496 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



3. Functions of the Spinal Cord and its Nerves. 



874. In commencing our more detailed examination into the func- 

 tions of the different parts of the Nervous system in Vertebrated ani- 

 mals, it seems best to commence with the Spinal Cord ; this being the 

 portion whose presence is most essential to the continuance of life. As 

 already mentioned, Infants are sometimes born without any Cerebrum 

 or Cerebellum ; and such have existed for several hours or even days, 

 breathing, crying, sucking, and performing various other movements. 

 The Cerebrum and Cerebellum have been experimentally removed from 

 Birds and young Mammalia, thus reducing these beings to a similar 

 condition ; and all their vital operations have, nevertheless, been so 

 regularly performed, as to enable them to live for weeks, or even 

 months. In the Amphioxus, as already remarked, we have an example 

 of a completely-formed adult animal, in which no rudiment of a Cere- 

 brum or Cerebellum can be detected. And in ordinary profound sleep, 

 or in apoplexy, the functions of these organs are so completely sus- 

 pended, that the animal is, in all essential particulars, in the same con- 

 dition for a time as if destitute of them. It is possible, indeed, to re- 

 duce a vertebrated animal to the condition (so far as its nervous system is 

 concerned) of an Ascidian Mollusc ( 850) ; for it may continue to exist 

 for some time, when not merely the Cerebrum and Cerebellum have 

 been removed from above, but when nearly the whole Spinal Cord has 

 been removed from below, that part only of the latter being left, which 

 is the centre of the respiratory actions, and which corresponds to the 

 single ganglion of the Tunicata. On the other hand, no animal can 

 exist by its Encephalon alone, the Spinal Cord being destroyed or re- 

 moved ; for the reflex actions of the latter are so essential to the con- 

 tinuance of its respiration, and consequently of its circulation, that if 

 they be suspended (by the destruction of the portion of the cord which 

 is concerned in them), all the organic functions must soon cease. 



875. Although the Spinal Cord was formerly regarded as little else 

 than a bundle of nerves proceeding from the Brain, yet its true rank, 

 as a distinct centre of nervous power, is now universally admitted. 

 That the actions prompted by it, when these do not originate in one 

 of the higher centres, are of a purely reflex nature, consisting in the 

 excitement of muscular movements in respondence to external impres- 

 sions, without the necessary intervention of sensation, appears to be 

 a necessary inference from the facts that have been brought to light by 

 experiment and observation. Experiments on the nature of this func- 

 tion are best made upon cold-blooded animals ; as their general functions 

 are less disturbed by the effects of severe injuries of the nervous sys- 

 tem, than are those of Birds and Mammals. When the Cerebrum has 

 been removed, or its functions have been suspended by a severe blow 

 upon the head, a variety of motions may be excited by their appropriate 

 stimuli. Thus, if the edge of the eyelid be touched with a straw, the 

 lid immediately closes. If a candle be brought near the eye, the pupil 

 contracts. If liquid be poured into the mouth, or a solid substance be 

 pushed within the grasp of the muscles of deglutition, it is swallowed. 





