SECT. 182.] VESSELS OF THE THYMUS. 399 



vessels run in the tissue of the thick walls of the lobule, ramifying 

 ultimately on the surface of the separate acini, where they form a 

 very close capillary network, with vessels measuring 0*003'" to 

 0"005'", and interstices of 001'" to o - 02'". In man, the expansion 

 of these vessels is situated so entirely in the interior of the acini, 

 that, even when distended to the utmost, not a single vessel is to 

 be met with upon the outer side of their enveloping membrane, 

 hut they all terminate in loops close beneath it. Besides these 

 blood-vessels, a small quantity of connective tissue seems to enter 

 into the composition of the thick walls of the acini • at least, there 

 is often found in their intimate structure a tolerably distinct mem- 

 brane supporting the larger vessels, and being analogous to that 

 which lines the central cavity. In other cases, in animals espe- 

 cially, no inner limiting membrane can be demonstrated, and the 

 cavities of the lobules are directly bounded by the granular sub- 

 stance uniting the vessels, while there is no trace of any fibre, 

 except a few delicate strise between the vessels. In no case do we 

 find an epithelium lining the cavities, and, accordingly, no com- 

 parison can properly be made between the innermost parts of their 

 walls and a mucous membrane. 



The common cavity, or central canal of the thymus, has the same 

 structure as the lobules, except that externally the fibrous layer 

 is thicker, and that internally we meet with a thinner granular 

 layer, with rather large vessels. In a fully developed thymus, the 

 central cavity contains, like all the accessory ones, a greyish-white 

 or milky fluid, of slightly acid reaction. This is often found in 

 large quantity, and in it, along with a clear juice rich in albumen, 

 there arc contained numerous nuclei, separate cells, and, under 

 certain circumstances, concentric bodies (see below). The lym- 

 phatics of the thymus are numerous, and the nerves can be readily 

 demonstrated upon its arteries, although they cannot be traced to 

 their terminations. 



Jendrassik, who confirms my results in most points, believes that the 

 arteries spread out into capillaries immediately after their entrance into the 

 acini. He regards the greater part, if not the whole, of the vessels on the 

 wall of the cavity of the lobule as of the nature of veins, and I am willing to 

 believe that he may be right ; it is certain, however, that vessels, which are 

 arteries, do proceed from the interior to the surface of the acini. Friedleben 

 has revived and confirmed an observation of Rostelli, according to which the 

 blood of the thymic vein contains a large quantity of the microscopic elements 

 of the gland-juice. 



Besides the above-described normal elements, there are found, especially at 

 the period of the involution of the organ, other peculiar round structures 



