TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 833 



umns, muscle sense through the homolateral dorsal column, while tactile 

 sensations pass partly by the uncrossed fibers of the dorsal column and 

 partly ~bij the opposite lateral columns. It is interesting that of these 

 two paths for tactile impulses the crossed one is alone closely associated 

 with the tract that carries pain (Holmes). 



Head and Thompson 11 have also found that the sensations are grouped 

 to the extent that those of one kind travel together, Avhether they are 

 from deep or superficial, from protopathic or epicritic receptors. When 

 the appreciation of cutaneous pain is lost, so also is that produced by 

 deep pressure; light touch and heavy touch are also lost simultane- 

 ously. The appreciation of all degrees of temperature is abolished at the 

 same time. The ability to discriminate between two points, the apprecia- 

 tion of weight, the recognition of the vibrations of a heavy tuning fork 

 applied to the skin all depend on impulses conducted through the 

 homolateral dorsal columns. 



Because the crossing in the cord of sensory fibers carrying certain sen- 

 sations occurs more promptly than that of those carrying others, and for 

 other less clearly understood reasons, the clinical findings are often difficult 

 of interpretation, especially when the lesions are only partial. The 

 senses of pain and temperature are undoubtedly lost much more readily 

 than those of cutaneous sensibility, though sometimes the reverse con- 

 ditions are found. If a partial lesion of one-half of the cord occurs 

 about the level of the twelfth dorsal segment, a very common symptom 

 is loss of pain and temperature on the opposite side, but not of touch 

 even when strong stimuli are applied. This crossed relation does not, 

 however, occur when the lesion is below the twelfth dorsal. 



Regarding . the number of segments necessary for the decussation of 

 each kind of sense fiber, observations on cases in which there is unilat- 

 eral injury of the cord are being collected, so that the upper limit of the 

 anesthetic area may be compared with the segmental level of the injury. 

 It appears that pain and thermal impulses cross quickly (i. e., within a 

 segment or two) in the middorsal region, but that those of touch cross 

 somewhat more gradually. In the upper segments the obliquity of 

 crossing of both kinds of fibers is greater, and in the cervical region 

 it may require five or six segments for the crossing of pain impulses. 

 With this increasing obliquity, a distinction appears in the crossing levels 

 of pain and temperature, for the latter cross a little more quickly. 

 This conforms with the clinical observation that thermal appreciation 

 may be disturbed without that of pain. Even the thermal impulses do 

 not all decussate at the same level, for anesthesia to heat may reach 

 higher up on the skin area than that to cold. 



When recovery occurs, the sensations gradually reappear caudalwards. 



