EFFECTS OF EXPERIMENTAL LESIONS 



837 



extremities it is not at first sight so clear, although it can be accurately 

 worked out, as is indicated in the accompanying diagram (Fig. 218). 



In attempting to determine the level of a lesion from the sensory paraly- 

 sis, some confusion often arises on account of the oblique course of the 

 decussation of the sensory fibers in the spinal cord, fibers for the different 

 sensations not crossing at the same levels. For example, the appreciation 

 of moderate temperature is often lost slightly higher than that of pain. 

 The appreciation of the vibrations caused by drawing the base of a 



Fig. 218. Diagram showing the segmental arrangement of the sensory nerves. (From Purves 



Stewart.) 



heavy tuning fork over the skin is often very useful in locating the 

 lesion, particularly in the abdomen. When this method is used on the 

 thorax, however, the skin should be pulled up in folds before the fork 

 is applied, since otherwise the thorax will act as a resonator and spread 

 the sensation. Section of two or more sensory roots produces a very 

 definite area of anesthesia, involving all the skin sensations as well as 

 several of those of deep sensation. 



// the severed roots include all of those going to one of the extremities, 

 there is not only an entire absence of sensation, but a marked interference 



