separated by a distinct cellular interspace, corresponding to the sterno-clavicular articulation: 1. from the anterior surface of the first bone of the sternum close to its clavicular joint, by a very strong flat tendon which is directed upward and backward for the space of more than an inch, before terminating in fleshy fibres; 2. from the upper edge of the inner third of the clavicle by a thin origin composed of parallel aponeurotic fibres, which directly become fleshy, and take a nearly vertical course. As these two bundles ascend, the sternal, more oblique in its course, seems to overlap the other, and, both by difference of direction and by a line of cellular separation, can often be distinguished from it in the lower two-thirds of the neck; but in approaching the mastoid process they are indistinguishably fused together. The insertion is, 1. by a strong and rounded tendon into the mastoid process, of which it seems to embrace the tip and anterior border; 2. by a thin aponeurosis along the posterior edge of the process, and about a third of the superior semicircular line, which is continued into it.
This muscle, to which I shall have abundant occasion to refer in speaking of the surgical anatomy of the neck, has very important relations: the space between its heads corresponds to the bifurcation of the arteria innominata; and the broad band-like muscle, as it ascends, crosses in succession the subclavian and carotid arteries, the jugular and subclavian veins, the hypo-glossal, pneumogastric, phrenic, sympathetic, spinal accessory nerves, and a portion of the cervical plexus; the sterno-hyoid, sterno-thyroid, omo-hyoid, scaleni, levator anguli scapulæ, splenius, and digastric muscles, besides many lymphatic glands and branches from several of the nervous and vascular trunks which have been enumerated. Its superficial aspect corresponds to the integuments and platysma, to the external jugular vein and superficial branches of the cervical plexus; its thick anterior edge bounds the anterior triangle of the neck, receives branches from the external carotid artery or from its thyroid branch, and corresponds above to the parotid gland and posterior aural artery; its thin posterior edge limits the other great triangle of the neck, is pierced by the spinal accessory nerve, corresponds to a chain of lymphatic glands, and is wound round by the nerves and vein which lie on the surface of the muscle.
The two sterno-mastoid muscles acting together directly bend the head on the chest, and their joint action is well illustrated in an endeavour to raise the head from the supine position. But when the head is thrown far back, a predominance is given to the posterior fibres of the muscle, which being attached behind the line of the occipito-vertebral articulation, become then capable of increasing this direction of the head. The sterno-mastoid of one side, acting singly, rotates the head and flexes it with a lateral inclination to its own side, so as to bring the side of the head nearer to the shoulder, and to turn the face in the opposite direction.
The platysma myoides (latissimus colli of Albinus) is a broad, thin, membraniforrn muscle, which covers the side of the neck and lower part of the face, and is in its whole course subcutaneous. It arises by scattered fibres in the superficial fascia below the clavicle, and covers by its origin the upper part of the pectoralis major and deltoid, as also the space between those muscles, which corresponds to the coracoid process. This origin does not extend within an inch or two of the median line, but reaches as far outwardly as the acromial process. The fibres become more closely aggregated as they ascend, and the muscle accordingly narrows. Its direction is obliquely upward and to the median line; it passes over the base of the lower jaw, and its fibres again spread to their insertion: those which are posterior lose themselves in the skin covering the parotid gland and masseter muscle; others from this neighbourhood bend forward toward the angle of the mouth, and in some subjects constitute a very distinct horizontal retractor anguli oris, which is generally known as the risorius Santorini: some fibres from the middle of the muscle obtain a more fixed insertion about the base of the jaw and into the skin covering it; while the anterior portion of the muscle, which is most constant in its relations, is inserted into the lower lip by blending its fibres with those of the depressor labii inferioris, and by decussating toward the border of the lip and in the substance of the chin with the mesial fibres of its fellow.
This muscle is subcutaneous in its whole extent, and by its extremities intimately attached to the deep surface of the skin which covers it. In approximating its extreme attachments, it wrinkles the skin in a direction transverse to that of its fleshy fibres. It is a single and partial relique in the human subject of that general muscular investment, which fulfils various functions in different orders of Mammalia, as an appendage of the tegumentary system: rolling the hedge-hog in a ball, erecting the quills of the porcupine, and the bristles of the boar, or dislodging insects from the hide of grazing cattle. Its relations to the deeper parts in the neck will be detailed hereafter: between it and the cervical aponeurosis lie chiefly, cutaneous nerves and veins; branches from the cervical division of the portio dura are distributed to its upper portion, reaching the deep surface just below the angle of the jaw, and branches from the cervical plexus crossing the sterno-mastoid partly supply the platysma, partly pierce it in their course to the skin; the superficial pectoral branches lie beneath it till they reach the clavicle; the external jugular vein lies immediately beneath this muscle, and runs nearly parallel to its fibres, crossing transversely those of the sterno-mastoideus.
II.—Fasciæ of the neck.
1. The superficial fascia, or subcutaneous areolar tissue, presents characters in common with thne same structure in other parts of the body, and is universally continuous with that
