168 Rev. P. Keith on the Susceptibilities of Living Structures. 



and the divided poi'tions will immediately contract towards 

 their points of insertion. Exhaust the lungs of air, and the 

 cells will immediately collapse. But though the contractility 

 or elasticity in question i-esides in the tissues of living fabrics, 

 it does not depend solely upon their life ; for their death does 

 not destroy it. It depends rather upon the texture of the 

 parts in which it inheres, and upon the arrangement of their 

 integrant molecules, and is not destroyed but by their putre- 

 faction. It is not even peculiar to living fabrics; for many life- 

 less and inorganic bodies possess it in a greater or in a less de- 

 gree, particularly such as are notably elastic, as caoutchouc or 

 Indian rubber. It seems to be merely a recovering of the 

 natural state of the fabric when its extension or compi'ession 

 ceases, and a regaining of its former shape or size. Hence the 

 recoil of a bent bow, and the shrinking of a muscle that is 

 divided by a transverse section even after death. 



Contraction par racornissement, or by crispation, — that is, 

 the peculiar shrinking up of organized substances when sud- 

 denly exposed to the action of fire or of highly concentrated 

 acids, — is merely a variety of tissual contractility confined to 

 animal fabrics. 



II. Vital Susceptibility. — Vital susceptibility, or tonicity, is 

 that capacity of the organs of vegetables, or of the nutritive 

 organs of animals, which enables them to receive and to obey 

 the impulse of s^/wzi/?', of the impression and operation of which 

 the individual knows nothing. The stimulant may be mecha- 

 nical, as in the case of the collapsing of the leaves of Dioficca 

 Muscipula if a fly but alights on them. It may be chemical, 

 as in the case of the lacteals when stimulated to action by the 

 contact of chyle. It may be visible, as when the sensitive plant 

 shrinks from the touch of the finger. It may be invisible, as 

 when an organ is excited by means of the agency of galvanism, 

 or stimulated to action by a volition : or, lastly, it may be al- 

 together unknown. 



If the impression is occasioned by the gentle operation of 

 natui'al stimuli, that is of stimuli suited to the constitution 

 of the organ, impelling it to sound and healthy action, and 

 terminating merely in the organ affected, without being con- 

 veyed to any common centre of feeling or organ of sensation, 

 it is a case of vital excitability, and is imperceptible. The 

 result of the impression may be either the extension or con- 

 traction of the organ, as the case may be ; for these opposite 

 conditions mutually involve one another, as well as a mean or 

 middle condition, in which the organ may be quiescent. In 

 animals it is the muscular fibre that is endowed with the sus- 

 ceptibility in question, the muscles that are thus affected being 



situated 



