PART FOURTH. 

 THE BLOOD AND ITS CIRCULATION. 



THE BLOOD. 



THE blood is a liquid which, circulating in the body from 

 the periphery to the centre and from the centre to the 

 periphery, diffuses throughout the system the elements ab- 

 sorbed by certain globules on the surface, and draws the 

 waste parts of the system in general towards other globules 

 on the surface, whose function it is to cast them. off. 1 In 



1 It is difficult to give an exact definition of the blood. Physi- 

 ologists consider it as an internal medium. ' ' The name of medium 

 is given to those conditions which surround a living being, and 

 supply it with all that is needful to develop, nourish, and exhibit 

 the life by which it is animated. . . . We must distinguish between 

 the cosmical mediums (air, water, food, temperature, light, electri- 

 city) and the internal mediums: the former surround the individual 

 in his perfect state ; the latter belong directly to the anatomical ele- 

 ments of which he is composed." (Cl. Bernard, " Proprietes des 

 Tissus Vivants.") But this general definition errs by being too 

 general; it is easy to see that in this way all the tissues play the 

 part of medium in reference to each other. It is better, therefore, 

 to consider the blood as a tissue, as is done by most histologists in 

 these days (Frey, Rouget) , and to define it as a cellular tissue con- 

 sisting of an intercellular fluid substance. It enters thus into one of 

 the four great classes of tissues : 



1. Cellular tissue, with little or no intercellular substance, 

 epitheliums and their derivatives (nails, hair, enamel, crystalline 

 substance) . 



2. Cellular tissues, whose substance is fundamentally liquid 

 (blood, lymph, chyle). 



3. Cellular tissues, with abundant fundamental substance, such 

 as mucus, hyaline, or fibrous (cartilage and all collagenous or 

 connective tissues). 



4. Tissue formed by globules, giving rise by their juxtaposition 

 to different kinds of tubes or fibres (muscles, nerves, vessels, etc.). 



