2 3 2 VERTEBRATA. 



form and mechanism so conspicuously met with in this great 

 division of the animal world. 



The nervous system of the Vertebrates consists of the brain 

 contained within the cavity of the skull, continued from which, and 

 lodged in a canal formed by the back-bone, is the spinal marrow 

 or spinal cord, whence are derived, at intervals, symmetrical pairs 

 of nerves : these escape from the spinal canal by appropriate ori- 

 fices, situated between the different bones of the vertebral column, 

 and are distributed to the voluntary muscles, and to the integu- 

 ment of the two sides of the body, thus constituting the medium 

 whereby the intimations of the will are communicated to every 

 part, and information received from the external world. The 

 nervous system may thus be compared to an electric telegraph, 

 of which the brain is the central office, and the nerves the wires, 

 along which travel with inconceivable rapidity the various com- 

 mands and reports from all parts of the complex system. 



With the increased development of the nervous system in the 

 Vertebrate classes, the organs of the senses assume a proportionate 

 perfection of structure. The eyes, now invariably two in number, 

 are lodged in cavities formed for their reception in the bony 

 framework of the face. The auditory apparatus, of which only 

 rudiments exist in the lower animals, gradually becomes more 

 and more completely developed. Organs of smell of variable 

 construction are generally present. The tongue becomes slowly 

 adapted to appreciate and discriminate savours, and the sense of 

 touch is especially conferred upon organs of different kinds pecu- 

 liarly fitted to exercise the faculty. Thus, with increased intelli- 

 gence, higher capabilities of enjoyment are allotted, and sagacity 

 developes itself in proportion as the nervous centres expand. 



The blood of all the Vertebrate races is red, and contains micro- 

 scopic corpuscles of variable form and dimensions in different 

 animals. In the class of fishes, owing to the as yet imperfect 

 condition of the respiratory apparatus, the temperature of the 

 body is scarcely higher than that of the surrounding medium; 

 and even in reptiles, such is the languid condition of the circu- 

 lation, and the incomplete manner in which the blood is exposed 

 to the renovating influence of oxygen, that the standard of animal 

 heat is still extremely low ; but in the higher classes, the birds 

 and mammals, the effect of respiration is increased to the utmost, 

 and pure arterial blood being thus abundantly distributed to all 

 parts, heat is more rapidly generated, the warmth of the body 

 becomes considerably increased, and such animals are perma- 



