Part 20

=The Tarsus.=--The articulations between the separate bones of the tarsus and between the tarsus and metatarsus have a considerable number of variously communicating articular capsules. The separate bones are connected by many ligaments. The ligaments of the ventral surface (=ligamenta plantaria=) are especially well developed. Here may be noticed particularly a very large =calcaneocuboid= ligament, and a large =calcaneocuneiform= ligament which passes from the sustentaculum tali to the medial cuneiform. Those on the dorsal surface (=ligamenta dorsalia=) are less strong and numerous. Many ligaments, longitudinal, transverse, and oblique, pass also between the separate bones (=ligamenta interossea=). =Ligamenta lateralia=, on the lateral and medial borders of the foot, are also distinguishable. Strauss-Durckheim enumerates ninety-four ligaments of the tarsus; an account of each of these does not form part of the plan of the present work.

The ligaments of the metatarsus and phalanges are of the same general character as in the forelimb. Of these Strauss-Durckheim enumerates thirty-six; they will not be described here.

THE MUSCLES.

=I. THE MUSCLES OF THE SKIN.= (Fig. 62.)

=M. cutaneus maximus= (Fig. 62, b).--This is a very large, thin muscle which covers almost the whole side of the body. It arises from the outer surface of the latissimus dorsi (Fig. 68, m) near its ventral end and from the bicipital arch (Fig. 65, t′) in the axilla; from the linea alba for a considerable distance (two or three inches) caudad of the base of the xiphoid process, and from the thorax over a line joining the axilla and the base of the xiphoid. Sometimes a few fibres take origin from the fascia which covers the pectoantibrachialis on the ventral side of the arm.

From their origin the fibres diverge. The cranial ones curve about the base of the forelimb and are inserted into the skin at or near the middle line caudad of the first thoracic vertebra. The most cranial fibres of all are inserted about one to one and a half centimeters from the middorsal line; thence the line of insertion approaches the middorsal line to reach it at about the eighth or ninth thoracic vertebra. The middle fibres run parallel to the middorsal line in the lumbar and sacral regions as far as the root of the tail, a small bundle passing onto the dorsal side of the tail, another onto the ventral side. The fibres of the caudal portion pass onto the thigh, the ventral ones running in the fold of skin which stretches from thigh to abdomen, and are finally lost in the fascia along a line connecting the knee and the root of the tail. A strong fascia connects the adjacent borders of the cutaneus and platysma and lies over the scapular region.

a, a′, a″, M. platysma; b, M. cutaneus maximus; c, M. orbicularis oris. Between the platysma and the cutaneus maximus are seen the following deeper muscles: d, M. acromio-trapezius; e, M. levator scapulæ ventralis; f, M. spino-deltoideus; g, M. acromio-deltoideus; h, M. clavobrachialis; i, caput laterale of M. triceps brachii; j, caput longum of M. triceps brachii; k, M. xiphi-humeralis; l, M. pectoralis minor. 1, external jugular vein.]

This muscle lies immediately beneath the integument. It covers the first layer of body muscles.

Action.--Moves the skin.

=M. platysma= (Figs. 62 and 64, a, a′, a″).--This muscle forms a thin layer of fibres covering the sides of the neck and face, in close relation with the integument. Several more or less distinct portions may be distinguished.

(1) Most of the fibres (a) arise from the middorsal line, from the occiput to the first thoracic vertebra, in a narrow fascia common to the muscles of the two sides. The most cranial fibres of this region arise as a small bundle from the external occipital crest, beneath the levator auris longus (Fig. 63, g, g′).

From this origin in the middle line the fibres pass craniolaterad. The most cranial fibres curve about the ventral side of the ear and pass toward the caudal angle of the eye, where they unite with fibres of the zygomaticus (Fig. 64, d) or corrugator supercilii lateralis (Fig. 64, k), or pass to the lower eyelid. Caudad of these the fibres cover the side of the face and become lost among the facial muscles, some passing to the lower eyelid, some to the fibrous pad which supports the vibrissæ, some to the angle of the mouth, some to the lower lip. The most ventral fibres meet the fibres of the opposite muscle just ventrad of the symphysis of the mandible.

The ventral free border of the platysma is separated on the ventral side of the neck from the border of the opposite muscle by a wedge-shaped area having its point at the symphysis menti.

The fibres of this portion of the muscle are interrupted by an attachment to the skin, along a line passing from the base of the ear to about the middle of the coracoid border of the scapula, The dorsal (a) and ventral (a′) portions of the muscle, separated by this line of attachment, are sometimes described as separate muscles (the =supercervicocutaneus= and =cervicofacial=, respectively, of Strauss-Durckheim).

(2) A band of fibres one or two centimeters across (a″) arises in the fascia of the side of the neck just craniad of the middle of the coracoid border of the scapula, and passes caudoventrad toward the manubrium, its fibres crossing the fibres of the first part of the platysma at right angles. These fibres become lost in the fascia ventrad of the manubrium, or pass across the middle line to intermingle with the corresponding fibres of the opposite side. This portion of the platysma is sometimes absent.

The platysma is everywhere subcutaneous, except at its dorsocranial angle, where a small bundle of fibres is covered by the levator longus auris. It covers the deeper muscles of the neck and head. Closely attached to its inner surface are the submentalis and depressor conchæ, whose fibres bridge over the ventral interval between the borders of the platysmas of opposite sides.

Action.--Moves the skin of the face and neck.

=II. THE MUSCLES OF THE HEAD.=

A. SUPERFICIAL MUSCLES.--The most superficial layer of muscles on the face and head is formed by differentiation of the fibres of the platysma. The muscles thus formed are not clearly distinct from each other; in this region sets of fibres differing in direction and in origin or insertion receive separate names even though the different sets of fibres are closely interwoven.

In the quadrangle on the dorsal surface of the head enclosed between the two eyes and the two ears, a thin superficial sheet of fibres is found, in which a number of different sets may be distinguished (Fig. 63). These have received the following names.

=M. intermedius scutulorum= (Fig. 63, a).--This consists of a broad thin sheet of transverse fibres between the two external ears. The fibres are attached at either end to the scutiform cartilage (1) of the two ears, and pass without interruption across the middle line. At its cranial edge this muscle is continuous with the corrugator supercilii medialis (b); at its lateral edge with the frontoscutularis; at its caudal edge with the levator auris longus (g).

Relations.--Outer surface with the integument. Inner surface with the galea aponeurotica (to which the muscle is closely united), the epicranius muscle (h) and the temporal muscle (n).

Action.--Draws the two ears dorsad, toward the middle line.

On the right side are shown the superficial muscles; on the left side the superficial muscles have mostly been removed, exposing the deeper muscles and the bone. a, M. intermedius scutulorum; b, M. corrugator supercilii medialis; c, M. orbicularis oculi; d, M. corrugator supercilii lateralis; e, cranial end fibres of M. platysma; f, M. adductor auris superior; g, g′, M. levator auris longus (g, cranial portion; g′, caudal portion); h, h′, M. epicranius (h, M. occipitalis; h′ M. frontalis); i, M. transversus auriculæ; k, M. auricularis superior (cut on the left side); l, M. abductor auris brevis; m, M. abductor auris longus; n, M. temporalis; o, cut origin of M. frontoscutularis; p, p′, M. levator labii superioris alæque nasi (p′, the origin from the maxillary bone); q, angular head of M. quadratus labii superioris. 1, scutiform cartilage; 2, external ear; 3, bones of the skull.]

=M. corrugator supercilii medialis= (b).--This consists of a thin sheet of scattered fibres lying craniad of the last and intermingling with it. The fibres take origin near the middle line, pass laterad, then curve craniad, converging, to be inserted into the whole extent of the upper eyelid, especially near the caudal angle. Here the fibres unite with those of the orbicularis oculi (c).

This muscle is continuous caudad with the intermedius scutulorum (a), craniad with the orbicularis oculi (c); laterad with the corrugator supercilii lateralis (d). Toward the medial side the fibres lose themselves in a tendinous sheet that joins the galea aponeurotica.

Relations.--Outer surface with the integument. Inner surface with the frontoscutularis and the skull.

Action.--Raises the upper eyelid.