THE SKIN AS AN ORGAN OF SENSATION 85 



blood vessels, and lymphatic vessels, and it contains great 

 numbers of sweat glands and oil glands. 



The epidermis, lacking blood vessels, does not bleed, 

 and the horny layer, lacking nerves, has no feeling; but so 

 fine are the networks of blood vessels and nerves in the 

 dermis that the finest needle cannot pass between them. 

 The whole surface of the dermis is thrown into innumer- 

 able projections called papillce, many of them supplied 

 with capillary blood vessels and nerve fibers. 



110. Sensation. When we become conscious of receiv- 

 ing an impression, that is, when we perceive that some 

 part of our nervous system is stimulated, we have what 

 is called sensation. It has already been shown that nerv- 

 ous stimulation may affect parts of the nervous system 

 and reflex action may follow without conscious reception 

 by the brain of any influence that is, without sensation. 



In order that there may be sensation there must be (1) 

 a stimulus, (2) a nervous end organ suited to receive the 

 stimulus, (3) a path to the brain for the impulse excited by 

 the stimulus, (4) a part of the brain to receive the impulse. 

 Still another condition of a different sort seems essential 

 to sensation, and that is an attitude of the mind which 

 we call attention. A person absorbed in thought may look 

 upon an object without being conscious of perceiving it, 

 may hear music without knowing it. That is, all the con- 

 ditions of sight and hearing may be present save the one 

 of attention, for which we have as yet no physiologically 

 descriptive terms. 



111. General and Special Sensations. There are various 

 vague, indefinable feelings which are not referred to any 

 particular portion of the body or to any external influence, 

 and which we know as general sensibility. Sensations of 

 fatigue, of restlessness, languor, weakness, and the like, 



