SENSORIAL FUNCTIONS. 161 



^ sioiogists under the name of the medulary substance. 

 This substance composes the greatest proportion of the 

 brain, spinal marrow, and nerves. These together, com- 

 pose that set of organs generally known under the name 

 of the nervous system. 



The brain is the primary and essential organ of sen- 

 sation, the nerves and spinal marrow being the instru- 

 ments, or media by which external impressions are con- 

 veyed to this source of perception. 



The nerves are bundles of white filaments or threads, 

 which, like the blood vessels, are divided into branches, 

 and finally into very minute fibres, which, in some instan- 

 ces are distributed to every part of the system ; there 

 being for instance not a portion of the skin which can 

 be touched by the finest point, where there is not a 

 nerve. 



The nerves thus pervading the whole system, are 

 those of touch ; and they are universally present in all 

 orders of animals, however low in the scale of exis- 

 tence. In the Mollusca and Polypi, these appear to be 

 the only organs of sensation, since no external cause 

 applicable to the other senses have the least effect upon 

 them. They close their shells, or recede when touched, 

 but often exhibit no other signs of life. 



As we rise higher in the scale of animal existence, 

 we find that the different orders are furnished with a 

 greater number of these instruments of sensation. Thus 

 some races have not only the nerves of touch but those 

 of sight, those of hearing and smell being denied them. 

 In the next step of organization, we may find all these, 

 w r ith perhaps the absence of taste ; and it is only when 

 we examine the highest orders of animals, that we find 

 the senses of touch, taste, smell, seeing and hearing, all 

 combined in the same animal. 



The appearance of the nerves are every where simi- 

 lar, those of the touch, or taste, or smell, not being dis- 

 tinguishable from each other, except by tracing them to 

 the organs of perception to which they refer. 



What composes the nervous system'? What part is the origin or 

 source of sensation! How are sensations conveyed to the brain 1 HOMT 

 do the nerves appear 1 What is said of the nerves of touch 1 



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