THE THYMUS 413 



part of the sternum. It enlarges fairly rapidly during the first 

 few years of life, but from infancy to the onset of puberty 

 its rate of growth is much slower. Thereafter it rapidly 

 decreases, and, in adult life, it may be represented merely by 

 a few fibrous strands in the superior mediastinum. It must 

 be remembered, however, that it is by no means uncommon 

 for the thymus gland to retain its original size throughout 

 life, and this condition is much commoner in females than 

 it is in males. 



Tumours of the Thymus Gland. Even the fibrous remnants 

 of the gland may give rise to a mediastinal tumour ; and such 

 a condition is frequently found in exophthalmic goitre. The 

 symptoms depend on the relations of the gland. Venous 

 engorgement is commoner on the left side of the head and 

 neck and upper limb, as the gland lies anterior to the left 

 innominate vein. Since the gland lies immediately behind the 

 manubrium sterni, the area of dulness to which it gives rise is 

 quite definite. Further, the gland is closely related to the 

 upper border of the aortic arch and to the large branches 

 which arise from it. On this account, it may be difficult to 

 distinguish the tumour from an aneurism, when it is palpated 

 in the jugular (supra-sternal) notch. Owing to the narrowness 

 of the area in which the examination is carried out, it may be 

 impossible to determine whether the pulsation is expansile or 

 whether it is merely transmitted from the vessels. 



Very little is known with regard to the functions of the 

 thymus, and its relation to such conditions as myasthmia 

 gravis, in which it is very frequently the seat of pathological 

 changes, is not as yet properly understood. 



The Glomus Caroticum (Carotid Body) is a small structure 

 which is situated on the dorsal aspect of the bifurcation of the 

 common carotid artery. It is richly supplied with blood and 

 lymph vessels and it receives numerous branches from the 

 sympathetic system. It contains numerous chromophil cells, 

 but the nature of the internal secretion is still unknown, and 

 the relation of the gland to disease has not been fully studied. 



