The term thymic asthma has been applied to another form of disturbed respiration due to a large thymus, which comes on suddenly in infants otherwise apparently healthy. Without warning, the child seems to choke, has great difficulty in breathing, with inspiratory stridor and indrawing of the epigastrium; he rapidly becomes cyanosed, and in the majority of cases dies in a few minutes—thymus death. No satisfactory explanation of the sudden onset of the symptoms is forthcoming, but it appears to be associated with something which suddenly narrows the mediastinal space, such as backward bending of the head, or venous engorgement of the thymus gland. Cases are recorded in which an attack has come on during the administration of a general anæsthetic; in some instances the patient has suffered from the generalised status lymphaticus.

Tumours of the Carotid Gland or Glomus Carotica (Potato-like tumour of the neck).—The carotid gland under normal conditions is about the size of a grain of corn, and lies to the posterior aspect of the bifurcation of the carotid. It is sometimes the seat of endothelioma. The tumour has a definite capsule, is moderately firm and elastic, increases in size slowly and gradually for a time, and then may grow more rapidly. Its relation to the vessels is characteristic: as it grows it envelops the common carotid and its branches, and becomes adherent to the internal jugular vein; and it may come to implicate the nerves in the neck, particularly the vagus and its recurrent branch, and the cervical sympathetic.

It gives rise to few symptoms, and in the majority of cases the surgeon is consulted on account of the disfigurement resulting from the presence of the swelling in the neck. This swelling is ovoid, smooth or slightly lobulated; it lies at the level of the bifurcation of the carotid, and tends to grow upwards rather than downwards; it is movable from side to side, but not up and down; it lies under the sterno-mastoid, and the skin is not implicated. There is transmitted pulsation in the tumour, but no expansion.

The diagnosis has to be made from lymphoma, adenoma, tuberculous glands, sarcoma, and carcinoma.

In a large proportion of the cases operated upon it has been necessary to ligate the carotids and to excise portions of the internal jugular vein, and as severe cerebral symptoms are liable to ensue the mortality has hitherto been high. Operation is therefore only to be recommended when the growth is rapid, or the symptoms have become urgent.

CHAPTER XXVII
THE THYREOID GLAND

Surgical Anatomy.—The thyreoid gland consists of two lateral lobes connected by an isthmus. The lateral lobes lie in contact with the side of the larynx up to the middle of the thyreoid cartilage, and with the sides of the first five or six rings of the trachea. The isthmus lies in front of the second, third and fourth rings of the trachea, and from it a process of gland tissue—the pyramidal lobe—passes up in the middle line towards the hyoid bone.

The gland lies under cover of the superficial muscles of the neck, and is surrounded by a process of the cervical fascia—the external thyreoid capsule of Kocher—which connects it with the larynx, trachea, and œsophagus, so that it moves with these structures on swallowing. In this capsule are numerous veins; and in the groove between the œsophagus and trachea the recurrent (laryngeal) nerve runs. Enclosing the gland substance is the capsule proper, which sends in processes to form its fibrous stroma. The arteries of supply—the superior and inferior thyreoids—are very large for the size of the gland, and enter it at its four corners. The thyreoidea ima, when present, goes to the isthmus. Isolated nodules of thyreoid tissue—accessory thyreoids—are sometimes met with in different parts of the neck; they are liable to the same diseases as the main gland.

The secretion of the gland is absorbed into the general circulation through the veins; it consists of a complex colloid substance which contains an iodine-albumin—iodothyrin—and plays an important part in maintaining the normal metabolism of the body, particularly of the central nervous and cutaneous tissues in adults, and of the bones in children. Disturbance of the function of the thyreoid gland plays a part in producing the symptoms characteristic of myxœdema, cretinism, and goitre.