THE SPINAL CORD 555 



called that the nerve-fibers composing the dorsal roots of the spinal 

 nerves which transmit nerve impulses to the nuclei of origin of the ascend- 

 ing tracts may be divided into two groups, viz.: (i) Those connected 

 peripherally with the skin and mucous membrane and (2) those connected 

 with muscles, tendons, joints and possibly bones. The fibers of the 

 first group receive their stimulation from agents in the external world 

 and are therefore termed exteroceptive; the fibers of the second group 

 receive their stimulation from the tissues, muscles, tendons, etc., and 

 are therefore termed proprioceptive. 



The exteroceptive nerves terminate centrally, for the most part, 

 around cells in the gray matter of the dorsal horn of the same side. These 

 cells, it will be recalled, give origin to the axons which, crossing to the oppo- 

 site side, turn upward and constitute the fibers of the lateral and ventral 

 spino-thalamic tracts and the spino-mesencephalic tracts (the spino-tectal 

 and the spino-olivary tracts). 



The axons composing the lateral and ventral spino-thalamic tracts 

 transmit some of the nerve impulses received from the skin by way of the 

 dorsal roots, to the lateral and ventral nuclei of the thalamus; thence by 

 axons composing the thalamo-cortical tract, by way of the internal capsule 

 to specialized groups of cells in the post-central convolution of the cerebral 

 cortex where they evoke sensations of pain and temperature (heat and cold) 

 touch and pressure. The pathway for the nerve impulses that give rise 

 to pain and temperature is the lateral spino-thalamic tract; the pathway 

 for the nerve impulses that give rise to touch and pressure is the ventral 

 spino-thalamic tract. Destructive processes of either tract is followed by 

 a loss of the corresponding sensations. 



The axons composing the spino-tectal and spino-olivary tracts trans- 

 mit other nerve impulses to cells in the tec turn and in the inferior olive 

 and call forth by way of efferent or descending tracts reflex coordinated 

 contractions of certain groups of muscles, the true character of which 

 cannot be stated with any degree of definiteness. Some of the nerve 

 impulses are directed to the nuclei of origin of different cranial nerves and 

 call forth reflex activities of the eye muscles at least. 



The proprioceptive nerves terminate centrally for the most part in 

 two different regions as follows. One group on entering the cord passes 

 upward in the dorsal funiculus on the same side, both in the fasciculus 

 of Goll and the fasciculus of Burdach, as far as the lower portion of the 

 medulla oblongata where they terminate around the cells of the clavate 

 and cuneate nuclei respectively. These cells it will be recalled give origin 

 to nerve-fibers which cross the median plane in the ventral region of the 

 medulla then turn upward, forming a tract, known as the medial lemniscus 

 or fillet. 



The axons composing the medial lemniscus transmit some of the nerve 

 impulses received from the muscles, tendons, etc., by way of the dorsal 

 roots to the thalamus, thence by axons composing the thalamo-cortical 

 tract by way of the internal capsule to specialized groups of cells in the 

 cerebral cortex where they evoke sensations relating to the muscle appa- 

 ratus and which give rise to the perception of the position of the body or 

 its individual parts, of the duration and direction of movements, of resist- 



