632 TEXT-BOOK OF PHYSIOLOGY. 



Whatever the conditions for their origin and whatever their character- 

 istics, 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 includes 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. 



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 deli- 

 cacy in different regions, though its structure is everywhere essentially the 

 same. As the physiologic anatomy of the skin has elsewhere been detailed 

 (page 486), 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 which serve as inter- 

 mediates between the stimulus, on the one hand, and the afferent nerves, 

 on the ether hand. By virtue of their structure they are far more irritable 

 than the nerve-fibers and hence respond more quickly to the physiologic 

 stimulus than the nerve-fiber itself. To these specialized organs, found not 

 only in the skin but in other sense-organs as well, the term peripheral or 

 terminal organ is given. It is these structures that are primarily excited 

 to activity by the physiologic stimulus, and that in turn arouse the nerve 

 to activity. Peripheral organs are to be regarded as special modes of termi- 

 nation of afferent nerves adapted for the impress of a specific stimulus. 

 The peripheral organs of afferent nerves found in the skin and oral mucous 

 membrane present a variety of forms, some of which are as follows: 

 i. Free Endings. These are pointed or club-shaped processes, the ultimate 

 terminations of afferent nerve-fibrils, found in and among epidermic 

 cells. 



