THE SPINAL CORD 557 



upward and constitute the fibers of the dorsal and lateral spino-cerebellar 

 tracts. The axons composing these tracts transmit nerve impulses re- 

 ceived from muscles and tendons by way of the dorsal roots, to the cere- 

 bellum. As the cerebellum is an organ for the coordination of muscle 

 groups these nerve impulses call forth reflex coordinated contractions of 

 groups of muscles which assist in maintaining the equilibrium of the body 

 both during station and progression. The course of some of the ascend- 

 ing fibers and their relation to the thalamus are shown in Fig. 229. 



The current views regarding the physiologic activities of the afferent 

 portion of the peripheral nerve system and its relation to the production of 

 different forms of sensibility have been enlarged by the results of the investi- 

 gations that have been made by Head. Thus, he has shown that the afferent 

 nerves consist of three systems, each of which when excited to activity evokes 

 in consciousness a different and distinct group of sensations as follows : 



1. One system of nerves which when stimulated evoke sensations of 



light touch or light pressure and sensations of moderate degrees 

 of temperature, e.g., 25 to 40 C., as well as slight variations of such 

 pressures and temperatures. The quality of the sensations evoked 

 by light touch or light pressure is accompanied by the power to 

 accurately localize them in an area of the skin corresponding to and 

 surrounding the part stimulated, a procedure known as cutaneous 

 localization. By reason of this latter fact there arises the ability not 

 only to recognize, but to distinguish at least two simultaneously 

 evoked sensations following the simultaneous application of two 

 different stimuli applied to the skin at varying distances one from 

 the other. To this ability the term discriminative sensibility has 

 been applied and usually tested by means of the compass points. 

 The facts observed in the study of lesions of the spinal nerves and 

 the diseases of the spinal cord it has come to be believed by neurologists 

 that the cutaneous localization of light touch and tactile discrimina- 

 tion are mediated by two different groups of nerve fibers. To this 

 form of cutaneous sensibility the term epicritic has been applied. 



2. A second system of nerves which when vigorously stimulated, as by the 



prick of a pin or by the application of a hot or cold object, evoke sensa- 

 tions of pain or heat and cold. To this form of cutaneous sensibility 

 the term protopathic has been applied. This form of sensibility is 

 unaccompanied by a definite appreciation of the locality stimulated for 

 the reason that the stimulus causes a widespread or radiating sensation 

 which at times is referred to parts far removed from the part stimulated. 



3. A third system of nerves which when stimulated evoke sensations of pres- 



sure, of the passive position and the movements of parts of the body, and 

 sensations of pain as well, if the stimulus (pressure) be severe, or if the 

 underlying structures are injured, e.g.^ the rupture of a joint. The 

 nerves subserving this form of sensibility are contained in the trunks 

 of the motor (muscle) nerves and are distributed to muscles, tendons 

 and joints. To this form of sensibility the term deep has been 

 applied. The nerves mediating these different sensibilities are 

 shown in Fig. 230. 



The pathways in the spinal cord that conduct afferent impulses to the 

 cortex and that call forth or evoke the various sensations, are not as 



