564 JOHN BERRY HAYORAFT. 



tissue outside the bone was found, in confirmation of this 

 surmise, to contain sensory nerve-fibres, for the application of 

 acetic acid, or of an interrupted galvanic stimulus, caused 

 definite reflex defensive movements, similar to those which follow 

 the application of acid to the frog's skin. 



Inasmuch, therefore, as sensory nerves evidently end quite 

 superficially, it became an interesting question to determine 

 their exact mode of termination in the curiously modified tissues 

 of the carapace. Portions of the carapace, generally taken 

 from the region of the costal plates, were softened in chromic 

 acid and nitric acid fluid, frozen, and cut with a thick-bladed 

 razor. In this way one can obtain fairly thin sections even of the 

 tough tortoise-shell. The sections were treated in various 

 ways, with a view of demonstrating nerves or nerve termina- 

 tions, and in no case was I able to discover any nervous struc- 

 ture in the tortoise-shell itself. 



In the subjacent connective- tissue layer, however, were 

 bodies which I, at first sight, thought were end organs (PI. 

 XLIII, fig. 1). They turned out to be the transverse sec- 

 tions of curiously modified nerve-fibres. These nerve-fibres 

 are easily distinguished from the blood-vessels (which in this 

 situation are devoid of a muscular coat) by their solid appear- 

 ance (fig. 2), strong connective-tissue covering, and by their 

 occasional transverse section, which is very characteristic 

 in appearance. 



Fig. 1 represents, in transverse section, two of these fibres 

 bound together by a common sheath of connective tissue (h). 

 Each fibre consists of an external layer of concentrically 

 arranged connective tissue, consisting of laminae of colloid 

 granular material with intervening connective-tissue corpuscles 

 (m). "Within this is a colloid-looking core (k), devoid of nuclei, 

 and also staining pink with picro- carmine. 



In the centre of the core is generally to be found a small 

 spot, probably an axis- cylinder, somewhat difi'ereutiated from 

 the rest of the core (g). 



No trace of medullary matter is to be found in connection 

 with any of these nerves, nor are ordinary medullated fibres 



