LESSON 24.] CELLS UPON FREE SURFACES OF THE BODY. 71 



cles would flow at a rapid rate, and simulate fever. These conditions 

 of the blood really occur under the influence of disease ; in a state 

 of health it appears to be imperative that the liquor sanguinis main- 

 tain a certain specific gravity, but still the weighty elements pour in ! 

 "What is now to be done ? 



To relieve this state of things it seems probable that the excess 

 of albumen and fibrine is rolled up, in the solid form, to get it out 

 of the way, and thus the specific gravity of the fluid is maintained. 



395. The observations, particularly of Mr. Paget, lead to the 

 conclusion that these colorless corpuscles gradually change into the 

 red corpuscles, and if this theory be correct, the whole phenomena 

 of their development is remarkably simple and beautiful. Firstly, 

 the colorless corpuscle, formed out of the excess of the materials 

 held in solution in the liquor sanguinis, takes on the solid form, 

 in which it is no longer soluble ; and secondly, the series of gradual 

 changes by which it becomes transformed, with new characteristics, 

 into the red corpuscle. Mr. Paget's observations regarding the 

 origin of the red corpuscle, confirm the experiments on the frog's 

 blood, in regard to the fibrinous character of the nucleus, and offer an 

 additional reason for believing that this body should and does exist 

 in human blood. 



396. The colorless corpuscles appear to possess great power to 

 repair injuries. If the web of the frog's foot be artificially inflamed 

 under the microscope, the colorless corpuscles will be seen to rush in 

 considerable quantity from all adjacent parts, to the seat of injury, 

 and there they remain till the disease disappear. 



If a boil, or other inflamed surface in the human subject, be 

 pricked with a needle, and the fluid thus procured be examined with 

 a microscope, it will appear to be almost composed of the colorless 

 corpuscles, thus demonstrating their activity to cure disease. 



LESSON XXIY. 



CELLS DEVELOPED UPON FREE SURFACES OF THE BODY. 



397. A great difference exists between the cells forming the ex- 

 ternal surface of the body, Epidermis (epi, upon; derma, skin), 

 or the cellular covering of the external surface ; and the Epithelium, 

 or the cellular covering of the internal mucous membrane. 



