SUMMARY 163 



quite as much as the epidermis, as showTi by the drawings and 

 photographs of sections of the mammalian skin, it is shown that 

 there is a structure which is, to a great extent, pecuhar to the 

 ventral surface of the hand and foot. This highly speciahsed 

 form of papillary layer of the corium can have no raison d'etre, 

 but that of increasing the dehcacy of the sense of touch. As 

 before stated there are certain other regions, such as the nipples, 

 edges of eyehds, and on the tip of the tongue where the papiUaB 

 are highly developed ; but in the greater part of the skin of the 

 body these are httle developed and the peculiar distribution of 

 Meissner's tactile corpuscles on the human hand is significant 

 of the great importance of the skin of this region as the organ 

 of touch. The very fact that the above three or four regions 

 are all that can be enumerated where the papilla) are highly 

 developed, makes the high degree of development of the papil- 

 lary layer of the corium of the hand and foot, all the more striking. 



There are certain established principles concerning the sense 

 of touch in man, to which reference should be made and which 

 will serve as indications of similar principles in the lower animals, 

 though as yet these have not been demonstrated at all fully. 



It has been well said that the sldn is a mosaic of tiny sensorial 

 areas, for minute examination of its surface shows that there are 

 small but definite intervals between sensory spots, where no 

 sensation is produced by a fine stimulus. Sensory spots of at 

 least four kinds have been distinguished ; touch-spots, cold-spots, 

 warmth-spots, and pain-spots, and it is probable that the diiier- 

 ence between these separate classes of sensory areas depend upon 

 the distribution in them of the five or six kinds of end-organs of 

 the skin and subcutaneous tissue which have been identified. 

 In the root- sheaths of hairs there is a fine network of nerve - 

 fibrils and also in the epidermis among the epithelial cells, a 

 plexus of non-medullated nerve-fibrils. The loss of the tactile 

 function of the hairs on the hairless palmar and plantar surfaces 

 of the hand and foot is more than compensated by the still more 

 delicate papillary ridges and highly developed papillae of the 

 corium. It is held, also, that in order to produce the necessary 

 stimulus for the organs of touch, there must be a deformation of 



