SENSOK1AL FUNCTION'S. 157 



physiologists under the name of the medullary substance. 

 This substance composes the greatest proportion of the 

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

 compose that set of organs generally known under the 

 name of the nervous system. 



470. The brain is the primary and essential organ of 

 sensation, the nerves and spinal marrow being the instru- 

 ments, or media, by which external impressions are con- 

 veyed to this source of perception. 



471. The nerVes are bundles of white filiments, or 

 threads, which, like the blood-vessels, are divided into 

 branches, and finally into very minute fibres, which, in some 

 instances, 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, \vhere there is not a nerve. 



472. The nerves thus pervading the whole system, are 

 those of touch ; and they are universally present in all or- 

 ders of animals, however low in the scale of existence. 

 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 ex- 

 hibit no other signs of life. 



473. 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, 

 with 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. 



474. The appearance of the nerves are everywhere 

 similar, those of the touch, or taste, or smell, not being 

 distinguishable 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 of-source 

 of sensation? How are the sensations conveyed to the brain? How do 

 the nerves appear' What is said of the nerves of touch? 



