Anatomy, Embryology, Comparative 

 Anatomy and Histology 



E. V. COWDRY 



NEW YOBK 



Anatomy 



The term "thymus" is apparently derived from two sources. Owing 

 to its bilobed shape, the organ is named after the word "thymus," which 

 is applied to a genus of plants with a two lipped calyx. Galen, hearing 

 in mind its close association with the heart, took the word from the 

 Greek, fyw> ? meaning courage. 



Size. The statements of authors conflict with regard to the average 

 size of the thymus gland. When it has attained its maximum develop- 

 ment, that it is say at about thirteen years, of age, it usually weighs 

 approximately thirty-eight grams, but variations of several hundred per 

 cent are by no means of rare occurrence. There is no reason to suppose 

 that there is any noticeable difference in size as between sexes, as is 

 commonly the case in endocrin organs. The weight of the gland is not 

 always indicative of the amount of thymic tissue, because the proportion 

 of fat and connective tissue varies within very wide limits. 



Gross Morphology and Relations. The thymus is a soft, white, fusi- 

 form mass, in which two lobes can easily be distinguished, of which the 

 left is often the larger. It is situated chiefly in the thorax, though it 

 may extend cephalad into the neck region to a variable extent, usually, 

 however, not more than two centimeters. The fourth costal cartilages 

 generally mark its caudal extremity ; but it has been known to extend down 

 as far as the diaphragm. Ventrally it is in relation with the sternum, the 

 origins of the sternothyroid muscles, the sternocostal and sternoclavicular 

 articulations, and the internal mammary arteries. Dorsally it comes into 

 contact with the aorta, superior vena cava, and their branches, the pul- 

 monary artery and the pericardium. It is bounded laterally by the pleural 

 sacs and phrenic nerves. A fairly dense capsule surrounds the gland, 

 portions of which penetrate in between the lobules accompanied by vessels 



361 



