DETECTION OF PHYSIOLOGICAL ANISOTROPY 113 



two points of an anisotropic organ may be simultaneously 

 excited, under a series of stimuli of uniform or increasing 

 intensity. 



This mode of excitation, by thermal shocks, will be found 

 in every way satisfactory and convenient. The Thermal 

 Variator, by which stimulation is effected, consists of a spiral 

 of german-silver wire, the diameter of the spiral being about 

 3 cm. The electrical circuit, through which the heating- 

 current is sent, is closed periodically for a definite length of 



FIG. 81. The Thermal Variator 



The anisotropic tissue-petiole of Musa is held in ebonite clip, c. E, E f , 

 electrodes connected with opposite sides. Specimen after adjustment 

 pushed inside heating- spiral, T, by slide, s. Spiral heated periodically 

 by closure of electric circuit by metronome, M. 



time, by means of a metronome (fig. 81). The thermal 

 variation within the coil can be controlled by a suitable 

 adjustment of the battery-power, or by the duration of 

 closure, or both. The experimental tissue is held in an 

 ebonite clip, C, fixed on a slide, S, on the same stand as the 

 heating-spiral. This slide is pulled out for the purpose of 

 adjustment. Square or circular pieces of wetted muslin make 

 contacts with equal areas on two opposite sides of the 

 experimental tissue, these pieces of cloth being connected 

 with non-polarisable electrodes, E a_nd E.' After the adjust- 



I 



