FORM AND POSITION OF THE THYMUS GLAXD. 



923 



lobes. In the foetus, and during early infancy, this organ is relatively larger than in 

 after life ; its proportion to the weight of the body in the new-born infant being that 

 of 1 to 240 or 400, whilst at the end of three weeks it becomes only 1 to 1160, and 

 in the adult 1 to 1800 (Krause). In advanced life the thyroid body becomes in- 

 durated, and frequently contains earthy deposit; its vesicles also attain a very large 

 size. 



2. THE THYMUS GLAND. 



The thymus gland or body (glandula thymus, corpus thymicum) is a tem- 

 porary organ which reaches its greatest size at about the end of the second 

 year, after which period it ceases to grow, and is gradually reduced to a 

 mere vestige. Its function, like that of the thyroid body, is unknown, 

 although it is probable that it is in some way connected with, the elaboration 

 of the blood in infancy. When examined in its mature state in an infant 

 under two years of age, it appears as a narrow elongated glandular-looking 

 body, situated partly in the thorax, and partly in the lower region of the 

 neck : below, it lies in the anterior mediastiual space, close behind the ster- 

 num, and in front of the great vessels and pericardium ; above it reaches 

 upwards upon the trachea in the neck. Its colour is greyish, with a pinkish 

 tinge ; its consistence is soft and pulpy, and its surface appears distinctly 

 lobulated. It consists of two lateral parts, or lobes, which touch each other 

 along the middle line, and are nearly symmetrical in form, though generally 

 unequal in size, sometimes the left, and at other times the right lobe being 

 the larger of the two. An intermediate lobe often exists between the two 

 lateral ones, and occasionally the whole body forms a single mass. The 

 forms of the smaller lobules also differ on the two sides. 



Fig. 647. ONE LOBE OF THE HUMAN THYMUS Fig. 647. 



GLAND (from Koliiker). 



The lower part presents a large cavity which 

 has been opened, and within it are seen nume- 

 rous apertures leading into the smaller lobes. 



Each lateral lobe is of an elongated 

 triangular form, its base being directed 

 downwards. The summit, or upper ex- 

 tremity, usually mounts up into the 

 neck, reaching as high as to the lower 

 border of the thyroid body. The base 

 rests on the upper part of the pericar- 

 dium, to which it is connected by areolar 

 tissue. The anterior surface, slightly 

 convex, is covered by the first and the 

 upper part of the second piece of the 

 sternum, reaching, in the infant at birth, 

 as low down as the level of the fourth 

 costal cartilage. It is attached to the 

 sternum by loose areolar tissue, but 

 opposite the upper part of that bone is 



separated from it by the origins of the sterno-hyoid and sterno-thyroid 

 muscles, which also cover it in the neck. The posterior surface, some- 

 what concave, rests, in the thorax, upon part of the pericardium, upon the 

 front of the aortic arch and the large arteries arising from it, and also on 



