5 86 TEXT-BOOK OF PHYSIOLOGY. 



a resulting difference in quality or rapidity of the nerve impulses 

 which would impress or excite the non-specialized cortex in such a 

 way as to call forth the characteristic sensation. It is possible, how- 

 ever, that neither supposition is wholly correct, and that the char- 

 acter of the sensation depends on the construction and adaptation 

 of the entire sense apparatus to the character of the stimulus. 



Whatever the conditions for their origin and whatever their 

 characteristics, sensations in themselves do not constitute knowledge; 

 they are but elementary states of consciousness, raw materials out 

 of which the mind elaborates conceptions and forms judgments as 

 to the character of any given object in comparison with former 

 experiences. 



THE SENSE OF TOUCH. 



The physiologic mechanism involved in the sense of touch in- 

 cludes the skin and the mucous membrane lining the mouth, the 

 afferent nerves, their cortical connections, and nerve-cells in the 

 cortex of the parietal lobe and the gyrus fornicatus (Figs. 226, 227). 



Peripheral excitation of this mechanism develops nerve impulses 

 which, transmitted to the cortex, evoke sensations of touch and 

 temperature. To the skin, therefore, is ascribed a touch sense 

 and a temperature sense. Of the touch sensations two kinds may 

 be distinguished: viz., pressure sensations and place sensations. 

 With the contact of an external body there arises the perception 

 not only of the pressure, but also the perception of the place or 

 locality of the contact. In accordance with this, it is customary to 

 attribute to the skin a pressure sense and a location sense. 



The specific physiologic stimuli to the terminal organs in the skin 

 and oral mucous membrane are mechanic pressure and thermic 

 vibrations. 



The Skin. The skin, which constitutes the basis for the sense 

 of touch, covers and closely invests the entire body. It varies in 

 thickness and delicacy in different regions, though its structure is 

 everywhere essentially the same. As the physiologic anatomy of the 

 skin has elsewhere been detailed (page 435), it is only necessary to 

 state here that it is divided into a deep and a superficial layer. The 

 former, known as the derma, consists of an inner layer of rather 

 loose connective tissue and an outer layer of condensed connective 

 tissue. The latter, known as the epidermis, consists of an inner 

 layer of pigment cells and a thick outer layer of epithelial cells. 

 The derma is characterized by the presence of elevations (papillae) 

 which are everywhere extremely abundant. Throughout the derma 

 ramify blood-vessels and nerves. 



The Peripheral or Terminal Organs. Between the contact 

 surface and the afferent nerves specialized structures are found 



