B. Lateral Group

The splenius is thin. It arises directly from the first three or four thoracic spines, and ends by a thin tendon on the occipital bone, the wing of the atlas, and the transverse process of the axis, blending with the mastoido-humeralis, trachelo-mastoideus, and omo-transversarius. The remaining muscles present no very marked differential features.

Muscles of the Thorax

The levatores costarum number ten or eleven pairs.

The diaphragm presents several important differential features. Its slope is much steeper and its width is greater than in the horse. The costal attachment extends almost in a straight line from the upper fourth of the last rib to the junction of the eighth rib with its cartilage, and along the latter to the sternum. The mid-line slopes from the twelfth thoracic vertebra obliquely as far as the vena cava, beyond which it is almost vertical. The right crus divides into two branches, which circumscribe the œsophageal opening, unite below, and then spread out in the tendinous center. The left crus is small. The œsophageal opening is situated about four to five inches (10 to 12 cm.) below the eighth thoracic vertebra, a little to the left of the median plane. The foramen venæ cavæ is a little more ventral and almost in the median plane. The other muscles resemble those of the horse.

Muscles of the Back and Loins

The serratus anticus is very thin. It is inserted on the fifth to the eighth ribs. It may be reduced to two or three digitations or may be absent. The serratus posticus is usually inserted on the last three or four ribs.

The transversalis costarum (Iliocostalis) has a distinct lumbar portion which is attached to the lumbar transverse processes and the external angle of the ilium.

The longissimus resembles that of the horse, but it is more fleshy anteriorly, and the spinalis dorsi is clearly distinguishable from the common mass. In the lumbar region the tendons meet across the summits of the spines.

Intertransversales are present in the back, and interspinales in the back and loins.

Muscles of the Tail

These resemble those of the horse; the coccygeus is, however, much more developed.

Abdominal Muscles

The obliquus abdominis externus is somewhat thinner and has a less extensive origin, which begins at the lower part of the fifth intercostal space and ends on the last rib above its middle. The direction of the fibers in the flank is horizontal, and they do not reach to the external angle of the ilium, nor as high as the lumbar transverse processes. (In this region the abdominal tunic has a strong attachment to the point of the hip and the lumbo-dorsal fascia.) The aponeurosis is intimately united with the abdominal tunic, and does not detach a layer on the inner surface of the thigh.

The obliquus abdominis internus is more developed and has an additional origin from the lumbo-dorsal fascia. The aponeurosis blends with that of the external oblique near the linea alba, and detaches a layer which assists in the formation of the internal sheath of the rectus.

The rectus abdominis arises on the lateral border of the sternum as far forward as the third costal cartilage. The two muscles are separated, except near the pelvis, by an interval varying from two to four inches (ca. 5 to 10 cm.), so that this part of the abdominal wall is entirely fibrous. (The umbilicus is in a transverse plane through the third lumbar vertebra.) There are five tendinous inscriptions, on the third of which is a foramen for the passage of the subcutaneous abdominal vein (milk vein). The prepubic tendon has, in addition to branches inserted into the ilio-pectineal eminences, a strong attachment to the median common tendon of the adductors of the thigh, so that the abdominal wall is strongly retracted and almost vertical at its junction with the pelvis.

The transversus abdominis presents no striking differential features. The transversalis fascia is strong and distinct except over the diaphragm.

Muscles of the Thoracic Limb