924 THE THYMUS GLAND. 



the left innominate vein, some areolar tissue being interposed between it 

 and these parts. In the neck, it lies upon the front and corresponding 

 side of the trachea. Its external border is in contact with the corresponding 

 layer of the mediastinal pleura, near the internal mammary artery, and 

 higher up (in the neck), with the carotid artery, or its sheath. The internal 

 border is in close contact with that of the opposite lateral lobe. The 

 dimensions of the thymus vary according to its stage of development. At 

 birth it measures above two inches in length, an inch and a half in width 

 at its lower part, and about three or four lines in thickness. Its weight at 

 that period is about half an ounce. Its specific gravity, which is at first 

 about 1 -050, diminishes as the gland continues to waste. 



Chemical Composition. The substance and fluid of the thymus contain 

 nearly eighty per cent, of water. Its solid animal constituents are com- 

 posed essentially of albumen and fibrin in large quantities, mixed witli 

 gelatine and other animal matter. The salts are principally alkaline and 

 earthy phosphates, with chloride of potassium. 



Structure. The lateral halves or lobes of the thymus gland are each 

 surrounded by a proper investment of thin areolar tissue, which encloses 

 in a common envelope the smaller masses composing it. This tissue being 

 removed, the substance of the gland is found to consist of numerous com- 

 pressed lobules, the most of them from two to five lines in diameter, con- 

 nected by a more delicate intervening areolar tissue. These primary 

 lobules, as they may be called, are each made up externally of smaller or 

 secondary lobules, of a compressed pyriform shape, placed close together 

 with their bases outwardly, and are arranged round an elongated central 

 stem (reservoir of the thymus, Cooper), running through each lateral half of 

 the gland, and more or less spirally twisted. 



On making a section of the thymus, there is obtained a milky substance 

 consisting of fluid rich in nuclei and small nucleated cells. The walls 

 both of the lobules and the larger stems are limited by a fine homogeneous 

 membrane (Simon) ; the substance in the interior of this appears at first 

 sight to be entirely composed of corpuscles of the kind just mentioned, 

 varying in diameter from -gjuo^ to Wofi** 1 ^ an i ncn > an( i having the 

 appearance of free nuclei ; but, on closer examination, according to Kolli- 

 ker and His, seen to be mostly contained within delicate cells. The sub- 

 stance contained within the limiting membrane is not, however, a mere 

 fluid with corpuscles, but possesses a delicate reticulum of connective tissue, 

 and, as was first pointed out by Kolliker, likewise capillary blood-vessels, 

 resembling in this respect the substance which occupies the interior of 

 Peyer's glands. 



According to Astley Cooper the central stem of the thymus presents a 

 continuous cavity, the ramifications of which pass into both primary and 

 secondary lobules. The existence of a central cavity has been since doubted 

 by some and aflirmed by others ; the difficulty, however, may now be re- 

 garded as cleared up, since the discovery of connective tissue and blood-vessels 

 within the lobulated structure ; for it is admitted that, in the centres of the 

 lobules, sublobules, and central stem, the capillaries and reticulum of con- 

 nective tissue are deficient and the corpuscles most abundant, while on the 

 other hand it is equally certain that there is no cavity bounded by epithelial 

 lining. Considered in relation to development, Cooper's view is correct ; for 

 it has been shown by Simon that the primitive form of the thymus gland is 

 a linear tube, from which, as it grows, lateral branched diverticula subse- 

 quently bud out, but that in the mature thymus this tube becomes obscure. 



