lYO COMPARATIVE PHYSIOLOGY. 



tion, by whicli a substance, proteid in nature, known as fibrin, 

 is formed, the antecedents of which are probably very variable 

 throughout the animal kingdom, and are likely so even in the 

 same group of animals, under different circumstances ; and a 

 substance abounding in proteids (as does also plasma), known 

 as serum, squeezed from the clot by the contracting fibrin. It 

 represents the altered plasma. 



Certain well-known inorganic salts enter iato the composi- 

 tion of the blood — ^both plasma and corpuscles — but the princi- 

 pal constituent of the red corpuscles is a pigmented, ferrugi- 

 nous proteid capable of crystallization, termed haemoglobin. It 

 is respiratory in function. 



