ENLAEGEMENT OF THYMUS. 295 



vein and superior vena cava to the back of the gland, where 

 they lie in grooves. Dulness over the manubrium is due 

 to the presence of the glandular mass immediately behind 

 the sternum, in the fore parts of the superior and anterior 

 mediastina. The gland reaches as low as the level of the fourth 

 costal cartilage, a rib below the position where the upper limit 

 of cardiac dulness may be expected in childhood. The cardiac 

 hypertrophy which has been found in some instances has been 

 attributed to compression of the ascending aorta or pulmonary 

 artery. Compression of the vagi has been held responsible for 

 certain irregularities of the heart's action. Dyspnrea is the most 

 important symptom of all. There is great diversity of opinion 

 as to the possibility of suffocation as the result of pressure by an 

 enlarged thymus. It is urged that many of the sudden deaths 

 which are attributed to suffocation are really due to syncope. 

 The anatomical arrangement certainly seems to favour the 

 possibility of tracheal compression when the gland is large. 

 The space between the back of the manubrium and the front 

 of the vertebral column measures little more than two centi- 

 meters (about seven-tenths of an inch), and is occupied by the 

 oesophagus, trachea, great vessels and important nerves. Even 

 if the size of the gland be insufficient to cause much compres- 

 sion of the trachea with the head in the normal position, if the 

 neck be suddenly bent backwards the thymus and surrounding 

 structures are further dragged up into the narrow thoracic aper- 

 ture, and the trachea may be completely occluded. It is possible 

 that this sequence of events might occur during life, owing to 

 weakness of the flexor muscles of the neck, and the softness of 

 the infant trachea. 



Tuberculosis of the thymus must be carefully distinguished 

 from caseation of the superior mediastinal glands, which are in 

 very close relation with the thymus. Careful dissection is 

 requisite to define the limits of the two structures. The tendency 

 of the lobes of the gland to break down into a fluid consisting of 

 lymphocytes has often given rise to an erroneous diagnosis of 

 suppuration. The investment of the gland by the anterior 



