THYMUS GLAND 103 



septa of connective tissue, along which the blood-vessels and lymphatics 

 pass to and from the lobules. Each lobule shows plainly, when 

 examined with the low power, a distinction into an outer cortical and 

 an inner medullary portion. The cortical part of the lobule is imper- 

 fectly divided into nodules by trabeculae of connective tissue, and is 

 very similar in structure to the lymphoid tissue of the lymphatic 

 glands and tonsils, but the medulla is more open in its texture, and 

 the reticulum is composed of larger, more transparent, flattened cells, 

 and contains fewer lymph-corpuscles. Moreover, there are found in 



FIG. 127. ELEMENTS OF THE THYMUS. (800 diameters.) (Cadiat.) 

 a, lymph-corpuscles ; b, concentric corpuscle. 



the medulla peculiar concentrically striated bodies (the concentric cor- 

 puscles, fig. 127), which are usually composed of a number of flattened 

 cells arranged concentrically around one or more central cells. Some- 

 times these corpuscles are compound, two or three being grouped 

 together and similarly enclosed by flattened cells. The lymphoid 

 tissue is abundantly supplied with capillary blood-vessels, and large 

 lymphatic vessels issue from the organ, but in what way the latter are 

 connected with the lobules has not been ascertained. 



Lymphoid tissue occurs in many other parts of the body in addition 

 to the lymphatic glands, tonsils, and thymus gland, although it may 

 not, as in these structures, constitute the bulk of the organ. Thus it is 

 found in many mucous membranes, such as those of the intestine and 

 of the respiratory tract, both in a diffuse form and also collected into 

 nodular masses which are like the cortical nodules of a lymphatic 

 gland, and may, like those, be partially surrounded by a lymph-sinus. 

 In the spleen also a large amount of lymphoid tissue is found sheathing 

 the smaller arteries, and also expanded into nodular masses (Malpighian 

 corpuscles of the spleen). In these organs it will, however, be studied 

 in subsequent Lessons. 



Lymphoid tissue also occurs in considerable amount in the serous 

 membranes, especially in young animals ; in the adult it is often trans- 

 formed into adipose tissue. The tissue is generally developed in con- 

 nection with lymphatic vessels, an accumulation of retiform tissue and 

 lymph-cells taking place either external to and around the lymphatic 



