Direction. A block having been placed at the back of the neck, and the face turned to the side opposite to that to be dissected, so as to place the parts upon the stretch, two transverse incisions are to be made: one from the chin, along the margin of the lower jaw, to the mastoid process ; and the other along the upper border of the clavicle. These are to be connected by an oblique incision made in the course of the Sterno-mastoid muscle, from the mastoid process to the sternum ; the two flaps of integument having been removed in the direction shewn in fig. i jo, the superficial fascia will be exposed.

The Superficial Cervical Fascia is exposed on the removal of the integument from the side of the neck; it is an extremely thin aponeurotic lamina, which is hardly demonstrable as a separate membrane. Beneath it is found the Platysma myoides muscle, the external jugular vein, and some superficial branches of the cervical plexus of nerves.

The Platysma Myoides is a broad thin flat plane of muscular fibres, of an irregular quadrilateral form, placed immediately beneath the skin on each side of the neck. It arises from the clavicle and acromion, and from the fascia covering the upper part of the Pectoral, Deltoid, and Trapezius muscles; its fibres proceed obliquely upwards and inwards along the side of the neck, to be inserted into the lower jaw beneath the external oblique line, some fibres passing forwards to the angle of the mouth, and others becoming lost in the cellular tissue of the face. The most anterior fibres interlace, in front of the jaw, with the fibres of the muscle of the opposite side; those next in order become blended with the Depressor labii inferioris and the Depressor anguli oris; others are prolonged upon the side of the cheek, and interlace, near the angle of the mouth, with the muscles in this situation, and may occasionally be traced to the Zygomatic muscles, or to the margin of the Orbicularis palpebrarum. The most posterior fibres, which are lost in the skin at the side of the face, are the rudiments of a remarkable accessory fasciculus, the Risorius Santorini, already described. Beneath this muscle, the external jugular vein may be seen descending from the angle of the jaw to the clavicle. It is essential to remember the direction of the fibres of the Pla-tysma, in connection with the operation of bleeding from this vessel; for if the point of the lancet is introduced in the direction of the fibres of this muscle, the orifice made will be filled up by its contraction, and blood will not flow; but if the incision is made in a direction opposite to that of the course of the fibres, they will retract, and expose the orifice in the vein, and so facilitate the flow of blood. This operation is now, however, very rarely performed.

Relations. By its external surface, with the integument, to which it is united closely below, but more loosely above. By its internal surface, below the clavicle which it covers, with the Pectoralis major, Deltoid, and Trapezius. In the neck, with the external and anterior jugular veins, the deep cervical fascia, the superficial cervical plexus, the Sterno-mastoid, Sterno-hyoid, Omo-hyoid, and Digastric muscles. In front of the Sterno-mastoid, it covers the sheath of the carotid vessels; and behind it, the Scaleni muscles and the nerves of the brachial plexus. On the face, it is in relation with the parotid gland, the facial artery and vein, and the Masseter and Buccinator muscles.

The Deep Cervical Fascia is exposed on the removal of the Platysma myoides. It is a strong fibrous layer, which invests the muscles of the neck, and encloses the vessels and nerves. It commences, as an extremely thin layer, at the back part of the neck, where it is attached to the spinous processes of the cervical vertebrae, and to the ligamentum nuchae; and, passing forwards to the posterior border of the Sterno-mastoid muscle, divides into two layers, one of which passes in front, and the other behind it. These join again at its anterior border; and, being continued forwards to the front of the neck, blend with the fascia of the opposite side. The superficial layer of the deep cervical fascia (that which passes in front of the Sterno-mastoid), if traced upwards, is found to pass across the parotid gland and Masseter muscle, forming the parotid and masseteric fasciaj, and is attached to the lower border of the Zygoma, and more anteriorly to the lower border of the body of the jaw; if the same layer is traced downwards, it is seen to pass to the upper border of the clavicle and sternum, being pierced just above the former bone for the external jugular vein. In the middle line of the neck, this layer is thin above, and connected to the hyoid bone; but it becomes thicker below, and divides, just below the thyroid gland, into two layers, the more superficial of which is attached to the upper border of the sternum and inter-clavicular ligament; the deeper and stronger layer is connected to the posterior border of that bone, covering in the Sterno-hyoid and Sterno-thyroid muscles. Between these two layers is a little areolar tissue and fat, and occasionally a small lymphatic gland. The deep layer of the cervical fascia (that which lies behind the posterior surface of the Sterno-mastoid) sends numerous prolongations, which invest the muscles and vessels of the neck; if traced upwards, a process of this fascia, of extreme density, passes behind and to the inner side of the parotid gland, and is attached to the base of the styloid process and angle of the lower jaw, forming the stylo-maxillary ligament; if traced downwards and outwards, it will be found to enclose the posterior belly of the Omo-hyoid muscle, binding it down by a distinct process, which descends to be inserted into the clavicle and cartilage of the first rib. The deep layer of the cervical fascia also assists in forming the sheath which encloses the common carotid artery, internal jugular vein, and pneumogastric nerve. There are fibrous septa intervening between each of these parts, which, however, are included together in one common investment. More internally, a thin layer is continued across the trachea and thyroid gland, beneath the Sterno-thyroid muscles; and at the root of the neck this may be traced, over the large vessels, to he continuous with the fibrous layer of the pericardium.

The Sterna- Cleido-Mastoid is a large thick muscle which passes obliquely across the side of the neck, being enclosed between the two layers of the deep cervical fascia. It is thick and narrow at its central part, but is broader and thinner at each extremity. It arises, by two distinct heads, from the sternum and clavicle. The sternal portion arises by a rounded fasciculus, tendinous in front, fleshy behind, from the upper and anterior part of the first piece of the sternum, and is directed upwards and backwards. The clavicular portion arises from the inner third of the upper surface of the clavicle, being composed of fleshy and aponeurotic fibres; it is directed perpendicularly upwards. These two portions are separated from one another, at their origin, by a triangular cellular interval; but become gradually blended, below the middle of the neck, into a thick rounded muscle, which is inserted, by a strong aponeurosis, into the outer surface of the mastoid process, from the apex to its superior border, and into the

136.-Muscles of the Neck, and Boundaries of the Triangles.

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