104 



THE NERVOUS SYSTEM 



The problem can be approached from the. experimental standpoint. The seventh lum- 

 bar dorsal root of the cat is especially adapted for such a test. This root as it approaches 

 the cord breaks up into a number of filaments which spread out in a longitudinal direction 

 and enter the cord along the posterolateral sulcus. Within each root filament, as it ap- 

 proaches this sulcus, the unmyelinated separate out from among the myelinated fibers and 

 take up a position around the circumference of the filament and along septa that divide it 

 into smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterally 

 into the dorsolateral fasciculus, constituting together with a few fine myelinated fibers the 

 lateral division of the root (Fig. 74). Almost all of the myelinated fibers run through the 

 medial division of the root into the cuneate fasciculus. A slight cut in the direction of the 



Exterior fin\lculus. 



Inmuel 'mated [tiers. 



Lissauers tract 

 .Dorsal Toot 



qela/tinosa. 



Lateral 

 funiculus 





Fig. 74. From a section of the seventh lumbar segment of the spinal cord of the cat, showing the 

 unmyelinated fibers of the dorsal root entering the tract of Lissauer. 



arrow, which as shown by subsequent microscopic examination divided the lateral without 

 injury to the medial division of the root, at once eliminated the pain reflexes obtainable 

 from this root in the anesthetized cat, such as struggling, acceleration of respiration, and 

 rise of blood-pressure. On the other hand, a long deep cut in the plane indicated by B, 

 Fig. 74, which severed the medial division of the root as it entered the cord, had little or no 

 effect on the pain reflexes. This series of experiments, the details of which are given else- 

 where (Ranson and Billingsley, 1916), furnishes strong evidence that painful afferent im- 

 pulses are carried by the unmyelinated fibers of the lateral division of the dorsal root. 



These fibers probably terminate in the substantia gelatinosa Rolandi, and, if so, it is 

 not unlikely that intermediate neurons are intercalated between them and the neurons 

 whose axons run in the ventral spinothalamic tra.ct. 



