Chap. V.] GENERAL HISTOLOGY. 65 



corpuscle, each containing a nucleus, seem to creep off in 

 opposite directions, the protoplasm between them getting more 

 and more constricted, till its continuity at last is broken. In the 

 capillaries of the frog's foot the white corpuscles will be seen to 

 creep along the capillary walls. 



The lymph is a colourless fluid containing, in the larger trunks, 

 leucocytes quite similar to the white corpuscles of the blood 

 In the lacteals, e.g. of the rabbit killed shortly after a full meal, 

 the lymph is milky in appearance, and is called chyle. The milki- 

 ness is seen under the microscope to be due to multitudes of very 

 minute fatty particles, which dissolve in ether, and are deposited 

 as oil-drops on the evaporation of the solvent. 



In the mesentery of the rabbit a number of lymphatic glands 

 will be seen. A section of such a gland will show an outer 

 portion or cortex, divided into compartments (alveoli), and an 

 inner portion or medulla, formed of interlacing strands. In the 

 alveoli of the cortex there are roundish masses or follicles of 

 adenoid tissue formed of a network of connective fibres, the inter- 

 spaces of which are crowded with leucocytes similar to the 

 white corpuscles of the blood. The strands of the medulla are 

 formed of similar tissue, also crowded with leucocytes. The 

 Peyer's patches are composed of lymphatic follicles aggregated 

 together and full of leucocytes. From such glands, and there 

 are many such, leucocytes pass into the lymph. 



2. Epithelium and Endothelium. The epithelium is the lining 

 membrane of such tubes or cavities as the mouth, stomach, 

 intestine ; the ducts and cavities of the salivary glands, testis, 

 kidney, etc. They are for the most part in communication with 

 the exterior. At the surface of the body the epithelium passes 

 into the epidermis. The endothelium is a special variety of 

 epithelium, and forms the lining membrane of such cavities 

 or tubes as the peritoneal cavity, lymph sinuses, heart cavities, 

 and blood-vessels. They are for the most part not in communi- 

 cation with the exterior. 



The epithelium cells vary in form and arrangement in different 

 parts of the body. Sometimes they form a single layer (simple 

 5 



