FIBER TRACTS OF THE SPINAL CORD 103 



fibers situated in the lateral funiculus under cover of the ventral spinocerebellar 

 tract. Together with the spinotectal and ventral spinocerebellar tracts it forms 

 the fasciculus anterolateralis superficialis (of Gowers). It mediates pain and 

 temperature sensations. 



Conduction of Painful Afferent Impulses in the Spinal Cord. Not all of the fibers of 

 the lateral spinothalamic tract reach the thalamus. According to May (1906), "Some of 

 these fibers certainly pass directly to the thalamus, while others terminate in the inter- 

 mediate gray matter, and thus, by means of a series of short chains, afford secondary paths 

 to the same end station, which may supplement the direct path, or be made available after 

 interruption of the direct path." It has been shown in many cases in man and animals that, 

 after a complete hemisection of the spinal cord, the loss of sensibility to pain on the op- 

 posite side of the body below the lesion was only temporary. In time there may occur a 

 more or less perfect restoration of pain conduction, showing that the homolateral side of 

 the cord is able to supplement or replace the heterolateral path. According to the researches 

 of Karplus and Kreidl (1914) and Ranson and Billingsley (1916) these short chains, which are 

 of secondary importance in man, are much better developed in the cat. In this animal 

 pain conduction through the spinal cord is bilateral and is effected to a large extent through 

 a series of short relays. 



According to Head and Thompson (1906) the path for pain in the spinal cord is the same 

 whether the impulses arise in the skin or in the deeper parts, such as the muscles and joints. 

 But Dejerine (1914) is of the opinion that painful impulses from the muscles may be trans- 

 mitted in the posterior funiculus and remain uncrossed as far as the medulla oblongata. 



Until recently we possessed no information as to which dorsal root fibers served as pain 

 conductors. But in the last few years evidence has been presented which points toward the 

 unmyelinated fibers of the spinal nerves and dorsal roots as the pain fibers (Ranson, 1915). 

 Space does not permit a detailed presentation of the evidence here. It should be noted, 

 however, that the unmyelinated fibers of the lateral division of the dorsal root terminate in 

 the gray matter almost immediately after their entrance into the spinal cord, and in this 

 respect correspond to the known course of the fibers carrying painful impulses. The un- 

 myelinated fibers are chiefly distributed in the cutaneous nerves, although a few run in the 

 muscular branches. This coincides with the much greater sensitiveness to pain of the 

 skin than of the deeper tissues. Furthermore, the median nerve at the wrist, a large nerve 

 supplying a relatively small area of skin richly endowed with the sense of touch, contains 

 relatively few unmyelinated fibers. On the other hand, nerves like the lateral cutaneous 

 of the thigh and the medial cutaneous of the forearm, which supply relatively large cutaneous 

 areas of low tactile sensibility, but not inferior to the fingers in sensitiveness to pain, are com- 

 posed in large part of unmyelinated fibers. This difference between the composition of the 

 median nerve and the medial cutaneous nerve of the forearm is just what should be expected 

 if the touch fibers are myelinated and the pain fibers unmyelinated. Head and his co-workers 

 (1905, 1906, 1908) have regarded the group of sensations (protopathic), to which according 

 to their classification cutaneous pain belongs, as primitive in character and the first to appear 

 in the phylogenetic series. It is well known that nerve-fibers in their earliest phylogenesis 

 are unmyelinated. If our conception is correct, a great many of the afferent fibers of mam- 

 mals remain in this primitive undifferentiated state and mediate a relatively primitive 

 form of sensation. In this connection it is interesting to note that Dejerine (1914) believes 

 that pain is conducted by the "sympathetic" fibers contained in the cutaneous and muscular 

 nerves. He does not state the evidence on which this belief is based; but if by "sympathetic" 

 he means to designate the unmyelinated fibers his view agrees perfectly with that presented 

 in the preceding paragraphs. 



