VOLUNTARY MOTION. 493 



voluntary muscles in which we have indulged may be performed 

 against our wishes in particular circumstances. 



Between the portion of the brain that wills and the muscle, 

 an intermediate portion of the nervous system exists. The will 

 is not exerted upon the muscles, but upon the motor nerves of 

 muscles at their extremity in the brain or spinal chord. Now 

 this extremity or the nerve in any part of its course may be 

 stimulated by other causes than the will, and thus the muscles 

 ordinarily stimulated by the will may be stimulated without it, 

 the will may neither voluntarily nor involuntarily have a share 

 in stimulating the muscles. Thus it is in chorea %, tetanus, and 

 other spasmodic diseases of the voluntary muscles. Even in 

 palsy of motion, we often observe spasms, especially in para- 

 plegia. Now, when a motion is naturally willed in conse- 

 quence of a sensation in a particular part, a peculiar relation 

 exists between the nerves of sensation of that part and its nerves 

 of motion, so that irritation of the former is communicated to the 

 latter. Thus at page 4-20. we saw that in 1788 Sir Gilbert 

 Blane found the hind legs of a decapitated kitten retract if the 

 paws were touched with a hot wire, and the tail move if so 

 touched after division of the chord below the last lumbar 

 vertebra. In cold blooded animals similar observations had 

 long before been made. Redi in 1687 h found in a large tor- 

 toise, which lived twenty-three days after losing its head, that 

 the fore and hind feet were forcibly convulsed whenever they 

 were pricked. Whytt, in 174-5, found that, if the toes of both 

 feet of a decapitated frog are stimulated, the feet are drawn up 



8 The exciting cause that influences the nervous system beyond the cerebral 

 part which wills may be opposed, and temporarily with success, by the will ; and on 

 the other hand the will may aid the exciting cause. So that under chorea persons 

 can frequently arrest the motions for a few seconds, or run when they cannot 

 walk ; and, again, they often seem to feel a pleasure in co-operating to produce 

 the morbid movements. 



h I mentioned, supra, p. 421., that, above a century and a half ago, Duverney 

 found a bird would move after losing its brain ; Dr. Kaau, in 1745, observed 

 a frog move all its limbs for half an hour after decapitation, and for a con- 

 siderable time after its body was divided in two. A viper, after losing its 

 head and bowels, moved towards a heap of stones where it had been accustomed 

 to hide itself. (Iinpetum faciens, No. 331.) Iledi extracted the brain of a 

 land tortoise through a hole in the skull, and it lived from November to May, 

 moving and walking about to the last. (Osservazioni intorno agli animali viventi, 

 &c. Napoli. 1687. p. 209. sq. Butterflies copulate and lay eggs after decapitation. 



