753 



LECTURE LIX. 



ON ANIMAL LIFE. 



The functions of animal life are not only more complicated in the sanu' 

 individual than those of vegetation, but also more diversified in the different 

 classes into which animals are divided; so that the physiology of each class 

 has its peculiar laws. We are indebted to Linnd for the first enlargement of 

 our views of the different classes of animals, and perhaps for the most conve- 

 nient arrangement, of the animal kingdom ; although his method has never 

 been universally adopted by our neighbours on the continent. 



A considerable portion of the bulk of all animals is composed of tubular 

 vessels, which originate in a heart ; the heart propels through the arteries, 

 with the assistance of their own muscular powers, either a colourless transpar- 

 ent fluid, or a red blood, into the extremities of the veins ; through which it 

 again returns to the origin of its motion. Both insects, and vermes, or worms, 

 have their circulating fluids a little warmer than the surrounding medium, and 

 generally colourless ; but insects have legs furnished with joints, and worms 

 have nothing but simple tentacula at most in the place of legs. Fishes have 

 cold red blood, which is exposed to the influence of the air contained in water, 

 by means of their gills. The amphibia receive the air into their lungs, but 

 their blood is cold, like that of fishes, and in both these classes the heart has 

 only two regular cavities, while that of animals with warm blood has four ; the 

 whole contents of one pair being obliged to pass through the lungs, in order 

 to arrive at the other pair. Of animals with warm blood, the oviparous are 

 birds, and are generally covered with feathers, the viviparous are either 

 quadrupeds or cetaceous animals, and are furnished with organs for suckling 

 their young. 



Each of these classes of animals is subdivided by Linn6 into different 

 VOL. I. 4y 



