348 EPITHELIAL AND ENDOTHELIAL TISSUE. 



of which furnishes the investment for the lymph-sinuses. The 

 sinuses unite to form what Toldt calls the " terminal sinuses." 

 From these sinuses two or more efferent vessels arise, which by 

 anastomoses gradually decrease in number. 



The arteries penetrate the lymph-ganglion both at the periph- 

 ery and at the hilus. Their ramules occupy the middle of the 

 interfollicular strings ; these also contain a few capillary blood- 

 vessels, all supplied with a distinct adventitial coat. The arteri- 

 oles form a wide capillary net- work, traversing the follicles and 

 follicular cords, and giving rise to the veins, which accompany 

 the arteries. The capillary system of the follicles and follicular 

 cords is, to a certain extent, independent of the Capillaries of the 

 interfollicular strings. The capillaries of the lymph-ganglia are 

 easily permeable to colored liquids injected into the artery. The 

 reasons for this have not been fully explained. 



The thymus body is a lymph-ganglion belonging especially to 

 foetal life. It reaches its highest state of development during 

 the first two years, remaining stationary up to the tenth year, 

 and at the time of puberty it has almost entirely lost its follicular 

 structure. The delicate fibrous investment of the thymus body 

 sends numerous prolongations into the interior, giving to it a 

 more or less lobular appearance. The follicular formations are 

 less distinct in the thymus than in other lymph-ganglia, and are 

 more closely aggregated at the periphery of a lobule than in its 

 center. Numerous small arteries enter the thymus body at its 

 periphery ; a few larger ones at its posterior portion. The course 

 which the lymphatics take in the interior of the thymus is 

 unknown. In the period of its involution peculiar concentrically 

 striated corpuscles (Hassal) make their appearance, kindred to 

 the amylaceous corpuscles of the brain. Apparently, these are 

 developed from the walls of the capillaries, most probably from 

 their endothelia. 



The thyroid body, in the earliest periods of embryonal develop- 

 ment, exhibits a glandular structure i. e., is composed of acini, 

 lined by cuboidal epithelia the acini are closed on all sides and 

 no excretory duct can be discovered. This body, in all probabil- 

 ity, is an elongation of the outer germ-layer the epiblast. At 

 the time of birth, the epithelial structure is often still preserved. 

 Sooner or later, however, the epithelia are transformed into indif- 

 ferent or embryonal corpuscles, exhibiting the characteristics of 

 lymph-corpuscles. We meet with small, homogeneous globules, 

 and also with larger ones of the size and the structure of nuclei, 





