THYMUS GLAND. 789 



mass ; and sometimes separated by an intermediate lobe. The thymus is of a 

 pinkish-gray color, soft, and lobulated on its surfaces. It is about two inches 

 in length, one and a half in breadth, below, and about three or four lines in 

 thickness. At birth, it weighs about half an ounce. 



Structure. Each lateral lobe is composed of numerous lobules, held together 

 by delicate areolar tissue; the entire gland being inclosed in an investing cap- 

 sule of a similar, but denser structure. The primary lobules vary in size from 

 a pin's head to a small pea. Each lobule contains, in its interior, a small cavity, 

 which is surrounded with smaller or secondary lobules, also hollow. The 

 cavities of the secondary and primary lobules communicate ; those of the latter 

 opening into the great central cavity, or reservoir of the thymus, which extends 

 through the entire length of each lateral half of the gland. The central cavity 

 is lined by a vascular membrane, which is prolonged into all the subordinate 

 cavities, and contains a milk-white fluid resembling chyle. 



If the investing capsule and vessels, as well as the areolar tissue connecting 

 the lobules, are removed from the surface of either lateral lobe, it will be seen 

 that the central cavity is folded upon itself, and admits of being drawn out into 

 a lengthened tubular cord, around which the primary lobules are attached in a 

 spiral manner, like knots upon a rope. Such is the condition of the organ, at 

 an early period of its development ; for Mr. Simon has shown, that the primi- 

 tive form of the thymus is a linear tube, from which, as its development pro- 

 ceeds, lateral diverticula lead outwards, the tube ultimately becoming obscure, 

 from its surface being covered with numerous lobules. 



According to Oesterlen and Mr. Simon, the cavities in the secondary lobules 

 are surrounded by rounded saccular dilatations or vesicles, which open into it. 

 These vesicles are formed of a homogeneous membrane, inclosed in a dense 

 capillary plexus. 



The whitish fluid contained in the vesicles and central cavity of the thymus, 

 contain numerous dotted corpuscles, similar to those found in the chyle. The 

 corpuscles are flattened circular disks, measuring about TTHHT f an i nc h i n 

 diameter. 



Vessels and Nerves, The arteries supplying the thymus are derived from the 

 internal mammary, and from the superior and inferior thyroid. The veins 

 terminate in the left vena innominata, and in the thyroid veins. The lymphatics 

 are of large size, arise in the substance of the gland, and are said to terminate 

 in the internal jugular vein. Sir A. Cooper believed that these vessels carried 

 into the blood the secretion formed in the substance of the tlrymus. The nerves 

 are exceedingly minute ; they are derived from the pneumogastric and sympa- 

 thetic. Branches from the descendens noni and phrenic reach the investing 

 capsule, but do not penetrate into the substance of the gland. 



Chemical Composition. The solid animal constituents of the thymus are albu- 

 men and fibrine in large quantities, gelatine and other animal matters. The 

 salts are alkaline and earthy phosphates, with chloride of potassium. It con- 

 tains about 80 per cent, of water. 



