FIBER TRACTS OF THE SPINAL CORD IOI 



Interoceptive fibers are present in the thoracic and upper lumbar dorsal 

 roots, but are either absent or very few in number in the others. We know 

 practically nothing about their intraspinal course in mammals. They will be 

 considered in the chapter on the Sympathetic Nervous System. 



Exteroceptive fibers carry cutaneous afferent impulses, and probably are 

 subdivided into several varieties. Most authors agree that there are separate 

 fibers for the impulses aroused by tactile and thermal stimuli; and Sherrington 

 (1906) has presented evidence for the existence of a separate group of fibers, 

 whose end organs are responsive only to agents capable of inflicting injury, 

 that is, to noxious or painful stimuli. 



Conduction of Tactile Impulses in the Spinal Cord. The phenomena of sen- 

 sory dissociation, characteristic of syringomyelia, show that the intraspinal 

 path for the sensations of touch is rather widely separated from that for pain 

 and temperature sensation (Fig. 73). In that disease a cavity is developed 

 within the gray matter of the spinal cord; and sensations of pain and tem- 

 perature may be abolished over a given cutaneous area which is still sensitive 

 to touch. The separation of these two lines of conduction occurs at the place 

 where the dorsal root fibers enter the cord. The fibers, mediating pain and 

 temperature sensations, end almost at once in the gray matter, while those for 

 touch ascend for some distance in the posterior funiculus of the same side (Head 

 and Thompson, 1906; Dejerine, 1914). As these fibers ascend in the posterior 

 funiculus they give off collaterals to the gray matter of the successive levels of 

 the spinal cord through which they pass. The tactile impulses from a given 

 root, therefore, do not enter the gray matter all at once, but filter forward through 

 the collaterals and terminals of these dorsal root fibers to reach the posterior 

 gray column in a considerable number of segments above that at which the 

 root enters the cord. Within the posterior gray column at these successive 

 levels the terminals and collaterals of the tactile fibers establish synaptic con- 

 nections with neurons of the second order. The axons of these neurons form the 

 ventral spinothalamic tract of the opposite side (Fig. 73). 



The ventral spinothalamic tract is an ascending bundle of fibers found in the 

 anterior funiculus. It consists of fibers which take origin from cells in the pos- 

 terior column of the opposite side, cross the median plane in the anterior white 

 commissure, and ascend in the ventral funiculus to end within the thalamus (Fig. 

 73). It is possible that many of the fibers do not reach the thalamus directly, 

 but terminate in the gray matter of the cord and medulla oblongata in rela- 

 tion to other neurons, whose axons continue the course to the thalamus. If 



