132 INTRODUCTION TO NEUROLOGY 



The sensory nerves which enter the spinal cord come either 

 from the deep tissues or from the skin, and both of these types of 

 nerves carry fibers of very diverse functional sorts belonging to 

 the somatic sensory group, in addition to visceral fibers which 

 will not be considered here. It will be recalled (see pp. 77, 79) 

 that the general somatic sensory group includes: (1) propriocep- 

 tive systems, concerned with motor coordination and the orien- 

 tation of the body and its members in space (muscle sense, ten- 

 don sense, etc.), and (2) exteroceptive systems, concerned with 

 the relations of the body to its environment (touch, temperature, 

 and pain sensibility). The first of these systems is served 

 chiefly by the deep nerves, and the second chiefly by the cutane- 

 ous nerves, though this is not rigidly true. In particular it 

 should be noted that, even though the skin be completely anes- 

 thetic, the nerves of deep sensibility can still respond not only 

 to their proprioceptive functions, but also to the ordinary clinical 

 tests for the exteroceptive qualities of touch, temperature, and 

 pain, though with a higher threshold than in the case of the 

 cutaneous end-organs of these senses. 



Henry Head and his colleagues have also separated the cuta- 

 neous fibers into a protopathic group (including cutaneous pain, 

 a diffuse non-localizable tactile sensibility, and the discrimina- 

 tion of extreme degrees of temperature) and an epicritic group 

 (light touch, cutaneous localization, discrimination of inter- 

 mediate degrees of temperature and some others) ; but there is 

 difference of opinion as to whether these groups represent two 

 distinct sets of nerve-fibers or different stages in regeneration 

 or different types of end-organs of the same fibers (see p. 84). 



Upon entering the spinal cord all of these functional types 

 of fibers effect two sorts of connections: (1) for intrinsic spinal 

 reflexes, and (2) for the transmission of their impulses upward to 

 the higher centers of the brain. We shall first take up the in- 

 trinsic connections. 



The simplest of these intrinsic connections is the direct motor 

 reflex illustrated by Fig. 1 (p. 25), but there are many more 

 complex forms of the connection between the dorsal and ven- 

 tral roots, some of which are indicated in Figs. 60 and 61. In 

 general, there is at least one neuron of the gray matter of the 

 spinal cord interpolated between the dorsal and the ventral root 



