The anatomy of the nervous system, from the standpoint of development and function . g paragraphs. io4 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 approachesthe cord breaks up into a number of filaments which spread out in a longitudinal directionand 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 andtake up a position around the circumfere
The anatomy of the nervous system, from the standpoint of development and function . g paragraphs. io4 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 approachesthe cord breaks up into a number of filaments which spread out in a longitudinal directionand 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 andtake up a position around the circumference of the filament and along septa that divide itinto smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterallyinto the dorsolateral fasciculus, constituting together with a few fine myelinated fibers thelateral division of the root (Fig. 74). Almost all of the myelinated fibers run through themedial division of the root into the cuneate fasciculus. A slight cut in the direction of the Posterior [utuculus. Unmuelirated, [ tractDorsal ,. Fig. 74.—From a section of the seventh lumbar segment of the spinal cord of the cat, showing theunmyelinated fibers of the dorsal root entering the tract of Lissauer. arrow, which as shown by subsequent microscopic examination divided the lateral withoutinjury to the medial division of the root, at once eliminated the pain reflexes obtainablefrom this root in the anesthetized cat, such as struggling, acceleration of respiration, andrise 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 noeffect on the pain reflexes. This series of experiments, the details of which are given else-where (Ranson and Billingslcy, 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
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