1320 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



Although arising from the branchial clefts one on each side of the neck, the 

 two portions become so closely associated that they are usually spoken of as one. 

 Each of these parts is ordinarily regarded as a lobe of the thymus [lobus, dexter 

 et sinister]. 



In coloiir the thymus is pinkish or reddish grey in the foetus and newborn, 

 becoming greyish white in the adult or yellowish as it undergoes involution. It 

 is composed of soft, jdelding tissue more friable than the thyreoid or spleen. 



In size the thymus varies greatly. Under normal conditions it appears to 

 attain its maximum size at about the age of puberty, and to continue large as 

 long as the body continues to grow and then to undergo a gradual involution. 



Fig. 1074. — Thymus Gland in a Child at Birth. 



Hyoid bone 



Hyo-thyreoid 

 membrane 



Thyreoid cartilage j^A\ 



Sterno-thyreoideusJ^ 

 Crico-thyreoid 

 membrane 

 Crico-thyreoid muscle 

 Thyreoid gland 



Right common carotid 

 artery 



Right vagus 



Right internal jugu- 

 lar vein 

 Level of sternum 



Section of clavicle 

 Section of first rib 



Section of sternum 



viThyreo-hyoideus 



Lateral portion crico- 

 thyreoid membrane 



Omo-hyoideus 



Sterno-cleido- 

 laastoideus 



Cricoid cartilage 

 First ring of trachea 



Trachea 



Left suspensory 



ligament 

 Left recurrent nerve 



(Esophagus 



Left innominate vein 



Left lobe of thymus 



Left internal mam- 

 mary artery 



Left lung 



Pericardium 



Section of fifth rib 

 cartilage 



Xiphoid process 



It is, however, very sensitive to any nutritive changes of the individual and becomes very 

 small, even in the infant, under the influence of wastinfj; diseases. It not infrequently exists in 

 the adult only as a vestige Ijut in sonic cases it may remain largo until middle age or later. At 

 birth it is usually from 50 to GO mm. long ccphalo-caudally and about half as broad. 



The weight varies with the size. It is given by Hammar as over 13 gm. at birth, increasing 

 to double this between the sixth and the tenth years and gaining its maximum of between 37 

 and 38 gm. between the eleventh and fifteenth years. From this time the weight decreases 

 until between the ages of fifty-six and sixty-five it weighs between 25 and 26 gm. and at seventy- 

 five years may be as light as gm. The involution of the gland is not accompanied by a cor- 

 rcHponding reduction in size and weight as tin; thymic tissue is gradually invaded by fatty tissue 

 which maintains to some extent the form of the organ. 



In shape the thymus is an elongated, spindle-shaped mass consisting of the 

 central portion or body and two extremities (figs. 1074, 1075). The body is the 



