BLOOD AND BLOOD-FORMING ORGANS. 



5. BEHAVIOR OF BLOOD-CELLS IN THE BLOOD CURRENT. 



In the circulating blood the behavior of the formed elements 

 varies. The more rapid axial current contains very nearly all the 

 erythrocytes, and as a consequence very few are found adjacent to the 

 walls of the vessels. In the peripheral current, on the other hand, 

 are found most of the leucocytes, and in a retarded circulation they 

 are seen to glide along the walls of the vessels. At the bifurcations 

 of the vessels, especially of the capillaries, the erythrocytes are 

 sometimes caught and elongated by the division of the current, the 

 one-half of the cell extending into the one and the other half into 

 the other branch of the vessel, while the corpuscle oscillates back 

 and forth. When again free the cell immediately resumes its original 

 shape. From this it is seen that erythrocytes are very elastic 

 structures. In the smaller vessels and capillaries, especially when 

 the latter are altered by pathologic conditions, the leucocytes may 

 be seen passing out of the vessels, and it would seem that they are 

 able to penetrate through the walls and even through the endo- 

 thelial cells lining the blood-vessels (compare also Kolossow, 93). 

 First, they' send out a fine process, which is probably endowed with 

 a solvent action. This penetrates the wall of the vessel, after which 

 the remainder of the cell pushes its way through slowly. 



B. LYMPHOID TISSUE, LYMPH-NODULES, AND LYMPH- 

 GLANDS. 



As to the origin of lymphoid tissue, the lymph-glands, and the 

 spleen, there is still considerable difference of opinion. Most 

 authors believe that these structures are developed from the middle 

 germinal layer (Stohr, 89 ; Paneth ; J. Schaffer, 91 ; Tomarkin). 

 Others believe in an entodermic origin (Kupffer, 92; Retterer; 

 Klaatsch ; C. K. Hoffmann, 93, II). 



The framework of lymphoid tissue is a reticular connective tis- 

 sue (adenoid connective tissue His, 61). This consists of a net- 

 work of fine fibrils of reticular and white fibrous connective tissue 

 and of cells (endoplasm and nuclei) which are situated on the 

 reticulum, often at nodal points. Within its meshes the lymph-cells 

 lie in such numbers and so densely arranged that on microscopic 

 examination the network is almost entirely covered unless very thin 

 sections are used. The cells may be removed from the meshes of 

 the reticulum by stippling and brushing section with a fine brush or 

 by placing sections in a test-tube partly filled with water and sub- 

 jecting them to vigorous shaking, or, still better, by subjecting 

 sections or pieces of lymphoid tissue to digestion with pancreatin. 



Lymph tissue may be diffuse, as in the mucous membrane of the 

 air-passages and as in that of the intestinal tract, uterus, etc. (vid. 

 Sauer, 96). Lymphoid tissue may be also sharply defined in the 



