416 PROPERTIES AND FUNCTIONS 



So when an object strikes the eye, or sound the ear, the 

 impression is communicated to the brain, and we then 

 see or hear. The will in like manner conveys its stimu- 

 lus along the nerve to the voluntary muscles, and imme- 

 diately they contract, and motion is produced. Three 

 things are necessary either for the mind to be acted upon 

 by external objects or in its turn to act upon them. As 

 it regards the senses there must be, 1st, an external sen- 

 sitive organ adapted to the property of the body to be 

 ascertained. 2d. There must be a nervous cord to trans- 

 mit this sensation to the central mass; and 3d, there 

 must necessarily be a central organ to perceive. As it 

 regards the action of the wilt upon the voluntary muscles, 

 there is an organ to will, a nervous cord to communicate 

 it to the muscle, and consequently a muscle to contract. 



To complete therefore the nervous system, three dis- 

 tinct organs are found in man and the most perfect 

 animals. For example, in the eye we have an optical 

 instrument, situated before a nervous agent to collect the 

 image of the object to be perceived and to picture this 

 upon the retina, the brain being made sensible of the 

 impression of this image by the agency of the optic nerve, 

 which is a medium of communication between the sense 

 and sensorium. An animal deprived of the external 

 senses may be said to live within himself, as he is desti- 

 tute of all communication with the world around him. 

 To him color, sound, heat and cold give no pleasure, 

 neither do they produce pain. The lower classes of 

 animals, as for example the zoophytes and worms, gene- 

 rally possess but one rudimentory sense, viz. that of feel- 

 ing, which is exercised by an imperfectly organized in- 

 tegument covering the body. The nervous system is, 

 strictly speaking, the most striking peculiarity of animals, 

 and constitutes more especially what is called the animal 

 life and the life of relation, by means of which the animal 

 extends his relations and lives not only within himself 

 but in connexion with surrounding objects. 



The human brain exceeds that of the most perfect 

 animals in the number and perfect development of its 

 parts, none being found in any animal which man has not, 

 while several parts found in the brain of man are either 



