If the splenius of one side contracts at the same time as that of the opposite, the extension takes place in a direct manner—that is to say, without any modifying lateral movement.
Infrahyoid Muscles
Having studied the lateral surfaces of the neck, we must now examine the anterior part of this region. Here, between the two sterno-mastoid muscles, we find a space broader above than below, in which are situated the larynx and the trachea, to the general arrangement of which is due the cylindrical form which this region presents. This space corresponds to that which in the neck of man is limited laterally by the sterno-cleido-mastoid muscles, below by the fourchette of the sternum, and above by the hyoid bone. In animals, as in man, it is called the infrahyoid region.
The hyoid bone in quadrupeds is situated between the two rami or branches of the lower jaw. Owing to this disposition, the region above this bone, instead of having its surface projecting a little beyond the inferior border of the maxillary bone, is depressed. This is especially so in the horse. It is there that we find in this animal the region known as the trough (auge); the larynx corresponds to that part known as the gullet.
The muscles which occupy the infrahyoid region are: the sterno-thyroid, the sterno-hyoid, and the omo-hyoid. There is also a thyro-hyoid, but because of its deep situation and its slight importance it offers no interest from our point of view.
Sterno-thyroid and the Sterno-hyoid Muscles.—These two muscles, long, narrow, and flat, arise from the anterior extremity of the sternum; then, covering the anterior surface of the trachea, they proceed to terminate, the one on the thyroid cartilage, and the other on the hyoid bone. The sterno-hyoid is superficial; it covers the sterno-thyroid, which, however, projects a little on its outer side.
Omo-hyoid.—This muscle does not exist in the dog or cat. It arises, in the horse, from the cervical border of the scapula, where it blends with the aponeurosis that envelops the subscapularis muscle, but in the pig and the ox it arises from the deep surface of the mastoido-humeral muscle. It is directed obliquely upwards and inwards, becoming superficial at the internal border of the sterno-mastoid, and is inserted into the hyoid bone.
The region in which are united the portion of the neck which we have just studied and the neighbouring part of the thorax—that is, the breast—has certainly, in our opinion, a form less expressive than the corresponding region in man.
In the latter, indeed, the fourchette of the sternum, with the hollow which it determines, the heads of the clavicles, and the sterno-cleido-mastoid muscles, by the elevations which they produce, and the trachea, by the situation which it occupies in the inferior part, constitute a whole in which are admirably indicated, not only the forms of the organs which constitute this region, but also the relations which these organs have one with another; and, to a certain extent, their respective functions.
In making an exception in the case of the ox, in which a fold of skin, the dewlap, which passes from the neck to the breast, constitutes an element of form which possesses some expressive value; in the horse and in the dog, which possess no sternal fourchette and no heads of clavicles, the bones and the muscles are found nearly on the same plane. This produces a uniformity which is evidently inferior, from an æsthetic point of view, to the modelling of the corresponding region of the human body. Such, at least, is our impression.