166 Rev. P. Keith om the Susceptibilities of Living Structures. 



may be owing to that part of the leflection of the ethereal 

 waves from the atoms, which is accompanied by change of 

 density; — not from the atoms at the surface only, but from 

 those at finite depths, for thus may the total effect become of 

 sensible magnitude. It might be shown, on this supposition, 

 that the reflection will be a maximum when the thickness of 



the medium is — , or -— , or -^, &c. ; and that it will be 



when the thickness is A, or 2X, or 3 A, &c., and this may ac- 

 count for the loss of half an undulation. The mean effect of 

 the other part of the reflected velocity, which is unaccom- 

 panied by change of density, may be to diminish in a given 

 ratio the velocities of the ethereal particles as soon as the waves 

 enter the medium, without producing change of density: so 

 that if the equations 



V = a. Nap. log q = m sm — {x—at) 



applied to the waves before intromission, the equations 



w = -^ Nap. log g = -^ sm — {x~ ^ t) 



will apply after intromission. This effect depends on the di- 

 minution of the velocity being always in the same ratio ; and 

 this again depends on the particular constitution of the me- 

 dium which determines it to be transparent. 



Papwoith St. Everard, Feb. 13, 1832. 



XXI. Of the Susceptibilities of Livi7ig Structures. By the 

 Rev. Patrick Keith, F.L.S.* 



THE susceptibilities of living structures are demonstrated 

 to the eye, or to the understanding, by the peculiar and 

 palpable movements which all such structures exhibit, whether 

 spontaneously or through the intervention of stirnuli ,- whether 

 in the vital or in the cerebral system, or in the tissues compos- 

 ing them. Hence the susceptibilities of living structures are 

 either tissual, vital, or cerebral. 



I. TissualSusceptibiliiy. — Tissual susceptibility, or elasticity, 

 is the physical capacity of extension, and of spontaneous con- 

 traction, which is found to be inherent in primary tissues or 

 fabrics. The cellular tissue, from its great abundance in all or- 

 ganized bodies, affords a good and ready example. You may 

 easily detect it, by placing under the microscope a fine and mi- 



* Communicated by the Author. 



nute 



