408 1NNERVATION. [CHAP. XIV. 



their bodies, from the medium in which they live, are provided with 

 a cutis of extraordinary toughness and density, as well as with a 

 growth of subcutaneous fat, called blubber, of prodigious thickness. 



Fat occurs very generally in the subcutaneous areolar tissue, 

 serving as a soft bed on which the skin may rest, and giving round- 

 ness and symmetry to the outline of the body. 



It is on the exterior surface of the cutis that the tactile papilla 

 are developed ; and it is here to be remarked that there is no neces- 

 sary relation between the degree of their development, and of 

 that either of the dermoid framework which supports, or the cuticle 

 which covers them. It is true that in the palm and sole all these 

 attain a large size, but, in the back, the tactile organ is well-nigh 

 absent, though the cutis is dense ; and in the tongue, on the con- 

 trary, this organ is highly developed ; while the areolar framework 

 is nothing more than a very thin expansion ; and the investment of 

 cuticle is so thin that the papillae form separate projections from the 

 surface. On the buccal surface of the lips and cheeks, too, the 

 cuticle is comparatively thin. 



In all parts of the cutaneous surface, as well as in some portions 

 of the internal mucous tracts, common sensation exists, attended 

 with a feeble discriminating power, which must be regarded as the 

 lowest condition of the sense of touch ; but the organ peculiarly 

 fitted for receiving tactile impressions is concentrated in a very 

 remarkable manner in certain portions of the integument, which in 

 other respects, whether from the precise and varied movements 

 they can perform, or from their peculiar position, are the best 

 adapted to be inlets of this kind of sensation. The palmar surface 

 of the hand and fingers, or the sole of the foot, may be selected for 

 description, as presenting the most highly developed form of the 

 organ of touch. 



The integument in these regions is finely and regularly furrowed 

 by grooves, separated from one another by corresponding ridges. 

 The direction of these grooves and ridges is various ; they run in 

 sweeping curves, frequently branch to adapt themselves" to the 

 inequalities of the general surface, and differ somewhat in width 

 and distinctness. These lines indicate the arrangement and develop- 

 ment of the tactile organ below. Each ridge is produced by a 

 single or double row of elongated conical processes, termed papillae, 

 projecting from the surface of the cutis into the epidermis. The 

 grooves are occasioned by the epidermis sinking in to occupy the 

 intervals between the rows af papillae. The papillae in each row 

 are usually arranged in pairs, the intervals between which are 



