INTRODUCTION. XX111 



I. Division. VERTEBRATA. 



The arrangement of the nervous centres in the highest or 

 Vertebrate division indicates the greatest possible concentra- 

 tion and development. The ganglionic masses assume a 

 very great proportionate size when compared with the nerves 

 which emanate from them, and are principally united into a 

 long chain denominated the cerebro-spinal axis or cord, which 

 is inclosed in a cartilaginous or bony canal occupying the 

 dorsal region of the animal. The anterior extremity of the 

 cerebro-spinal axis is made up of those ganglia which are 

 more especially in relation with the principal senses and the 

 higher powers of intelligence, forming a mass denominated, 

 from its position in the skull which incloses it, the encephalon. 

 It is with the increased proportionate development of this 

 portion that the intelligence of the animal becomes augment- 

 ed ; in the lower tribes the cerebral masses scarcely exceed in 

 size those which form the rest of the central chain of ganglia, 

 but in advancing from fishes towards the higher forms of the 

 Vertebrata, they are observed to preponderate more<and more 

 in bulk, until at last in man they assume that extraordinary 

 development adapted to the exalted position which he is des- 

 tined to occupy. It is in the cerebral ganglia, therefore, that 

 is found the representative of the supra-oesophageal masses of 

 the articulated and molluscous classes, which, as already ob- 

 served, preside especially over the senses, and correspond in 

 their proportions with the capabilities of the tribes of animals 

 included in those divisions. The spinal cord, as the rest of 

 the central axis of the nervous system.of Vertebrata is denom- 

 inated, is made up of a succession of ganglia, in communica- 

 tion with symmetrical pairs of nerves connected with them, 

 and which preside over the generally diffused sense of touch 

 and the voluntary motions of the body. But besides the 

 cerebro-spinal system, there is in the vertebrated classes an- 

 other set of nervous centres to which nothing corresponding 

 has been satisfactorily identified in the lower divisions ; name- 

 ly, the sympathetic system, which mainly controls the involun- 

 tary movements of the body connected with the vital functions. 



The Vertebrata are further distinguished by the possession of 

 an internal organized skeleton, composed of either cartilage or 

 bone, made up of several pieces, serving to support the frame, 

 and forming a series of levers upon which the muscles act. 



