SECT. T. NERVOUS SYSTEM. 9 



external impressions, but without sensation, and is en- 

 tirely independent of the will ; the vegetative functions 

 of respiration, the contractions of the heart, circulation 

 of the blood, and digestion, are carried on under every 

 circumstance, even during sleep. The reason of their 

 being independent of sensation and the will is, that 

 the nerves in the organs performing these functions 

 never reach the brain, which is the seat of intelligence 

 and sensation, but they form what is called the reflex 

 system ; for any impressions made upon them are 

 carried to the upper part of the spinal cord alone, and 

 are reflected back again to the muscles of the heart, 

 lungs, &c., which, by their contractions, produce these 

 involuntary motions. For instance, the flow of blood 

 into the cavities of the heart while dilating, acts upon 

 the nerves, and these excite a rhythmical movement in 

 the muscular fibres of the heart. For there is a vital 

 contractility in muscular tissue which is one of the 

 most universal attributes of living beings, and is pro- 

 bably the sole cause of motion in the lowest grades of 

 life, and the movements produced by it in the higher 

 grades are in all cases the most directly connected with 

 the vegetative functions. The involuntary reflex system 

 of nerves constitutes the chief locomotive power in a 

 number of the lower animals ; but it forms a continually 

 decreasing portion of the whole nervous system in pro- 

 portion as animals rise in the scale of life, till in man 

 its very existence has been overlooked. If the spinal 

 cord were destroyed, instant death would be the conse- 

 quence; whereas infants born without brain have sucked 

 and lived for a day or two. 



There are numerous actions, especially among the 

 lower animals, as little under the influence of the will or 

 intelligence as the reflex nerves, which nevertheless de- 

 pend upon sensation for their excitement. The sensa- 

 tion may call the muscular apparatus into action with- 



