350 



Dr. H. Watney. 



[Feb. 1.6, 



in the bird it disappears generally before adult life; in the reptile 

 and the fish it is found even in very large specimens, although in 

 such cases it is in great measure transformed into connective-tissue. 



The fully developed gland is divided into lobes, lobules, and follicles • 

 the follicles, except in earliest embryonic life, are composed of cortex 

 and medulla ; the follicles are generally attached to one another, either 

 by cortical or by medullary tissue. 



When first developed the thymus is composed at its upper end of a 

 single tube, and below of a number of tubes, some of which are solid, 

 the others hollow ; the cells in cross section of the tube vary from two 

 or three to many in number, and are epithelioid in character ; the 

 lumen of the tube is at times closely packed with blood-corpuscles, 

 and appears to be a vessel. The follicle first grows by pushing out 

 processes of epithelioid cells. In a subsequent stage the connective- 

 tissue (ensheathing the vessels which enter and leave the follicles) 

 invades the follicles and divides them up ; at the same time the follicles 

 increase in size : there is thus an increase in the number of the 

 follicles, which are partially united to one another. 



The cortex when first formed is small, but in the fully developed 

 gland is more than twice as large as the medulla ; in involution the 

 outer part of the follicle disappears much the more rapidly of the 

 two. As further the cortex is composed chiefly of cells (thymic corpus- 

 cles), similar to those which are formed in the lymphatics of the 

 thymus, this portion is the more important. 



The blood-vessels are disposed in two rings, one of which surrounds 

 the" follicle, the other lies just within the margin of the medulla ; the 

 cortex contains only fine vessels arranged in a radiating manner ; the 

 blood in these vessels flows chiefly to the inner circle of vessels. The 

 centre of the medulla contains only few and fine vessels during growth 

 and the period of full development, but in involution there are many 

 more and larger vessels. The blood-vessels met with in the invading 

 processes of connective-tissue during involution are surrounded by an 

 adventitia of epithelioid cells, so that they look somewhat as if com- 

 posed of proliferating endothelium ; this probably is not the case, as 

 their lumen is unaltered. 



There are no lymphatic vessels in the cortex of the follicle, although 

 perivascular sheaths are found on the vessels during the period of 

 involution. 



The cortex is composed in great measure of lymphoid cells, sup- 

 ported by a delicate reticulum. If sections of the cortex are shaken 

 a second retiform tissue is brought into view, i.e., a branched network, 

 composed of finely branched cells, and of coarse threads ; these 

 together form an adventitia to the vessels. This network of cells 

 is permanent, and is found in the thymus of adult animals when the 

 organ is undergoing involution. In specimens stained in indigo- 



