Muscles of the neck.—We have already, in treating of the trapezius, studied the muscular structure and form of the posterior region of the neck, and the details have been described of the upper part of the side of the neck, and the oblique longitudinal groove between the anterior border of the trapezius and the posterior border of the sterno-cleido-mastoid (pages 208 and 210). It now remains to examine the inferior part of this groove and all the anterior region of the neck. This study should begin with that of the sterno-cleido-mastoid muscles, which have the most important influence on the form of this region, and which by their direction map out on the anterior surface of the neck a muscular interval or space in which it will be easy to study the deeply placed muscles.

The sterno-cleido-mastoid muscles are two in number, one on each side of the neck, extending from the upper limit of the thorax, obliquely upwards and backwards, to the base of the skull (19, Fig. 88). The muscle arises below by two heads—one internal or sternal, which arises by a strong tendon from the front of the first piece of the sternum (18, Fig. 70, page 197), the other external or clavicular, which arises in the form of a thin fleshy band from the inner third of the upper surface of the clavicle, above the origin of the clavicular portion of the great pectoral muscle (Fig. 68, page 194). These two heads are directed upwards and backwards, separated at first by a narrow triangular space, of which the base corresponds to the sterno-clavicular joint (Fig. 68); they then unite to form a single muscle, broad and thick, which ascends obliquely towards the skull, passing behind the angle of the lower jaw and the ear to be inserted into the surface of the mastoid process of the temporal bone and the superior curved line of the occipital bone (16, Fig. 73).

This muscle, being inserted into the skull at a point which is situated behind the axis of the movements of flexion and extension of the head, acts by extending the head on the neck, but to this movement, usually little marked, it adds the power of flexing the neck on the trunk. When both these muscles contract at the same time, they produce extension of the head on the neck and flexion of the neck on the thorax; therefore we see both clearly marked beneath the skin in a person lying down who raises his head (by flexion of the neck). When one muscle only contracts, it turns the face to the opposite side; therefore in the living model whose face is turned to the right the face turns to this side by the contraction of the left sterno-cleido-mastoid muscle, of which the prominence is well shown beneath the skin as a broad band, passing from the sternum to the mastoid process of the temporal bone. There are various attitudes in which this prominence is particularly remarkable, as when we carry the head quickly to one side to answer a question or give an order; or in the act of listening intently, when we concentrate our attention to one side and extend the head a little, turning the region of the ear upwards and forwards, an attitude in which the sterno-mastoid becomes particularly prominent beneath the skin of the neck.

From their insertions and direction we see that these two muscles are very close to each other below and wide apart above. Stretching diagonally across the side of the neck, each muscle forms the interval between two triangular spaces—the posterior triangle behind, bounded by the sterno-mastoid, trapezius, and the clavicle (below); and the anterior triangle in front, bounded by the sterno-mastoid, the middle line of the neck in front, and the lower jaw above. The anterior triangles of the two sides comprise together the space between the two sterno-cleido-mastoid muscles. This space is in the form of a triangle, with its apex below, corresponding to the episternal notch, and its base above, formed by the whole breadth of the lower jaw. This triangle is the anterior region of the neck, divided into two parts by the presence of the hyoid bone—a small bone without direct connection with the rest of the skeleton, and placed transversely above the prominence of the larynx (23, Fig. 88). The lower part forms a vertical plane, oblique downwards and backwards, and sinking behind the sternum; this is the infra-hyoid region, containing the infra-hyoid muscles. The upper part of the area forms a plane approaching more or less the horizontal, proceeding from the hyoid bone to the chin and the circumference of the lower jaw; this is the supra-hyoid region, or that of the chin, containing the supra-hyoid muscles.

Before entering further into the description of these regions we must point out the presence of several organs which, placed in front of the vertebral column, project forwards in the space between the sterno-mastoid muscles. In the middle line of the neck, the region above the hyoid bone corresponds to the floor of the mouth, in which the tongue lies, having on each side large salivary glands partly concealed by the lower jaw; below the depression corresponding to the hyoid bone appear the cartilages of the larynx—the thyroid (pomum Adami), projecting forwards, and more marked in the male than in the female, and the cricoid cartilage, to which the windpipe or trachea is attached. Separating the tongue and larynx from the vertebral column is the cavity of the pharynx, which is continuous below the cricoid cartilage, with the gullet or œsophagus, situated behind the windpipe or trachea. The œsophagus is soft and fleshy, with a cavity effaced when empty. The trachea, which on account of its functions is always open, is formed of incomplete cartilaginous rings, which give it an almost cylindrical form and render it prominent below in the middle line of the neck. Crossing the upper part of the trachea is the median part of a gland known as the thyroid body, which in some cases (especially in the female) may form a slight rounded projection in the infra-hyoid region of the neck.

The infra-hyoid muscles ascend from the back of the sternum, inside the thorax, towards the lower border of the hyoid bone. They are four in number—two superficial (the omo-hyoid and sterno-hyoid) and two deep (the sterno-thyroid and thyro-hyoid).

The omo-hyoid (24, Fig. 88; 17, 18, Fig. 89) is a small muscle, long and slender, with a very remarkable course. It consists of two fleshy bellies with an intermediate tendon. It arises from the upper border of the scapula near the root of the coracoid process, and is directed at first obliquely forwards and inwards behind the clavicle; after crossing the posterior triangle it turns on itself, and is directed upwards beneath the sterno-mastoid and through the anterior triangle to be inserted into the lateral part of the lower border of the hyoid bone. This muscle, covered at first by the trapezius and then by the sterno-mastoid, is thus visible on the model in only two parts of its course—in front of the sterno-mastoid, where its anterior belly appears over the trachea, and again in the posterior triangle, where its posterior belly occupies the lower portion of the groove which separates the trapezius from the sterno-mastoid. Although deeply placed, this muscle becomes visible beneath the skin, for it is brought into sharp relief during certain actions. Evidently, from its slender form, we cannot expect it to be an important elevator of the scapula; but it serves to depress the hyoid bone. The most important function of the muscle is by its contraction, especially during spasmodic efforts in respiration, as in the strong inspiration in sighing or sobbing; in such cases it prevents the skin and loose aponeurotic tissue from being too strongly depressed in the supra-clavicular fossa by atmospheric pressure. Therefore, if the neck is thin and the fossa well marked, as in an aged female, during the movements of sobbing or abrupt respiration a cord corresponding to the posterior belly of the omo-hyoid muscle may be clearly seen in the supra-clavicular fossa.

The sterno-hyoid (25, Fig, 88; 14, Fig. 89) forms a long, thin, fleshy band, extending from the back of the sternum and clavicle upwards to the lower border of the hyoid bone. Both these muscles are contiguous to each other by their internal borders above, but below, at the deep median fossa corresponding to the episternal notch, they are separated by a narrow angular space, in which appear the trachea and the inner border of the sterno-thyroid muscle.

The two deep muscles of this region (the sterno-thyroid and thyro-hyoid) are continuous with one another. They lie beneath the sterno-hyoid and omo-hyoid muscles, and cover the trachea and larynx in the front of the neck. The sterno-thyroid muscle arises from the posterior part of the first piece of the sternum, and ascends vertically beneath the sterno-hyoid. It appears at the inner border of the sterno-hyoid at the lower part of the neck on each side of the middle line (Fig. 68); reaching the thyroid cartilage, it is inserted into an oblique line on its external surface. The thyro-hyoid muscle arising from this oblique line on the thyroid cartilage ascends to be inserted into the hyoid bone.

The supra-hyoid muscles proceed from the hyoid bone to the base of the skull and lower jaw, and by their contraction elevate this bone, as can be easily observed in those who bring into action the pharynx or larynx, as in the act of singing or swallowing. The first of these supra-hyoid muscles is the digastric muscle, composed of two fleshy bellies connected together by an intermediate tendon (1, 2, Fig. 89). The posterior belly connects the hyoid bone with the under-surface of the mastoid process; the anterior belly connects the hyoid bone to the lower border of the lower jaw near the chin. Next come two other muscles one behind, lying alongside the posterior belly of the digastric, and called the stylo-hyoid (4, Fig. 89), and another in front, called the mylo-hyoid (5, Fig. 89); this muscle lies underneath the anterior belly, and extends between the hyoid bone and the deep surface of the body of the lower jaw. It thus forms a partition closing in the floor of the mouth on each side of the tongue.

The digastric muscle arises from the under-surface of the mastoid process of the temporal bone. From this origin its fusiform posterior belly descends obliquely downwards and forwards, and near the hyoid bone is replaced by a round tendon. This tendon is attached by a fibrous pulley to the hyoid bone, and, taking its fixed point here, it turns abruptly, so as to be directed obliquely upwards and forwards towards the chin; at the same time this tendon is soon replaced by the anterior belly, also fusiform, which is inserted on the posterior surface of the symphysis of the chin, into a small depression called the digastric fossa. We see that this muscle, with its two bellies, is admirably arranged to raise the hyoid bone, and consequently the whole of the larynx; for the anterior belly carries the hyoid bone upwards and forwards, the other upwards and backwards, and if the two contract at the same time they will raise the hyoid bone directly upwards.

The stylo-hyoid is a small muscular fasciculus which accompanies the posterior belly of the digastric (21, Fig. 88). Arising from the styloid process of the temporal bone, this muscle passes downwards and forwards beneath the posterior belly of the digastric, with which it is closely connected; at the level of the hyoid bone the tendon of the digastric pierces the stylo-hyoid, and the latter terminates by an aponeurotic slip which is inserted into the lateral part of the hyoid bone. On the model the stylo-hyoid and digastric are united in one cylindrical bundle (Fig. 88).

The mylo-hyoid muscle (27, Fig. 88) forms the floor of the cavity of the mouth. It constitutes a quadrilateral fleshy plane, attached by its upper border to the inner surface of the body of the lower jaw in a prominent oblique line; by its inferior border it is inserted into the hyoid bone. Above the hyoid bone the mylo-hyoid muscles are also inserted in the middle line into a fibrous raphe, so that the two muscles, that of the right and left side, form practically a single layer constituting the floor of the mouth. This muscle is covered on its deep surface by the mucous membrane of the mouth and by the fleshy fibres of muscles which are not visible externally, and which, arising from the small tubercles developed on the posterior surface of the symphysis of the chin (genial tubercles), are inserted either into the hyoid bone (genio-hyoid) (6, Fig. 89) or into the tongue (genio-hyo-glossus). The mylo-hyoid muscles, like the previous muscles, assist in raising the hyoid bone and larynx; it is to be noted, too, that the supra-hyoid muscles also act in depressing the lower jaw and opening the mouth.