256 SITUATION AND STRUCTURE 



the right and left lobe of the lungs. The superior part of the 

 gland seldom rises higher than the upper edge of the first bone 

 of the sternum. Sometimes one and sometimes two processes 

 of the same glandular structure arise from the upper part of 

 the thymus, and ascend on the fore-part of the neck almost as 

 high as the glandula thyroidea, and lie between the trachea 

 and carotid arteries ; T but this is a circumstance that rarely 

 occurs. The two sides of the gland are placed next the 

 lungs, and the inferior part or basis of the triangle extends 

 downwards, (sometimes much lower than is expressed in the 

 Plate,) lying on the upper and outer part of the pericardium, 

 to which it is attached by the reticular substance. The supe- 

 rior and posterior part of the thymus lies on that part of the 

 aorta called sinus aortse, which arises from the left ventricle of 

 the heart to form the curvature, the fore-part of which and the 

 common trunk of the right carotid and subclavian arteries it 

 generally embraces. The inferior posterior part is always con- 

 nected with the upper part of the pericardium. 



SECT. 24. The thymus receives two small arteries, called 

 arteriae thymicse, which most commonly originate from the 

 sinus aortse; sometimes they arise from the curvature of the 

 aorta, and I have seen some instances of one small artery 

 coming from the common trunk of the right carotid and sub- 

 clavian arteries to the upper part of this gland. Besides these, 

 the thymus receives several small arterial branches on its fore- 

 part, from the mammary and other arteries, that supply the 

 mediastinal space with blood. 



SECT. 25. The veins are always more regular in their ter- 

 mination, and are generally two in number. They open into 

 the trunk of the jugular and subclavian veins on the left side, 

 of which a part is covered by the superior portion of the 

 thymus. 



SECT. 26. The thymus is a gland of the conglomerate kind, 

 and, like others of that class, has no particular centre of rami- 

 fication for its blood-vessels, but they enter promiscuously at 

 different parts of it. It is formed of a great number of small 

 lobules, or acini, united to each other by blood-vessels and the 

 tela cellulosa; and over the whole, giving it greater compact- 

 ness, is a condensed cellular substance, which forms a kind of 

 1 Hall. torn, i, p. 115. 



