=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.)
[Illustration: FIG. 75.--MUSCLES ON THE LATERAL SURFACE OF THE ARM.
_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.
[Illustration: FIG. 76.--LATERAL SURFACE OF THE SCAPULA, WITH THE
AREAS OF ATTACHMENT OF MUSCLES.
_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.