THE BLOOD. 



It is a matter of common observation that the loss of the 

 whole, or a very large part, of the blood of the body entails 

 death ; while an abundant haemorrhage, or blood-disease in any 

 of its forms, causes great general weakness. 



The student of embryology is led to inquire as to the neces- 

 sity for the very early appearance and the rapid development 

 of the blood-vascular system so prominent in all vertebrates. 



An examination of the means of transit of the blood, as 

 already intimated, reveals a complicated system of tubes dis- 

 tributed to every organ and tissue of the body. These facts 

 would lead one to -suppose that the blood must have a tran- 

 scendent importance in the economy, aiid such, upon the most 

 minute investigation, proves to be the case. The blood has 

 been aptly compared to an internal world for the tissues, an- 

 swering to the external world for the organism as a whole. 

 This fluid is the great storehouse containing all that the most 

 exacting cell can demand ; and, further, is the temporary re- 

 ceptacle of all the waste that the most busy cell requires to dis- 

 charge. Should such a life-stream cease to flow, the whole vital 

 machinery must stop — death must ensue. 



Comparative. — It will prove more scientific and generally 

 satisfactory to regard the blood as a tissue having a fluid and 

 flowing matrix, in which flow cellular elements or corpuscles — 

 a view of the subject that is less startling when it is remem- 

 bered that the greater part of the protoplasm which makes up 

 the other tissues of the body is of a semifluid consistence. In 

 all animals possessing blood, the matrix is a clear, usually more 

 or less colored fluid. Among invertebrates the color may be 

 pronounced : thus, in cephalopods and some crustaceans it is 

 blue, but in most groups of animals and all vertebrates the 

 matrix is either colorless or more commonly of some slight 

 tinge of yellow. Invertebrates with few exceptions possess 



