THE OUTER TISSUES OF ANIMALS. 299 



and producing, more frequently, thickening of the epidermis.* 

 It is no doubt true that the sensation which pressure causes, 

 propagated to the spinal chord, and reflected thence through 

 the vaso- motor nerve going to the spot, aids the process 

 by exciting a wave of contraction along the minute arteries, 

 thereby helping them to refill the capillaries the instant the 

 pressure is taken off; and doubtless, as alleged, the excessive 

 exudation that forms a blister when the intermittent com 

 pressions are violent and long-continued, is attributable to 

 this reflex nervous action. But it is clear that the nervous 

 action is secondary, and cannot of itself produce the effect ; 

 for m the absence of intermittent pressure no exudation takes 

 place, however acute and persistent the sensation may be. 

 Continued pressure produces absorption instead of exudation. 

 In animals therefore, as in plants, the external mechanical 

 actions to be resisted, are themselves directly instrumental in 

 working in the tissues they fall upon, the changes which fit 

 those tissues to meet them. And it needs but to contemplate 

 the process of thickening described, to see that it will go on 

 until the shield produced suffices to protect the capillaries 

 from excessive pressures will go on, that is, until there is 

 equilibrium between the outer and inner forces. 



294. Dermal structures of another class are developed 

 mainly, if not wholly, by the actions of external causes 

 on species rather than on individuals. These are the 



* An inquiry into the causes of these differences of result, brings further 

 evidence to light. The condition under which only the hypertrophy can 

 arise, is that the pressure intermits sufficiently to allow the capillaries to 

 refill frequently. The epidermis thickens where the pressures are habitually 

 taken off so completely, that the capillarns next the surface can refill, as in the 

 hands. If we consider what happens where the instep is pressed by a tight 

 boot, we shall see that the variations of pressure which occur in walking, dc 

 Hut suffice to relieve the quite superficial vessels and allow them to refill ; but 

 in consequence of the slight mobility and elasticity of the tissues, the vessels 

 at some distance beneath the surface are able to refill, and hence the thicken 

 ing occurs round them. 



