66 THE VERTEBRATED ANIMALS. 



each other, all of which have a cylindrical opening 

 in the middle, forming together a canal -which con- 

 tains that general mass of the nervous system, called 

 the spinal marrow. The spine frequently passes 

 the lower extremity, and is prolonged into a tail. 



The ribs are semicircular, and secure the sides of 

 the cavity of the trunk. They are most commonly 

 articulated by one extremity to the vertebrated 

 chain, and by the other to the sternum. In some 

 species they are scarcely visible. 



They have never more than two pair of limbs. 

 Sometimes, indeed, one or other of these pairs is 

 deficient, and sometimes both. They assume such 

 forms as are adapted to the peculiar motions they 

 have to perform: thus the anterior limbs may be 

 converted into hands, feet, wings, or fins; the 

 hinder into hands, feet, or fins. 



The blood of these animals is always red, and 

 seems, by its composition, adapted to sustain the 

 energy of sensation, and the muscular vigour which 

 characterize them. The different degree, however, 

 of this character of their blood, and its corre- 

 spondence with the portion of respiration necessary 

 to the several kinds of vertebrated animals have sug- 

 gested the subdivision of them into four classes. 



The external organs of sense in all the verte- 

 brated animals are two eyes, two ears, two nostrils, 

 the teguments of the tongue, and the teguments of 

 the entire body. The nerves run into the marrow 

 through apertures in the vertebrae, and in the cra- 

 nium. They all appear to unite in a double bundle, 



