66 THE NERVOUS SYSTEM 



myelinated fibers representing the branches of the axons of the spinal ganglion 

 cells. Both types of fibers can be followed through the dorsal roots into the 

 spinal cord, as well as distally into the nerves. In the latter they mingle with 

 the large myelinated fibers coming from the ventral roots (Fig. 40). When 

 traced distally in the peripheral nerve the unmyelinated fibers are found to go 

 in large part to the skin, though a few run in the muscular branches (Ranson, 

 1911 and 1915). 



Classification of the Somatic Afferent Fibers According to Function. 

 Sherrington (1906) in an instructive book on "The Integra tive Action of the 

 Nervous System" has furnished us with a useful classification of the elements 

 belonging to the afferent side of the nervous system. He designates those 

 carrying impulses from the viscera as interoceptive, and subdivides the somatic 

 afferent elements into exteroceptive and proprioceptive groups. The extero- 

 ceptive fibers carry impulses from the surface of the body and from such sense 

 organs, as the eye and ear, that are designed to receive stimuli from without. 

 These fibers, therefore, are activated almost exclusively by external stimuli. 

 The proprioceptive fibers, on the other hand, respond to stimuli arising within 

 the body itself and convey impulses from the muscles, joints, tendons, and the 

 semicircular canals of the ear. Each group has receptors or sensory endings 

 designed to respond to its appropriate set of stimuli, and for each there are 

 special connections within the brain and spinal cord. 



Exteroceptive fibers and sensory endings are activated by changes in the 

 environment, that is to say, they are stimulated by objects outside the body. 

 The impulses, produced in this way and carried by these fibers to the spinal 

 cord, call forth for the most part reactions of the body to its environment; 

 and, when relayed to the cerebral cortex, they may be accompanied by sensa- 

 tions of touch, heat, cold, or pain. The receptors are, for the most part, located 

 in the skin; yet it is convenient to include in the exteroceptive group the pressure 

 receptors which are closely allied to those for touch, but which lie below the 

 surface of the body. At this point it should be noted that sensibility to those 

 forms of contact which include some slight pressure, such as the placing of a 

 finger on the skin, is not abolished by the section of all of the cutaneous nerves 

 going to the area in question, since the deeper nerves carry fibers capable of 

 responding to such contacts (Head, 1905). This deep contact sensibility, which 

 for lack of a better name we may call "pressure- touch," must not be overlooked 

 in the analysis of cutaneous sensations. 



The balance of evidence is in favor of the assumption that each of the vari- 



