730 



PHYSIOLOGY. 



do not possess such elaborate apparatus. In the case of many nerves, 

 the axis-cylinder splits up into fibrils which are arranged in the form 

 of a network. From this somewhat deeply placed network very fine 

 fibrils or fibrillae are given off to terminate in the tissues to be sup- 

 plied. The fibrillae have their terminus -in free ends lying between 

 the epithelial cells. In many cases the free ends are seen to be pro- 

 vided with small enlargements. These latter are known as tactile 

 cells. 



Knowledge Gained. By the sense of touch one feels the con- 

 tact of bodies and their temperature, whether these bodies be solid, 

 liquid, or gaseous. This special sense also defines at the same time 

 the locality of the impression made by the external agent. The judg- 



Fig. 298. Krause's Corpuscle. (HEDON.) 

 a, Nerve fiber. 6, Corpuscle. 



ment of locality is not, however, free from error. It is really exact 

 for but a few points; that is, wherever the touch is delicate. On 

 the other parts of the skin the individual never exactly divines the 

 point pressed upon; so that he makes mistakes of millimeters, centi- 

 meters, and even decimeters. 



In sensory nerve-trunks there exist different kinds of nerve- 

 fibers; some administer to painful impressions and others to tactile 

 impressions. Sensations of temperature, sensations of pressure, and 

 of muscular sense belong to the latter group. 



There are, then, four sense qualities in skin-sensations: sensa- 

 tions of pain, temperature, pressure, and muscle-sense, and each one 

 has its own nerve-fiber. 



SENSE SPOTS. The surface of the skin is found by experimenta- 

 tion to be composed of very small sensorial areas. Between these 



