An Introduction to Zoology. 27 



The blood, then, becomes a fluid of immeuse importance in the animal 

 oeconomy, and to its formation, circulation, and renovation we shall 

 shortly see the principal solids of the body to be directly subservient. 



The blood varies considerably in colour, consistency, temperature, and 

 composition, in different animals ; or under different circumstances in the 

 same animal. It is either Avhite or red. In the Crustacea and moUusca, it 

 resembles a gelatinous lymph, totally dissimilar to that fluid which we re- 

 cognize as blood in the higher animals, but assuming, upon evaporation, 

 somewhat of a fibrous structure. In the insects, the blood is also colour- 

 less ; but in certain of the annelida, it is red. In the vertebrated animals, 

 fishes, reptiles, birds, and mammalia, the blood is also red ; but in the two 

 former it is not readily coagulable, nor does the coagulum, when formed, 

 bear any considerable proportion to the rest. In the birds and the mam- 

 malia, the blood is red, highly coagulable, and forming a large and firm 

 fibrous coagulum. 



The blood of the higher anitnals is found to exist under two conditions ; 

 before it has been submitted to atmospheric influence in the lungs, and ' 

 afterwards : being in the former case of a florid colour, and termed arterial j 

 in the latter, of a dark purple, and termed venous. The latter, however, 

 upon exposure to the atmosphere, assumes gradually the arterial hue. 



The blood of the higher animals, when permitted under ordinary cir- 

 cumstances to stand, separates into a central coagulum or clot, the cras- 

 samentum, and a surrounding straw-coloured fluid, the serum ; and if the 

 clot be submitted to the action of water, the colouring matter is washed 

 out, and the mass remains of a tough fibrous consistency, and a buft gray 

 colour. This substance is the same with that of which muscle is composed, 

 and is called ^6nn. The serum is found to consist almost wholly of albumen, 

 which coagulates on the application of heat or an acid, and leaves an un- 

 coagulable fluid termed the serosity of the blood. If that portion which, 

 as colouring matter, has been washed out from the crassamentum, be sub- 

 mitted to the microscope, it is found to be composed of a number of bodies 

 more or less spherical, called the " globules of the blood." 



In the lower vertebrated animals, as the fishes and reptiles, these globules 

 arc of larger size than in the mammalia, and their figure is oval ; and in some 

 cases it is said to be lenticular. The size of these globules in the birds is 

 still larger, and their form oval ; in the higher animals they are circular, and 

 gradually diminish in size, being at their minimum in some of the mammalia. 



These globules arc described as consisting of two distinct parts ; a 

 transparent vesicle, and a central solid body within it. 



'I'hc temperature of the blood, like that of the animals in whom it flows, 

 is variable ; all, it is probable, possess more or less animal heat, but the 

 quantity in the lower animals is too minute to be appreciated by our 

 ordinary means. 



Those aninials whose powers of producing beat arc feeble, and whose 



