Fig. 84, _e_) which passes through the groove on the radial side of the dorsal surface of the distal end of the radius.

_Insertion_ (Fig. 84, _c_).--The tendon passes across the carpus and is inserted onto the dorsal surface of the base of the second metacarpal on the radial side just distad of the groove for the radial artery.

_Relations._--Outer surface at the proximal end with the caput laterale of the triceps (Fig. 75, _h_), farther distad with the integument and the brachioradialis (_k_); the distal tendon with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the brachialis (Fig. 75, _i_) and the integument. Ulnar surface with the extensor carpi radialis brevis (Fig. 77, _p_), and near the proximal end with the extensor communis digitorum (Fig. 75, _m_).

_Action._--Extensor of the hand.

=M. extensor carpi radialis brevis= (Fig. 77, _p_).--A slender muscle from the humerus to the third metacarpal.

_Origin_ (Fig. 81, _l_) from the distal part of the lateral supracondyloid ridge distad of the extensor longus. Near the carpus the muscle ends in a slender tendon which passes through the groove with the extensor longus tendon. The tendon then diverges from the extensor longus and has its

_Insertion_ (Fig. 84, _f_) into the radial side of the dorsal surface of the base of the third metacarpal.

_Relations._--Outer surface with the extensor longus (Fig. 77, _o_), the integument, and distad with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the extensor longus. Ulnar surface with the extensor communis digitorum (Fig. 75, _m_) and the extensor brevis pollicis. Inner surface with the pronator teres (Fig. 77, _q_), supinator (Fig. 85, _b_), and extensor brevis pollicis (Fig. 85, _a_).

_Action._--Extensor of the hand.

_a_, M. extensor communis digitorum; _b_, M. extensor lateralis
digitorum; _c_, M. extensor indicis; _d_, M. extensor brevis pollicis;
_e_, M. extensor carpi radialis longus; _f_, M. extensor carpi
radialis brevis; _g_, M. extensor carpi ulnaris; _h_, tendons of the
interossei. 1, transverse ligament of the wrist.]

=M. extensor digitorum communis= (Fig. 75, _m_).--A slender muscle from the lateral supracondyloid ridge of the humerus to the phalanges of the second, third, fourth, and fifth digits.

_Origin_ (Fig. 81, _m_).--The distal surface of the lateral supracondyloid ridge dorsad of the origin of the extensor carpi radialis brevis.

At the junction of the middle and distal thirds of the radius the muscle passes into a large, flat tendon (Fig. 75, _m′_) which passes through the groove on the middle of the dorsal surface of the distal end of the radius. The groove is converted into a canal by a strong transverse ligament. The tendon divides near the groove into four portions (Fig. 84, _a_) which diverge and pass to the four ulnar digits.

_Insertions._--Each division of the tendon as it passes over the dorsal surface of the first phalanx has its border connected by strong fascia to the dorsal surface of the phalanx. It finally passes through the groove at the distal end of the first phalanx and is inserted mostly into the base of the second phalanx; a portion continues distad to be inserted into the third phalanx.

_Relations._--Outer surface with the integument and at the origin with the caput laterale (Fig. 75, _h_) of the triceps. Radial surface with the extensor longus (_l_) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (_n_). Inner surface with the supinator (Fig. 85, _b_) and the extensor pollicis brevis (Fig. 85, _a_).

_Action._--Extensor of the four ulnar digits.

=M. extensor digitorum lateralis= (Fig. 75, _n_). (This corresponds in position to the extensor digiti quinti proprius of man.)

_Origin_ (Fig. 81, _n_) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis.

The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis (Fig. 75, _m_) and the extensor carpi ulnaris (_o_). Near the carpus it ends in a tendon (_n′_), which divides immediately into three or four (Fig. 84, _b_). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon.

_Insertion_ (Fig. 84, _b_).--The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (_a_) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (_c_) and may also give a branch to the radial side of the base of the first phalanx of the third digit.

_Relations._--Outer surface with the integument. Radial border with the extensor communis digitorum (_m_); ulnar border with the extensor carpi ulnaris (_o_). Inner surface with the supinator (Fig. 85, _b_) and the extensor brevis pollicis (Fig. 85, _a_); distad with the extensor indicis (Fig. 85, _c_).

_Action._--Extensor of the four ulnar digits.

=M. extensor carpi ulnaris= (Fig. 75, _o_).

_Origin._--(1) By a short, broad tendon from the distal portion of the lateral epicondyle of the humerus (Fig. 81, _o_) distad of the origin of the extensor lateralis, and (2) by a smaller tendon from the ulna at the dorsal tip of the semilunar notch (Fig. 86, _e_).

The muscle passes along the ulnar side of the forearm dorsad of the extensor digitorum lateralis (_n_). It ends near the junction of the middle and distal thirds of the forearm in a large, flat tendon (_o′_) which is closely adherent to the ligaments of the wrist and which passes over the distal end of the ulna to its

_Insertion_ into the tubercle on the ulnar side of the base of the fifth metacarpal (Fig. 84, _g_).

_Relations._--Outer surface with the integument. Radial (or ventral) border with the extensor digitorum lateralis (Fig. 75, _n_); ulnar (or dorsal) border with the flexor profundus digitorum (_q_). Inner surface with the radius, the extensor indicis (Fig. 85, _c_), and the extensor pollicis brevis (Fig. 85, _a_).

_Action._--Indicated by its name.

=M. extensor indicis (proprius)= (Fig. 85, _c_; Fig. 75, _p_) (includes extensor pollicis longus also).

_Origin_ (Fig. 86, _f_) by short, fleshy fibres from the lateral surface of the ulna between the semilunar notch and the junction of the third and fourth (distal) fourths.

The muscle-fibres pass obliquely distad and toward the radial side and join the tendon (Fig. 85, _c′_), which runs nearly the whole length of the radial surface.

_Insertion_ (Fig. 84, _c_).--The tendon passes through the groove on the ulnar side of the dorsal surface of the base of the radius and divides into two. Both of these usually go to the base of the second phalanx of the second digit, but one may pass to the pollex, or there may be three divisions going to the first three digits.

_a_, M. extensor brevis pollicis (_a′_, its tendon); _b_, M.
supinator; _c_, M. extensor indicis (_c′_, its tendon); _d_, tendon of
M. extensor carpi radialis brevis; _e_, tendon of M. extensor carpi
radialis longus. 1, humerus; 2, olecranon; 3, styloid process of ulna;
4, distal end of radius; 5, head of radius.]

_Relations._--Outer surface with the integument, the extensor carpi ulnaris (Fig. 75, _o_), and the extensor digitorum lateralis (Fig. 75, _n_). Dorsal border with the flexor profundus digitorum (Fig. 75, _q_). Inner surface with the extensor brevis pollicis (Fig. 85, _a_).

_Action_ as indicated by the name.

=M. supinator= (Fig. 85, _b_).--A flat muscle wrapped spirally about the proximal end of the radius.

_Origin_ by a short, strong tendon from the lateral side of the annular ligament of the radius and by tendinous fibres from the radial collateral ligament passing from the humerus to the radius. From the tendon of origin the fibres diverge, passing toward the radial side and distad.

_Insertion_ (Fig. 86, _g_).--The dorsal and part of the ventral surface of the proximal two-fifths of the radius to within five millimeters of the articular face of the head.

_Relations._--Outer surface with the extensor carpi radialis brevis, extensor communis digitorum (Fig. 75, _m_), and extensor lateralis digitorum (Fig. 75, _n_). Radial border with the pronator teres (Fig. 77, _q_). Inner surface with the radius.

_Action._--Supinator of the hand.

_a_, long portion of caput mediale of M. triceps; _b_, caput longum of
M. triceps; _c_, caput laterale of M. triceps; _d_, M. anconeus; _e_,
M. extensor carpi ulnaris; _f_, M. extensor indicis; _g_, M.
supinator; _h_, M. extensor brevis pollicis; _i_, M. pronator teres;
_j_, M. flexor profundus digitorum; _k_, M. biceps.]

=M. extensor brevis pollicis= or extensor ossis metacarpi pollicis (Fig. 85, _a_) (includes M. abductor longus pollicis of man).

_Origin_ (Fig. 86, _h_) by fleshy fibres from the ventral half of the lateral surface of the shaft of the ulna from the semilunar notch to the styloid process; from the ulnar half of the dorsal surface of the radius from the bicipital tubercle to a short distance distad of the middle of the bone; and from the interosseous membrane between these two areas. The fibres converge to form a strong, flat tendon (Fig. 84, _d_) which passes obliquely over the tendons of the extensor carpi radialis longus (_e_) and brevis (_f_) to its

_Insertion_ into the radial side of the base of the first metacarpal. The radial sesamoid bone of the wrist is imbedded in the tendon at its insertion.

_Relations._--Outer surface with the extensor carpi radialis brevis, the extensor communis digitorum (Fig. 75, _m_), extensor lateralis digitorum (Fig. 75, _n_), and extensor indicis (Fig. 85, _c_). Inner surface with the radius and ulna, and distad with the tendons of the two extensores carpi radiales (Fig. 84, _e_ and _f_).

_Action._--Extends and abducts the pollex.

B. MUSCLES ON THE RADIAL AND VENTRAL SIDE OF THE FOREARM (flexors and pronators).

=M. pronator teres= (Fig. 77, _q_).

_Origin_ (Fig. 82, _l_) by a short strong tendon from the extremity of the medial epicondyle of the humerus.

_Insertion_ (Fig. 86, _i_) by fleshy fibres and short tendinous fibres along the medial border of the radius, at its middle. The radial edge of the muscle is continuous with the strong deep layer of the antibrachial fascia.

_Relations._--Outer surface with the superficial fascia. Radial border with the tendon of the biceps (Fig. 79, _l′_), with the extensor carpi radialis brevis (Fig. 77, _p_), and the supinator (Fig. 85, _b_). Ulnar border with the flexor carpi radialis (Fig. 77, _r_) and part of the flexor profundus digitorum (Fig. 77, _u_).

_Action._--Pronates the hand by rotating the radius.

=M. flexor carpi radialis= (Fig. 77, _r_).--A slender fusiform muscle.

_Origin_ (Fig. 82, _m_) from the tip of the medial epicondyle of the humerus.

_Insertion._--The slender tendon passes through a deep groove between the os magnum and the first metacarpal. The groove is converted into a canal by the overlying tendons and muscles. The tendon is finally inserted into the bases of the second and third metacarpals.

_Relations._--Outer surface with the pronator teres (Fig. 77, _q_), the superficial fascia, and the third head of the flexor profundus (_u_). Radial border with the pronator teres (_q_). Ulnar border with the third and fourth heads of the flexor profundus. Inner surface with the ulna proximad and the fifth head of the flexor profundus (_u′_) distad.

_Action_ indicated by the name.

=M. palmaris longus= (Fig. 77, _s_).--A flat fusiform muscle beneath the fascia on the medial border of the forearm.

_Origin_ (Fig. 82, _p_) by a short flat tendon from the distal part of the medial surface of the medial epicondyle of the humerus.

About one centimeter from the wrist the muscle ends in a flat tendon which passes through the transverse ligament and divides on the hand into four or five (or sometimes only three) tendons (Fig. 77, _s′_) which diverge to the insertions. The ulnar portion of the flexor sublimis digitorum (Fig. 77, _x_) takes origin from the common tendon before its division.

_Insertion._--Each tendon except the first gives off a branch which spreads out in the trilobed pad in the palm. The middle two of these may be traced to the integument covering the pad. The tendon is then inserted (Fig. 88, _a_) near the base of the first phalanx onto the outer surface of the perforated portion of the flexor sublimis tendon (_b_, _b′_) at its side, except that to the thumb, which divides near its distal end into two which are inserted into the sesamoid bones at the base of the first phalanx. The tendons of insertion are closely united to the fibrous pulley-ring at the base of the first phalanx, and each may send a slip to the base of the phalanx at either side of the ring.

_Relations._--Outer surface with the superficial fascia and one head of the flexor sublimis (Fig. 77, _x_). Radial border with the flexor carpi radialis. Ulnar border with the flexor carpi ulnaris and a part of the flexor profundus digitorum. Inner surface with the flexor carpi radialis (_r_), the flexor carpi ulnaris (_t_), the flexor profundus digitorum (_u_), and the radial head of the flexor sublimis digitorum.

_Action._--Flexor of the first phalanx of each of the digits.

=M. flexor carpi ulnaris= (Fig. 77, _t_, _t′_).

_Origin._--There are two heads. The first or humeral head (_t′_) takes origin in common with the second part of the flexor profundus from the median surface of the distal end of the humerus just distad of the medial epicondyle (Fig. 82, _q_). The second or ulnar head (_t_) takes origin by fleshy fibres from the lateral surface of the olecranon and from the dorsal border of the ulna from the olecranon to a point distad of the semilunar notch (Fig. 87, _f_). The two heads join proximad of the middle of the forearm. The muscle passes obliquely across the ventral surface of the forearm to its

_Insertion_ by fleshy and tendinous fibres into the proximal surface of the pisiform bone.

_Relations._--Outer surface with the integument and the palmaris longus (Fig. 77, _s_). Inner surface and ulnar border with the flexor profundus digitorum.

_Action_ indicated by the name.

=M. flexor sublimis digitorum= (or perforatus).--This muscle is in two parts, which are given a common name only because of the similar structure and insertions of their tendons.

The =ulnar part= (Fig. 77, _x_) is a conical muscle taking origin from the outer surface of the tendon and muscle of the palmaris longus and from the adjacent ligament. It gives rise to two or three tendons (Fig. 88, _b_) which arise from distinct slips of the muscle; these pass to the ulnar two or three digits. That passing to the fourth or fifth digit receives an accessory slip from a small mass of fibres attached to the transverse ligament on the radial side of the palmaris tendon.

The =radial part= is a small flat triangular muscle which takes origin from the outer surface of the tendon formed by the junction of the tendons of the first and second parts of the flexor profundus. It divides into two slips, each giving rise to a tendon. These tendons (Fig. 88, _b′_) pass to the second and third digits. That to the third digit may divide into two, one of which goes to the fourth digit.

Each of the four tendons (Fig. 88, _b_ and _b′_) is perforated by a tendon of the flexor profundus (Fig. 88, _c_) as it passes through the fibrous pulley-ring at the base of the first phalanx. It then continues beneath the flexor profundus tendon through the second pulley-ring (2) and is inserted into the base of the second phalanx.

_Relations._--Outer surface of the ulnar part with the integument; inner surface with the palmaris longus (Fig. 77, _s_). Outer surface of the radial part with the palmaris longus; inner surface with the flexor profundus.

_Action._--Flexor of the second phalanx of digits 2-5.

=M. flexor profundus digitorum= (or perforans).--This arises by five heads, the tendons of which join one another at the wrist to make the strongest and deepest of the flexor tendons.

The _first_ or _ulnar head_ has _origin_ (Fig. 87, _g_) from the dorsal half of the medial (radial) surface of the ulna from the proximal lip of the semilunar notch to within a centimeter of the styloid process. Its fibres converge to a large flat tendon which forms the lateral (ulnar) and superficial part of the common tendon.

_a_, M. pronator quadratus; _b_, tendon of M. biceps; _c_, conjoined
tendon of M. brachialis and M. clavobrachialis; _d_, insertion of
caput longum of M. triceps; _e_, insertion of short portion of caput
mediale of M. triceps; _f_, origin of M. flexor carpi ulnaris; _g_,
origin of first head of M. flexor profundus digitorum; _h_, origin of
fifth head of M. flexor profundus digitorum; _i_, intermediate portion
of caput mediale of M. triceps.]

The _second head_ has _origin_ (Fig. 82, _n_) from the distal end of the medial epicondyle of the humerus between the flexor ulnaris and the third and fourth heads of the profundus. It may be very closely attached to these muscles at their origins. Its tendon joins the radial border of the tendon of the first head. From this junction the radial part of the flexor sublimis arises.

The _third head_ (Fig. 77, _u_) has _origin_ (Fig. 82, _o_) by a strong tendon from the medial epicondyle of the humerus, between the palmaris (_p_) and flexor radialis (_m_). Its tendon forms the radial part of the common tendon.

The _fourth head_ has _origin_ by a strong tendon from the medial epicondyle of the humerus just ventrad of the origin of the second head and the flexor ulnaris, to which many of its fibres are attached. Its tendon forms the middle and superficial part of the common tendon.

The _fifth_ or _radial head_ (Fig. 75, _q_; Fig. 77, _u′_) has _origin_ (Fig. 87, _h_) from the middle third of the ventral surface of the shaft of the radius over an area limited by two oblique bony ridges; from the adjacent parts of the interosseous membrane; and from the ventral part of the medial surface of the shaft of the ulna between a point about two centimeters distad of the semilunar notch and the junction of the middle and distal thirds of the shaft. Its very thick tendon forms the middle and deep part of the common tendon.

The common tendon (Fig. 88, _c′_) covers the carpus and metacarpus ventrally. It divides into five tendons (_c_) which pass to the five digits and are _inserted_ into the bases of the terminal phalanges. Those of the first four digits perforate the tendon of the flexor sublimis (_b_) at the base of the first phalanx of each digit. At the same place each passes through a fibrous pulley-ring attached to the base of the phalanx. Each then passes through a second pulley-ring (2) near the head of the phalanx and is finally inserted into the base of the terminal phalanx.

_Relations._--Outer surface with the pronator teres (Fig. 77, _q_), flexor carpi radialis (Fig. 77, _r_), palmaris longus (_s_), flexor carpi ulnaris (_t_), integument, and extensor carpi ulnaris (Fig. 75, _o_). Inner surface with the radius, ulna, interosseous membrane and pronator quadratus (Fig. 87, _a_).

_Action._--Flexor of all the digits.

The integument, fibrous pads, and the palmaris longus (except the ends
of its tendons) have been removed; the flexor sublimis has been cut.
_a_, cut ends of tendons of M. palmaris longus; _b_, tendons of M.
flexor sublimis digitorum, ulnar division; _b′_, radial part of M.
flexor sublimis digitorum; _c_, tendons of M. flexor profundus
digitorum (_c′_, the common tendon); _d_, M. abductor digiti quinti;
_e_, M. flexor brevis digiti quinti; _f_, Mm. lumbricales (_f′_, their
tendons). 1, transverse ligament of wrist; 2, annular ligaments.]

=M. pronator quadratus= (Fig. 87, _a_).--A thick quadrangular muscle whose fibres run obliquely between the distal ends of the ulna and radius.

_Origin_ by fleshy fibres from about the distal half of the ventral (flexor) surface or border of the ulna and from the interosseous membrane adjacent to the area. The fibres pass obliquely distad toward the radial side to their

_Insertion_ by fleshy fibres into the ventral (flexor) surface of the radius distad of the area of origin of the fifth head of the profundus.

_Relations._--Outer surface with the flexor profundus. Inner surface with the radius, ulna, and interosseous membrane.

_Action_ as indicated by the name.

4. =Muscles of the Hand.=

A. BETWEEN THE TENDONS.--=Mm. lumbricales= (Fig. 88, _f_, _f′_).--Four small muscles in the palm of the hand.

_Origin_ by fleshy fibres from the outer (palmar) surface of the tendon common to the ulnar four parts of the profundus (_c′_).

_Insertion_ (_f′_).--The four slips are flat at their origin. Each becomes cylindrical and curves about the base of one of the four ulnar digits and is inserted into the radial side of the base of the first phalanx close to its ventral border.

_a_, M. flexor brevis pollicis; _b_, M. adductor pollicis; _c_, M.
interosseus of second digit (M. flexor brevis digiti secundi); _d_, M.
adductor digiti secundi; _e_, M. interosseus of third digit; _f_, M.
interosseus of fourth digit; _g_, M. opponens digiti quinti; _h_, M.
interosseus of fifth digit (M. flexor brevis digiti quinti); _i_, M.
abductor digiti quinti. 1-5, the digits in order.]

_Action._--Bend the digits toward the radial side.

B. MUSCLES OF THE THUMB.--=M. abductor brevis pollicis= (Fig. 77, _w_) (includes also the opponens pollicis of man).--A very minute, probably rudimentary muscle.

_Origin_ from the transverse ligament (1) which connects the prominent fibrous, hairless projection that lies over the pisiform bone with the radial border of the head of the radius. The muscle passes radiodistad and ends in a very slender tendon, the insertion of which is into the base of the first phalanx of the thumb.

=M. flexor brevis pollicis= (Fig. 89, _a_).

_Origin_ by fleshy fibres from the adjacent borders of the os magnum and scapholunar bones and from the fascia which bridges the intervening groove for the flexor carpi radialis.

_Insertion_ by a short tendon into the base of the first phalanx of the pollex.

_Action._--Flexor of the thumb.

=M. adductor pollicis= (Fig. 89, _b_).

_Origin_ by fleshy fibres from the ventral border of the os magnum. It curves about the base of the first metacarpal on its ulnar side to its insertion.

_Insertion_ by fleshy fibres into the base of the first phalanx of the pollex on its ulnar side.

C. MUSCLES LYING BETWEEN THE METACARPALS.--=Mm. interossei= (Fig. 89).--Small stout muscles lying on the palmar surfaces of the metacarpals of the second, third, fourth, and fifth digits.

_Origin_ by fleshy fibres from the ventral or lateral surfaces of the bases of the metacarpals. Near the distal end of the metacarpal each divides into two masses which pass onto the lateral surfaces of the metacarpal, so as to leave the ventral surface of the distal end exposed.

_Insertion_ partly onto the lateral surfaces of the base of the first phalanx and its sesamoids, and partly by a slender tendon (Fig. 84, _h_) which is continued dorsad to join the extensor tendon of the digit.