1 T* Rev. P. Keith on the Susceptibilities of Living Structures. 



chiefly those that are composed of bundles of red, minute, and 

 parallel ribres, symmetrical, operating as levers, opposed by 

 antagonists, and often attached to bones. We do not at 

 present inquire into the mode of muscular contraction, we 

 merely avail ourselves of the fact. The brain wills, and the 

 volition is instantaneously conveyed through the medium of 

 the fit and appropriate nerves — the nerves of motion — to the 

 fit and appropriate muscle. The muscle by its contractility in- 

 stantaneously obeys the impulse, and the organ instantaneously 

 acts, whether it be the hand to handle, the foot to walk, the 

 ear to listen to the modulations of melody, or the throat and 

 mouth to utter articulate sounds. In either case the impulse 

 is accompanied with a consciousness of the state and degree 

 of the contractility of the muscle that is stimulated, or of 

 the operation that is performed : and hence the contractility 

 of volition, or the function of which it is the cause, is also 

 essentially perceptible. 



This is what Bichat calls animal contractility, in contra- 

 distinction to what he calls also organic contractility, — the 

 former originating in the will, the latter in tissues, or organs. 

 He regards them as corresponding to the two sensibilities, — 

 the organic and animal, — with this difference, that the one can- 

 not be transformed into the other, as we have seen that the 

 sensibilities may*. Yet this rule, though sufficiently general 

 and sufficiently well founded, is not without its exceptions. 

 For on the one hand, undue and violent contractions of the 

 voluntary muscles do often occur, or may be excited in the 

 system, as in the case of spasm, paralysis, convulsion, which 

 are not at all I'egulated by the operations of the will ; and, 

 on the other hand, it appears that there are some persons who 

 have the faculty of vomiting or regurgitating when they please, 

 — thus converting an involuntary into a voluntary movementf . 



The several species of the contractility of living fabrics are 

 thus briefly characterized by Bichat. — If a muscle is put into 

 action by a volition, it is animal contractility ; if by a physi- 

 cal or mechanical stimulus applied to it, it is organic sensible 

 contractility or irritability ; if by the contact of nutritive fluids, 

 it is insensible contractility or tonicity ; and if by a transverse 

 section, it is the contractility of tissuej. Such are the leading 

 susceptibilities that are discoverable in living fabrics, — the tis- 

 sual, the vital, and the cerebral : the two former common both 

 to plants and to animals, and connected chiefly with nutrition ; 

 the latter peculiar to animals, and connected with sensation. 

 [To be continued.] 



• Reckerchcs Phij.s. p. 73. f Magendie : by Milligan, p. 286. 



t Reckerchcs PIn/s p. 77. 



XXII. 0,1 



