THE THYMUS BODY. 



1797 



the aorta and the pulmonary artery after they have emerged from the heart-sac. It 

 is in contact with a large part of the arch of the aorta, and is grooved on the posterior 

 surface by the innominate veins and the superior vena cava. If strongly developed, 

 its highest part may rest on the trachea and even on the oesophagus where this tube 

 appears on the left of the former. It extends laterally on each side into the interval 

 between the pericardium and the pleura. At the time of its greatest size, a hori- 

 zontal section in this region shows the thymus as a thick crescent ( Fig. 1518), which 

 becomes thinner as the organ atrophies. Behind the very top of the sternum its out- 



FIG. 1516. 



Common carotid artery, 

 Pneumogastric nerve 



Internal jugular 

 vein 



Clavicle 

 I rib' 



Right lobe 

 of thymus 



Lung 



Larvnx 



Thyroid body 



uspensory ligament 



Trachea 



Left lobe of 

 thymus 



Dissection of new-born child, showing thyroid and thymus bodies in situ. 



line on section is roughly quadrilateral. One or more fibrous bands from the thyroid 

 body to the capsule of the thymus are known as the suspensory ligaments. The 

 internal mammary vessels run in front of it. 



Weight and Changes. According to Friedleben, the average weight of the 

 thymus at birth is 13.75 gm. ; the statements of authors, however, vary widely, Sappey 

 giving 3 gm. and Testut, from twenty observations, an average of 5 gm. When heavi- 

 est, about puberty according to Hammar, its average weight is 37.52 gm. Atrophy 

 and the replacement of thymus tissue by fat set in while growth in length is still pro- 



