THE HYOID BONE.

I'he pos The lateral surfaces after middle life are joined to the greater cornua. In early life they are connected to the cornua by cartilaginous surfaces, and held together by liga¬
ments, and occasionally a synovial membrane is found between them.

The Greater Cornua (thyro-hyal) project backward from the lateral surfaces of the body; they are flattened from above downward, diminish in size from before, backward, and terminate posteriorly in a tubercle for the attachment of the lateral thyro-hyoid ligament. The outer surface gives attachment to the Hyo-glossus, their upper border to the Middle constrictor of the pharynx, their lower border to
part of the Thyro-hyoid muscle.

The Lesser Cornua (cerato-ltyals) are two small, conical-shaped eminences attached by their bases to the angles of junction between the body and greater cornua, and giving attachment by their apices to the stylo-hyoid ligaments.' The smaller cornua are connected to the body of the bone by a distinct diar¬throdial joint, which usually persists throughout life, but occasionally becomes
ankylosed.

THE PLEURAE.

Tracheotomy may be performed either above or below the isthmus of the thyroid body, or ".
this structure may be divided and the trachea opened behind it.
The isthmus of the thyroid gland usually crosses the second and third rings of the trachea;
along its upper border is frequently to be found a large transverse communicating branch between the superior. thyroid veins; and the isthmus itself is covered by a venous plexus formed between the thyroid veins of the opposite sides. Theoretically, therefore, it is advisable to avoid dividing
this structure in opening the trachea.
Above the isthmus the trachea is comparatively superficial, being covered by the skin, super-
ficial fascia, deep fascia, St.erno-hyoid and Sterno-thyroid muscles, and a second layer of the deep fascia, which, attached above to the lower border of the hyoid bone, descends beneath the muscles to the thyroid body, where it divides into two layers and encloses the isthmus.
Below the isthmus the trachea lies much more deeply, and is covered by the Sterno-hyoid and the Sterno"thyroid muscles and a quantity of loose areolar tissue in which is a plexus of veins, some of them of large size; they converge to two trunks, the inferior thyroid veins, which descend on either side of the median line on the front of the trachea and open into the innomi-nate veins. In the infant the thymus gland ascends a variable distance along the front of the trachea, and opposite the episternal notch the windpipe is crossed by the left innominate vein. Occasionally also, in young subjects, the innominate artery crosses the tube obliquely above the level of the sternum. The thyroidea ima artery, when that vessel exists, passes from below up"
ward along the front of the trachea.
From these observations it must be evident that the trachea can be more readily opened
above than below the isthmus of the thyroid body.
Tracheotomy above the isthmus is performed thus: the patient should, if possible, be laid
on his back on a table in a good light. A pillow is to be placed under the shoulders and the head thrown back and steadied by an assistant. The surgeon standing on the right side of his
patient makes an incision from an inch and a half to two inches in length in the median line of the neck from the top of the cricoid cartilage. The incision must be made exactly in the middle line, so as to avoid the anterior jugular veins, and after the superficial structures have been divided the interval between the Sterno-hyoid muscles must be found, the raphe divided, and the muscles drawn apart. The lower border of the cricoid cartilage must now be felt for, and the upper part of the trachea exposed from this point downward in the middle line. Bose has recommended that the layer of fascia in front of the trachea should be divided transversely at the level of the lower border of the cricoid cartilage, and, having been seized with a pair of forceps pressed downward with the handle of the scalpel. By this means the isthmus of the thyroid gland is d<,pressed, and is saved from all danger of being wounded, and the trachea cleanly exposed. The trachea is now transfixed with a sharp book and drawn forward in order to steady It, and is then opened by inserting the knife into it and dividing the two or three upper rings from below upward. If the trachea is to be opened below the isthmus, the incision must be made from a little below the cricoid cartilage to the top of the sternum.

THE THYMUS GLAND.

Vessels and Nerves.-The arteries supplying the thyroid are the superior and inferior thyroid, and sometimes an additional branch (thyroidea media or ima) from the innominate artery or the arch of the aorta, which ascends upon the front of the trachea. The arteries are remarkable for their large size and frequent anastomoses. The veins form a plexus on the surface of the gland and on the front of the trachea, from which arise the superior, middle, and inferior thyroid veins, the two former terminating in the internal jugular, the latter in the innom-inate vein. The lymphatics are numerous, of large size, and terminate in the thoracic and right lymphatic. ducts. The nerves are derived from the middle and inferior cervical ganglia of the sympathetic. 

Surgical Anatomy.-The thyroid gland is subject to enlargement, which is called goitre. 

This may be due to hypertrophy of any of the constituents of the gland. The simplest (parenchymatous goitre) is due to an enlargement of the follicles. The fibroid is due to increase of the interstitial connective tissue. The cystic is that form in which one or more large cysts are formed from dilatation and possibly coalescence of adjacent follicles. The pulsating goitre is where the vascular changes predominate over the parenchymatous, and the vessels of the' gland are especially enlarged. Finally, there is exophthalmic goitre (Graves's disease), where there is great vascularity and often pulsation, accompanied by exophthalmos, palpitation, and 

rapid pulse. 

. For the relief of these growths various operations have been resorted to, such as injection 

of tincture of iodine or perchloride of iron, especially applicable to the cystic form of the disease, ligature of the thyroid arteries, excision of the isthmus, and extirpation of the whole or a part of the gland. This latter operation is one of difficulty, and when the entire gland has been removed the operation has been followed by a condition resembling myxredema. In removing the organ great care must, be taken to avoid tearing the capsule, as if this happens the gland-tissue bleeds profusely. The thyroid arteries should be Jig-atured before an attempt is made to remove the mass, and in ligaturing the inferior thyroids the position of the recurrent laryngeal nerve must be borne in mind, so as not to include it in the ligature. A large number of cases of what were formerly supposed to be goitre are now known to be cases of adenomatous enlarge-ment, where an adenoma, starting in one part of the gland, gradually spreads and involves the 

whole organ.