20 INTRODUeTION. 



The action received by the external organs is continued by 

 the nerves to the central masses of the nervous system^ which, 

 in the higher animals, consists of the brain and spinal mar- 

 row. Tiie more elevated the nature of the animal, the more 

 voluminous is the brain and the more is the sensitive power 

 concentrated there ; the lower the animal, the more the me- 

 dullary masses are dispersed, and in the most imperfect genera, 

 the entire nervous substance seems to melt into the general 

 matter of the body. 



That part of the body which contains the brain and princi- 

 pal organs of sense, is called the head. 



When the animal has received a sensation, and this has oc- 

 casioned volition ; it is by the nerves, also, that this volition is 

 transmitted to the muscles. 



The muscles are bundles of fleshy fibres whose contractions 

 produce all the movements of the animal body. The exten- 

 sion of the limbs and every elongation, as well as every flexion, 

 and abbreviation of parts, are the effects of muscular contrac- 

 tion. The muscles of every animal are arranged, both as re- 

 spects number and direction, according to the movements it 

 has to make; and when these motions require force, the 

 muscles are inserted into hard parts, articulated one over an- 

 other, and may be considered as so many levers. These parts 

 are called bones in the vertebrated animals, where they are in- 

 t-ernal, and are formed of a gelatinous mass, penetrated by par- 

 ticles of phosphate of lime. In the Mollusca, the Crustacea, 

 and Insects, where they are external, and composed of a cal- 

 careous or horny substance that exudes between the skin and 

 epidermis, they are called shells, crusts and scales. 



The fleshy fibres are attached to tlie hard parts by means 

 of other fibres of a gelatinous nature, which seem to be a con- 

 tinuation of the former, constituting what are called tendons. 



The configuration of the articulating surfaces of the hard 

 parts limits their motion, which are also restrained by cords or 

 envelopes, attached to the sides of the articulations, called 

 ligaments. 



It is from the various arrangements of tliis bony and mus- 

 cular apparatus, and the form and proportion of the members 



