THE THYMUS GLAND. 981 



arteries which they accompany, and communicates with a network in the capsule 

 of the gland. Baber has found in the lymphatics of the thyroid a viscid material 

 which is morphologically identical with the normal constituent of the vesicle. 



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 heave 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 myxoedema. In removing 

 the organ great care must be takAi to avoid tearing the capsule, as if this happens the gland- 

 tissue bleeds profusely. The thyroid arteries should be ligatured 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. 



Parathyroids. These are small rounded, brownish-red bodies, with an average 

 diameter of about a quarter of an inch, situated in or near the thyroid gland, from 

 which, however, they differ in structure, being composed of masses of cells arranged 

 in a more or less columnar fashion with numerous intervening capillaries. They 

 are divided from their situation into external and internal. The former, usually 

 two in number, are situated, one on each side, in relation to the postero-internal 

 surface of the lateral lobe ; sometimes they are duplicated. The latter, also 

 usually two in number, are placed one in each lateral lobe, generally near its mesial 

 surface. 



THE THYMUS GLANDS. 



The thymus gland is a temporary organ, attaining its full size at the end of the 

 second year, when it ceases to grow, and gradually dwindles, until at puberty it 

 has almost disappeared. If examined when its growth is most active, it will be 

 found to consist of two lateral lobes placed in close contact along the middle line, 

 situated partly in the superior mediastinum, partly in the neck, and extending 

 from the fourth costal cartilage upward as high as the lower border of the 

 thyroid gland. It is covered by the sternum and by the origins of the Sterno- 

 hyoid and Sterno-thyroid muscles. Below, it rests upon the pericardium, being 

 separated from the arch of the aorta and great vessels by a layer of fascia. In 

 the neck it lies on the front and sides of the trachea, behind the Sterno-hyoid 

 and Sterno-thyroid muscles. The two lobes generally differ in size; they are 

 occasionally united so as to form a single mass, and sometimes separated by an 

 intermediate lobe. The thymus is of a pinkish-gray color, soft, and lobulated on its 

 surfaces. It is about two inches in length, one and a half in breadth below, and 

 about three or four lines in thickness. At birth it weighs about half an ounce. 



