A. BREAST-MUSCLES (connecting the arm and thorax) (Fig. 65, page 109).
The muscles connecting the arm with the sternum, corresponding to the
pectoralis muscles of man, fall into a considerable number of not always
clearly separated bundles in the cat. We shall distinguish by name four
such bundles; these might be, and frequently are, further subdivided.
=M. pectoantibrachialis= (Fig. 65, _m_).--This is a small flat bundle
about one or two centimeters wide, forming the most superficial portion
of the pectoralis group.
_Origin_ on the lateral surface of the manubrium. The muscle passes
laterad onto the arm and is _inserted_ by a flat tendon into the
superficial fascia of the dorsal border of the forearm, near the
elbow-joint. The tendon is continuous craniad with the border of the
clavobrachial (_k_), so that a bundle of fibres may pass from this
muscle to be inserted with the clavobrachial, or a bundle from the
clavobrachial to be inserted with this. The tendon is continuous caudad
with that of the epitrochlearis (_r_). The fascia of insertion may be
traced to an attachment into the dorsal border of the ulna. This muscle
usually receives near its insertion a slip, sometimes called its caudal
division, from the deep layer of the pectoralis major.
_Relations._--Outer surface with the integument. Inner surface with both
divisions of the pectoralis major (_l_) on its medial two-thirds, and
with the biceps (_t_) and bicipital arch (_t′_) in its lateral
one-third. The cranial border is closely applied to that of the
clavobrachial (_k_), except at the two ends.
_Action._--Draws the arm mediad.
=M. pectoralis major=, or =ectopectoralis= (Fig. 65, _l_, _l′_).--In
this it is usually possible to distinguish a superficial and a deep
layer.
The =superficial= (_l′_) layer is a flat band of fibres one or two
centimeters wide.
_Origin_ from a raphe in the midventral line, along the cranial half of
the manubrium and for five to ten millimeters craniad of it.
_Insertion_ (Fig. 81, _f_).--The pectoral ridge of the middle third of
the shaft of the humerus, slightly dorsad of the line of insertion of
the deep portion.
_Relations._--Outer surface with the clavobrachial (Fig. 65, _k_) and
integument, the medial end caudally with the pectoantibrachialis (_m_),
and the distal end with the brachialis (_v_). Inner surface with the
deep portion (_l_), the fibres of which it crosses obliquely.
The =deep portion= (_l_) is a flat band of parallel fibres about three
times as broad as the superficial portion.
_Origin._--The ventral surface of the manubrium and of the first three
divisions of the sternum, and the median raphe for about one centimeter
craniad of the manubrium. It passes directly laterad.
_Insertion_ (Fig. 81, _e_) along a line which begins at the
infraspinatus fossa of the great tuberosity, and runs parallel to the
deltoid ridge until it reaches the pectoral ridge at the junction of the
second and third fourths of the humerus and then continues in the
direction of the pectoral ridge as far as the junction of the third and
fourth fourths of the humerus. Some of its fibres may pass caudad of the
biceps and be inserted with the epitrochlearis. Into the part of the
line which is parallel to the deltoid ridge the muscle is inserted by a
flat tendon; into the remainder of the line it is usually inserted
directly by muscle-fibres, though at its caudal end its insertion may
again be tendinous.
_Relations._--Outer surface with the cutaneus maximus, the outer layer
of the pectoralis major (Fig. 65, _l′_), the pectoantibrachialis (_m_),
and the clavobrachial (_k_). Inner surface with the pectoralis minor
(_o_), the proximal end of the humerus, the coracobrachialis (Fig. 77,
_f_), and the supraspinatus (Fig. 77, _d_) at the insertion of the
latter. At the cranial part of its origin it touches the sternomastoid
(Fig. 65, _g_) and sternohyoid (Fig. 65, _e_).
_Action._--Draws the arm mediad and turns the foot forward.
=M. pectoralis minor=, or =entopectoralis= (Fig. 65, _o_).--This is a
fan-shaped mass of fibres, flat but thicker than the pectoralis major.
_Origin_ from the lateral half of the first six divisions of the body of
the sternum and sometimes from the xiphoid process. The fibres are
divisible into several bundles which vary in extent and distinctness and
are sometimes described as separate muscles. They pass craniolaterad and
converge so that the line of insertion is about one-half as long as that
of the origin.
_Insertion_ (Fig. 81, _d_) into the humerus along a line which forms the
ventral border of the bicipital groove at the proximal end of the bone,
passes thence onto the pectoral ridge and continues in the direction of
the pectoral ridge nearly as far as the middle of the length of the
humerus. The cranial and caudal fibres are inserted by means of a thin
tendon; the middle fibres directly. The cranial border of the tendon of
insertion is continued as a thin tendon to the coracoid process; it is
closely united with the insertion of the supraspinatus.
_Relations._--By its outer surface with the pectoralis major (Fig. 65,
_l_) over its cranial half; with the cutaneus maximus, latissimus dorsi
(Fig. 65, _q_), and integument over its caudal half. By its inner
surface with the xiphihumeralis (Fig. 65, _p_), rectus abdominis (Fig.
73, _k_), and a mass of fat in the axilla. The tendon of the
xiphihumeralis is also connected with its inner surface near its
insertion. Its cranial border is free. Its caudal border is in relation
at its medial end with the xiphihumeralis, at its lateral end with the
tendon of the latissimus dorsi (Fig. 65, _q_).
_Action._--Draws the arm toward the middle line.
=M. xiphihumeralis= (Fig. 65, _q_).--A long, thin, narrow muscle which
may be considered as part of the pectoralis minor.
_Origin_ a median raphe along the xiphoid process or at an angle to the
median line on the rectus abdominis muscle. It passes craniad, becoming
gradually smaller, and about two centimeters from its insertion ends in
a thin tendon.
_Insertion._--It ends in a flat tendon which in passing is connected by
a strong fascia with the tendon of the latissimus (Fig. 65, _q_). It
passes along the inner surface of the pectoralis minor (_o_) to be
inserted with its cranial fibres near the ventral border of the
bicipital groove.
_Relations._--Outer surface with the cutaneus maximus at its medial end,
with the pectoralis minor (_o_) at its distal end. Inner surface with
the external oblique (Fig. 68, _p_), the rectus abdominis (Fig. 73,
_k_), and a mass of fat in the axilla. Borders free. Some of the fibres
of the cutaneus maximus are attached to its outer surface.
_Action._--Assists the pectoralis minor.