II. THE TISSUES OF THE BODY. 



A. Tissues composed of Simple Cells with Fluid 

 Intermediate Substance. 



1. The Blood. 



65. 



In the blood-vessels of our body, a closed system (except in the case of 

 the spleen) of intercommunicating canals, into which, however, the lym- 

 phatic and lacteals discharge their contents, there exists an extremely com- 

 plex fluid, " the blood," which is constantly in motion during life. And 

 just as on the one hand no pause takes place in its continuous circulation 

 while life remains, so on the other hand is this fluid unceasingly engaged 

 in a lively interchange of matter. The walls of the blood-vessels being 

 formed of membranes permeable to endosmotic currents, and processes of 

 nitration further occurring in glands, the blood is constantly being robbed 

 by the organs and tissues of certain of its constituents in the form of 

 watery solutions, while other substances similarly dissolved are rendered 

 back to it again. It receives also bulky additions of other complex fluids 

 in the shape of lymph and chyle poured into it. 



Notwithstanding this coming and going of material which constitutes 

 the blood the centre of the vegetative processes of life, the fluid in ques- 

 tion is always singularly unvarying, both in regard to chemical and ana- 

 tomical composition, any deviations from the normal standard being 

 rapidly compensated. 



Human blood is a thickish, opaque fluid with a peculiar faint odour, 

 alkaline reaction, temperature of about 38 C., and a red colour, light 

 cherry-red in the arteries, but somewhat deeper in the veins. The amount 

 of blood contained in any one body cannot be estimated at present with 

 anything like accuracy, and we find statements on this point very various 

 as regards the human system. It appears probable that the weight of 

 the blood averages in man about a twelfth or thirteenth of that of the 

 whole body. 



REMAKKS.^-Compare Nasse's article "Blut" in the Hundwb'rterbuch der Physiol., 

 Bd. i.p. 75, and Milne Edwards, Lemons sur Tanat. et laphysiol. comparee, Paris, 1857, 

 tome i. p. 36 ; as also the various handbooks of histology. 



66. 



If we examine the anatomical composition of the blood with the aid 

 of a high microscopic power, we find it to be made up of a transparent 



