The posterior triangle of the neck is bounded in front by. the posterior border of the sterno-mastoid, behind by the anterior border of the trapezius, and below by the clavicle, beneath which and between it and the first rib it communicates with the axilla. Superficially, branches of the cervical plexus, the external jugular vein, and some lymphatic glands are found. The omo-hyoid lies a little deeper, and divides the triangle into two, the lower or subclavian being bounded in front by the sterno-mastoid and-deeper-the scalenus anticus, below by the clavicle and subclavius, and above and behind by the omo-hyoid. The pretracheal fascia, which en-sheaths the omo-hyoid, is continued down to the clavicle, to which, together with the sheath of the subclavius and of the subclavian vein, it is attached, and sometimes binds the omo -hyoid so closely to the .clavicle as almost to obliterate the subclavian triangle. Under this fascia lies a chain of lymphatic glands, which communicate with the mediastinum, axilla, and deep parts of the neck, and the suprascapular and transversalis colli arteries cross the triangle, the former behind the clavicle and the latter behind the omo-hyoid. This triangle is of importance, as the subclavian artery in its third stage is ligatured in it for axillary aneurism, wounds, etc., by an incision made along the clavicle, the skin having previously been pulled down. The skin is then pulled up, and the incision deepened until the triangle is exposed. The anterior scalene is made out, the artery lying behind it and the vein in front of it, and the finger is run down its external border till it rests at its insertion on the scalene tubercule (tubercle of Lisfranc) of the first rib, which is the guide to the artery. The height of the artery above the clavicle varies greatly in different individuals, and in different positions, occasionally being so prominent as to suggest the presence of an aneurism. The best position for ligature is with the shoulder depressed and the arm thrown behind the back, while the head is turned to the opposite side. The needle is passed from above downwards and backwards, taking care not to confuse a cord of the brachial plexus or the omo-hyoid muscle for the artery-mistakes which are frequently made in operations on the cadaver. When the posterior scapular artery presents in the wound, it is better to ligature it also, lest secondary haemorrhage occur. The collateral circulation is by suprascapular and posterior scapular, with acromio-thoracic, subscapular, and dorsalis scapulae; and superior and aortic intercostals and internal maxillary, with long thoracic and scapular arteries. The pleura lies close to the subclavian artery, and, indeed, the apex of the lung extends some 1/2 inch above the clavicle, behind the sterno-mastoid, particularly on the right side. Thus injury to the pleura may result from ligature of the artery, or removal of a tumour or adherent tubercular glands, while it also has been injured by stabs in the neck, severe fracture of the clavicle, etc. The lung may occasionally produce an appreciable swelling in the neck during severe coughing.

The portion of the posterior triangle which lies above the omo-hyoid is known as the occipital triangle. It is bounded in front by the posterior border of the sterno-mastoid, behind by the anterior border of the trapezius, and below by the omo-hyoid. The floor is formed from above downwards by the sraenius capitis, levator anguli scapulae, and scalenus medius and posticus. Superficially it is covered by the skin and fascia, with platysma in the lower portion, and presents superficial branches of the cervical plexuc-namely, lesser occipital, great auricular, transverse cervical (all emerging from posterior border of sterno-mastoid), and supraclavicular branches (in lower part of the triangle). Lying deeply along the posterior border of the sterno-mastoid are the glandule concatenate.

The anterior portion of the neck is formed embryonically by the growing forward of five branchial arches, with clefts- or, in the human embryo, grooves-both external and internal (called 'recesses '), between them. The first arch forms the lower jaw and malleus. The second, or hyoid arch, which forms the incus, styloid process, and part of the hyoid, grows rapidly, and, as flexion of the neck occurs at this period, soon overlaps the other arches, which become buried, and ultimately lose their epiblast. A cervical fistula leading down to these arches persists for a time, however, and in some cases permanently, as a fine channel, with small opening a little above the sterno-clavicular articulation, which penetrates a varying distance. Various forms of cysts, also, with epithelial lining, may arise from the included epiblast of these buried arches, and are sometimes alluded to as hygromas, or hydroceles of the neck, and occur most frequently in the posterior triangle. The first external groove forms the external auditory meatus, the membrane between it and the internal groove forming the tympanic membrane. The internal grooves between the arches disappear early, with the exception of the first, which persist as the tympanic Eustachian passage. The second leaves traces in the fossa of Rosenmuller in the pharynx and the supratonsillar fossa, and the fourth in the pyriform fossa at the pharyngo-laryngeal junction. Cervical ribs occasionally occur, especially in connection with the seventh cervical vertebra-sometimes anchy-losed to the transverse process, and sometimes movable ; sometimes short and resembling an exostosis, at others long and well formed. In the latter case the subclavian artery passes over the rib, and so projects, simulating aneurism, and the scalene muscles may be attached to it. Even the smaller forms may, however, present a projection in the neck if the subject be thin, and they sometimes give rise to trouble from pressure on the cords of the brachial plexus.

The neck communicates with the thorax anteriorly, many of the structures passing directly from the one to the other. On the right side the innominate vessels are comparatively superficial, and the vagus nerve passes down in front of the subclavian artery. On the left side the separate carotid and subclavian are much deeper, and the vagus passes down between them. The innominate artery is about 1½ inches long, and extends up and outwards to the sterno-clavicular articulation, where it divides, being at first in front of the trachea, and then to its outer side. In front of the vessel are the left innominate and inferior thyroid veins. To its right side are the right innominate vein, vagus nerve, and. pleura, while to its left are the left cornu of hyoid, carotid artery, and trachea. It is occasionally affected by aneurism, causing dyspnoea, cough from pressure on the recurrent laryngeal, difficulty in swallowing, etc. The condition is best treated by needling, but ligature has been done through an incision along the lower end of the anterior border of the sterno-mastoid, and then out along the inner one-third of the clavicle, cutting the sterno-mastoid, hyoid, and thyroid muscles, and the anterior jugular vein. Collateral circulation would take place by the superior with aortic intercostals ; internal maxillary with deep epigastric and aortic intercostals ; circle of Willis, etc. Between the sterno-mastoid and the scalenus anticus lie the phrenic nerve, omo-hyoid muscle, transversalis colli, and suprascapular arteries, external jugular, and subclavian veins. Behind the scalenus anticus, and between it and the other scalenes, pass the subclavian artery and cords of the brachial plexus. The subclavian artery arises from the innominate on the right and the aorta on the left, and reaches the inner border of the scalenus anticus in its first stage, passes behind the muscle in its second, and in its third extends to the outer border of the first rib, beyond which point it is known as the axillary. The first stage, therefore, differs considerably on the two sides. On the left it is larger, deeper, and more vertical ; the internal jugular vein and vagus nerve run parallel instead of crossing it at right angles ; the recurrent laryngeal nerve lies to the inner side, instead of looping round the deep aspect ; and the oesophagus and thoracic duct only form relations on this side. In its first stage it gives off (i) the vertebral artery, which ascends between the scalenus anticus and longus colli to the foramen in the transverse process of the sixth cervical. It has been ligatured in some cases of epilepsy by an incision along the posterior border of the sterno-mastoid, just above the clavicle, the carotid tubercle of the transverse process of the sixth cervical forming a guide. This vessel is surrounded by vaso-motor nerves from the inferior cervical ganglion, and higher up is in close connection with the hypoglossal and suboccipital nerves. (2) The internal mammary, which originates at the inner border of the scalenus anticus, and passes down behind the first costal cartilage to the thorax, where it runs parallel to and ½ inch from the sternal border. It may be wounded, and give rise to fatal haemorrhage, and is most easily treated through the second intercostal space. (3) The thyroid axis, which arises opposite the internal mammary, and divides into inferior thyroid, suprascapular, and transversalis colli. (4) The superior intercostal arises from either first or second stage, passes upwards and backwards over the pleura, and then descends into the thorax in front of the neck of the first rib, and supplies the first two intercostal spaces. It anastomoses with a branch of the occipital. The subclavian is occasionally affected by aneurism, especially on the right side, in its third part, forming a pulsating tumour in the posterior triangle, and, as it increases, produces pressure symptoms, including spasm of the diaphragm from pressure on the phrenic nerve. When it is necessary to ligature the vessel, this should, where possible, be done in the third part, as already described. Ligature of the first part is difficult and dangerous.