§ 4. NERVOUS SYSTEM AND SENSATION. 593 



determine, with any degree of precision, where animal life 

 terminates, and vegetable existence begins. The Lichen 

 which grows on the stone, and the Zoophyte attached to the 

 rock, present but little difference to the common observer : 

 both are permanently fixed to the spot on which they grow, 

 from the earliest period of their existence to their dissolu- 

 tion ; and in the vegetable dried by the heat of the sun, and 

 the coralline shrivelled up from the absence of moisture 

 during the ebb of the tide, he would seek in vain for those 

 characters which would assign the one to the vegetable, and 

 the other to the animal kingdom. 



4. Nervous system and sensation. — My limits will 

 not permit me to dwell on the distinctions which exist be- 

 tween animals and vegetables in their chemical composition, 

 nor on the different influence produced on the atmosphere 

 by their respective agency : and I must content myself with 

 explaining the remarkable endowment which is supposed to 

 be peculiar to animal organization, namely, that of sensation, 

 and to be dependent on that structure which is termed the 

 Nervous, or medullary system. 



When any object comes in contact with my finger I am 

 sensible of its presence, and the finger is said to possess 

 sensation : but if I compress, or cut across the nerve which 

 passes from the spinal marrow to the arm, this faculty of 

 sensation is suspended or destroyed: the same object may 

 come in contact with my finger as before, but no feeling is 

 excited indicating to me its presence. This phenomenon 

 is well known, for every one must sometimes, in lying or 

 sitting, have compressed the nerve of the arm or thigh, 

 and occasioned a temporary numbness and loss of accurate 

 feeling in the limb. I perceive, then, by my own ex- 

 perience, that sensation is inseparably connected with the 

 presence and condition of the nerves ; and that in Man and 

 the vertebrated animals, the nervous influence is developed 

 and transmitted by means of the brain and spinal marrow. 



