space. This nerve, where crossed by the external jugular vein, gives one or two minute twigs, which follow its direction toward the angle of the jaw.
The supra-clavicular and super-acromial are the two superficial branches in which the plexus terminates: as they descend, they divide into a lash of twigs, which diverge in the posterior triangle of the neck, and at various heights pierce its fascia, become subjacent to the platysma, and contribute to supply it. Their ultimate branching takes a very wide range: the inner filaments obliquely cross the clavicular origin of the sterno-mastoid; the outer, the anterior fibres of the trapezius; the middle ones, the clavicle itself; and are distributed, in their respective regions, to the integuments of the scapula, shoulder, chest, and sternum.
The branch from the portio dura, which enters the neck, is the lower division of its cervico-facial part. From near the angle of the jaw, where it traverses the fascia, it passes toward the hyoid bone, and supplies the platysma from its deeper side. Some of these twigs, approaching the cutaneous surface of the muscle in the anterior triangle of the neck, communicate with filaments of the anterior cervical nerve.
1. Mesial region of the neck.—This presents different relations, as considered above or below the level of the os hyoides.
Above the os hyoides, and extending from the body of that bone to the symphysis of the lower jaw, is the narrow space which separates the anterior bellies of the digastric muscles. It is an elongated triangle, broadest below—where the tendons of the digastrics are kept apart by the body of the hyoid bone—having its apex above, where these, having expanded into the fleshy anterior bellies, are infixed side by side at the median line of the jaw. The platysmata in their cellular sheath cover this space, and sometimes decussate across it with each other. The cervical aponeurosis likewise extends over it, adhering to its bony limits, and strengthened by the tendinous slip, which is derived from the digastric. Deeper than the digastrics are seen the fibres of the mylo-hyoid muscles, meeting in the median raphe, which runs along the space. The natural direction of this raphe is almost antero-posterior, and that of the fibres which meet in it almost horizontally transverse: but when (as in any operation on this part of the neck) the head is thrown back and the chin elevated, the raphe presents a considerable downward slope, and the fibres of the mylo-hyoid have a corresponding obliquity. The same observation applies to the deeper fibres which course from the tubercles within the symphysis to the body of the hyoid bone—those, namely, of the genio-hyoid and genio-hyo-glossal muscles. This little region can hardly be said to have any special surgical relations; it contains neither vessels nor nerves of size; its injuries only assume importance when they extend beyond it into the adjoining digastric space; its diseases derive no peculiarities from their situation, and for the most part belong to the integuments, which are vascular, highly folliculated, and in the male densely bearded: sycosis often extends to them, and they are a frequent seat of sebaceous tumours.
Below the os hyoides, the anatomy, which involves the surgical relations of the larynx and trachea, becomes of extreme importance. Between the two layers of the fascia superficialis the platysma no longer intervenes; they accordingly lie together and are blended. The vaginal processes of cervical fascia, which have isolated the sub-hyoid muscles, become united into a strong and single raphe along the middle line, from above to within a short distance of the sternal notch; but here the layers remain distinct, a superficial one fixing itself to the notch and to the interclavicular ligament, while the deeper one descends with the muscles into the mediastinum. The interval contains loose cellular tissue, and sometimes (as Burns noticed) an absorbent gland. Accordingly, in the very median line, an operator may expose the larynx, trachea, or thyroid body without dividing or displacing any portion of muscle; but a lateral deviation from this imaginary line would imply an exposure of the sub-hyoid muscles on one side or on the other. Indeed, the muscles so nearly approach to the line in question, and constitute in their laminar arrangement so useful a guide to the subjacent parts, that the bare possibility of avoiding them is wisely neglected, and the surgeon learns from them his nearness to the organs which they cover.
In tracing, from the hyoid bone downward, the irregular profile of the air-tube, the finger may distinguish through the integument the following changes of outline. 1. A horizontal semicircular notch, limited below by the prominent angle of the thyroid cartilage, and corresponding, in the interval of the muscles, to the thyro-hyoid membrane; the lateral parts of this give passage, as we shall presently see, to the laryngeal artery and nerve, but its mesial part, with which alone we are now occupied, has only a small twig from the thyroid artery ramifying over it: the membrane is thick, and composed of strong vertical fibres in the median line; it becomes weaker and of laxer tissue in proceeding backward. Its deep aspect contributes to the skeleton of the pharynx, and corresponds to the epiglottis, from the attached portion of which it is separated only by cellular tissue and the epiglottidean gland; while, above, the mucous membrane, in being folded forward to the epiglottis, intervenes between it and the membrane. This notch is frequently invaded by the knife of the suicide; and there is perhaps no part of the neck on which a gash may be inflicted with less serious injury: the large vessels are far removed, and the larynx lies below the blade, which may, if near to the hyoid bone, enter the pharynx above the epiglottidean fold of mucous membrane, leaving the epiglottis unhurt, or, if more nearly approached to the thyroid border of the space, may partly or entirely sever that cartilage from its inferior attachments. No special surgical operation belongs to the space; if indeed we
