114 HISTOLOGY. 



example, a corpuscle may be pressed against the point of 

 division of a capillary, or be crowded with many others in a 

 narrow vessel. This may easily be observed in the mesentery 

 of a recently killed frog. 



When blood is spread in a thick layer on a slide and exam- 

 ined while fresh, one almost always sees the so-called rouleaux 

 i e., cells lying upon one another, as coins do. This con- 

 dition is probably never present in circulating blood. On 

 being shed, the blood-corpuscles seem to be changed in some- 

 way that causes them to adhere to one another. In a 

 properly prepared thin specimen of blood rouleaux never are 

 seen. 



The number of blood corpuscles present in 1 cubic milli- 

 metre of blood varies in different animals, and ranges from 

 33,000 in Proteus to 19,000,000 in Gapra hircus. 



In man the number of erythrocytes in 1 cubic millimetre is 

 about 5,000,000. This varies under normal conditions. The 

 number is greatest just after birth, and decreases slightly with 

 age. With low atmospheric pressure it increases. Marked 

 changes are noticed in various diseases. 



White (colorless) blood-corpuscles (also called leucocytes) are 

 nucleated cells possessing no cell membrane. The protoplasm 

 may be abundant, or very small in quantity. During life these 

 cells have no constant form, being amoeboid, but after death 

 they assume a round outline. Their size varies considerably, 

 from about that of a red blood-corpuscle to three times that 

 size. Their number in man is from 7000 to 9000 in 1 cubic 

 millimetre. This number undergoes changes under physiolog- 

 ical conditions. It is dependent on the food taken in. After 

 several hours of fasting it is much smaller, while after a full 

 meal there is what is known as a digestion-leucocytosis. There 

 is similarly a leucocytosis (an increase in leucocytes) in many 

 febrile diseases. In rabbits the blood from peripheral vessels 

 contains more leucocytes than that from the central ones. 



Similar cells are found in the lymph, adenoid tissue, bone- 

 marrow, and distributed throughout connective tissue and 

 epithelial tissue (wandering cells). The white corpuscles are 



