OF THE NEBVOUS SYSTEM. 91 



and you will soon discover that some are obviously volun- 

 tary, or directed according to the will of the animal. Another 

 class of movements may also be observed, over which the 

 animal does not seem to possess the same influence ; these 

 are the involuntary. These phenomena imply contractility 

 and volition ; but there is to be added another remarkable 

 faculty, sensibility, by which the animal perceives the pre- 

 sence of surrounding objects, and becomes conscious of their 

 presence. 



These three faculties seem to be common to all animals ; 

 but there are others. Certain animals construct with the 

 most admirable art dwellings for their young and for them- 

 selves, and this they do independent of all instruction from 

 the parent. Others proceed on distant voyages and journeys, 

 traversing the air as certainly as if the point to be attained 

 were before their eyes. To this faculty the name of instinct 

 has been given; it leads animals to perform certain acts 

 which are not the effects of imitation, and which are not the 

 result of reasoning. The phenomena are sometimes very 

 simple, and sometimes incomprehensibly complex. 



To other beings are given the faculty of recalling previous 

 sensations, of comparing them with each other, analysing the 

 past and present, and drawing conclusions; these are the 

 intellectual faculties. 



Finally, some animals can communicate to others a know- 

 ledge of the ideas they possess, by movements or sounds. 



These varied phenomena, by which animals place them- 

 selves in relation with others, may be reduced to six prin- 

 cipal faculties, namely, sensibility, contractility, will, in- 

 stinct, intelligence, and expression. 



In the simplest animals, these various faculties of the life 

 of relation are not the appanage of any organ in particular ; 

 but in man and in the immense majority of animals, the 

 exercise of these faculties is dependent on the action of the 

 nervous system. 



* a, brain; b, little brain; c, spinal marrow; d, facial nerve; e, brachial 

 plexus, caused by the union of several nerves coming from the spinal mar- 

 row ; f, median nerve ; g. cubital nerve ; h, internal cutaneous nerve of the 

 arm ; i, radial and musculo-cutaneous nerve of the arm ;.;', intercostal nerves ; 

 Ic, femoral plexus ; I, sciatic plexus ; m, tibial nerve ; , external peroneal 

 nerve; o, external saphenous nerve. 



