154 BLOOD. 



astronomy were still incapable of interpreting the phenomena of 

 the animal organism It is only from a comparatively recent 

 period that we can date the first moderately accurate microscopical 

 investigations of the blood-corpuscles, and the first attempts to 

 investigate their origin, function and destiny, or an accurate and 

 systematic mode of analyzing the blood, &c. In the present 

 day the investigation of the blood has been conducted with the 

 most earnest attention and the most zealous activity; yet, notwith- 

 standing the devotion of so much labour, the theory of the blood 

 is yet in the first stage of its development. But it must be re- 

 membered that the mass of correctly or incorrectly observed facts 

 and of more or less ingenious hypotheses is abundant in propor- 

 tion to the recent date of a science and to its want of fixed and 

 reliable points of support. Such has been the fate of the theory 

 of the blood. Its right comprehension has been rendered nearly 

 impracticable amidst the accumulation of the innumerable en- 

 quiries which have been instituted in reference to its physical and 

 chemical relation in physiological and pathological conditions, and 

 amidst the multifarious and contradictory views promulgated 

 regarding its progressive and regressive metamorphosis and the 

 functions of its various constituents individually and collectively ; 

 so that it is now alike impossible to afford a clear and succinct 

 exposition of the theory of the blood, and to sift facts from con- 

 clusions, or the positive from the merely hypothetical. In a 

 chemical point of view this somewhat unpromising prospect must 

 be referred to an imperfect knowledge of the true basis of the 

 whole enquiry ; and all who have attentively followed our obser- 

 vations, in the first volume, on the protein-compounds, the mineral 

 substances of the animal body, the pigments, &c., will clearly 

 comprehend that no thorough knowledge of the blood can ever be 

 obtained until we shall succeed in throwing some degree of light 

 upon these obscure departments of zoo-chemistry. 



The blood, as it flows in the vessels of the higher animals, is 

 a somewhat tenacious fluid, heavier than water, and presents 

 various shades of red ; in the arteries, however, it is constantly 

 somewhat brighter than in the veins ; it is only transparent in 

 very thin layers. Immediately after its removal from the circula- 

 tion, it becomes more tenacious and gelatinous, and finally separates 

 into a firm, dense red mass, and a clear faintly yellow fluid. 



From accurate inquiries regarding the physical properties of 

 the blood, it has been ascertained that the specific gravity of 

 normal human blood averages 1'055, its physiological limits being 

 1-045 and 1'075 ; in women it is rather less than in men, and in 



