Rev. P. Keith on the Susceptibilities of Living Structures. 169 



situated internally; and being chiefly vascular, as the heart, 

 stomach, and intestines, occupying the cavities of the thorax 

 and abdomen, not symmetrical in their shape or form, not 

 obedient to the will, not attached to bones. In plants it is 

 the organs generally, whether central or superficial. Thus 

 the flowers of many plants, stimulated by the agency of light, 

 open their petals when the sun shines, and shut them when 

 he shines no longer. Thus the fluids that ai"e lodged in the 

 soil, exciting the susceptibilities of the extreme fibrile, or 

 spongeolee, of the root, are first admitted into, and then pro- 

 pelled and distributed throughout, the plant, carrying nutri- 

 ment to every part, and giving energy to every organ. Thus 

 also the stomach of animals is excited to action by the pre- 

 sence of aliment, and the heart by the ingress of blood ; and 

 thus are digestion, circulation, and nutrition maintained in 

 the fabric, without either our knowledge or our notice. The 

 stomach is indeed susceptible or sensible to the presence of 

 aliment, and the heart to the ingress of blood, and the flowers 

 and rootlets of plants to the access of light and of mois- 

 ture ; but the susceptibility is merely organic, as terminating 

 in the organ aff'ected, and is accompanied with no sensation 

 and no perception of change. This is the organic sensibility 

 of Bichat, having in the animal subject the heart for its prin- 

 cipal organ*. 



If the impression is occasioned by the violent operation of 

 un-natural causes, that is, of stimuli foreign to the constitu- 

 tion of the organ or fabric, or of natural stimuli in excess, it 

 is a case of vital irritability ; in the plant imijerceptible, — in 

 the animal, often perceptible. Thus although the leaf of 

 Dionaca Muscipula is not consciously sensible to the impression 

 of the fly that alights on it, nor the leaflets of Mimosa to the 

 touch of the finger, and though they cr^.nnot be irritated so as 

 to excite sensation ; yet the lungs, stomach, and intestines of 

 the living animal, though altogether insensible to the natural 

 action of chyle, food, or inhaled atmospheric air, are found 

 to be very irritable and very easily irritated by means of the 

 action of emetics, purgatives, or inhaled carbonic acid gas, — 

 stimulants that we must regard as un-natural; as well as by 

 means of the action of an enormous load of food that gluts 

 and oppresses them, a stimulant not un-natural, but admini- 

 stered in excess. 



III. CercbralSusceptibility. — Cerebral susceptibility is that 

 capacity of the sensorial organs of animals which enables them 

 to receive and to obey the impulse of stimuli and to transmit 



• Considerat. Gen. p. 53. 

 N.S. Vol. 11. No. 63. March 1832. Z the 



