404 INNEBVATION. [CHAP. xiv. 



cated in its impressions and mechanism. On these accounts it will 

 be the first treated of. 



The nerves of touch are the same, or at least are derived from 

 the same part of the cerebro-spinal centre as those of common sen- 

 sation. They are the posterior roots of the spinal nerves, and some 

 fibres of the eighth and fifth encephalic nerves. The peripheral organ 

 of touch to which they are distributed is a tissue everywhere diffused 

 over the sentient surface, but which in most situations is elevated 

 into papillae more or less distinct from one another, and closely set, 

 according to the tactile power. The nerves of touch are remark- 

 able for the ganglia which are formed upon them on their emer- 

 gence from the vertebral canal, and for the subsequent admixture 

 with most of them of nerves of motion. In these respects they 

 differ from those of the other special senses, except taste, which 

 ranks next to touch in the ascending scale. 



In accordance with our general plan, we shall commence with an 

 anatomical description of the skin, which is the principal seat of 

 touch ; and it will be convenient to include with it an account of 

 the various glands and appendages found in connexion with this 

 organ, whether they have any relation to the sense in question or 

 not. We must premise, however, that this external integument is 

 a part only of a great physiological system, which comprehends also 

 the mucous membranes, and the true or secreting glands ; all of 

 which, taken together, and reduced to their most simple expression, 

 are a continuous membrane, more or less involuted, more or less 

 modified in the elementary tissues which compose it or are in con- 

 nexion with it, and within which all the rest of the animal 

 contained. This expanse consists of two elements; a base) 

 tissue composed of simple membrane, uninterrupted, homogeneous, 

 and transparent, covered by an epithelium, or pavement, of nucleated 

 particles. Underneath the basement membrane, vessels, nerves 

 and areolar tissue are placed (p. 47). 



The sense of touch exists only in those regions of this great sys- 

 tem which are exposed to the contact of foreign bodies, and where 

 it is essential to the comfort or preservation of the animal that the 

 presence and qualities of external objects should be perceive 

 These regions, however, demand a greater protection, for the same 

 reason ; and hence it happens that the development of this sense 

 is found to be generally accompanied with the most remarkable 

 increase and transformation of the epithelial element, and of the 

 areolar tissue lying under the basement membrane. In the skin 

 the thick and hard epithelium is termed cuticle or epidermis, and 



