THE GREAT AFFERENT SYSTEMS 



right side of the body, and optic stimuli produced by light waves coming from 

 the right half of the field of vision, are propagated to the cortex of the left hemi- 

 sphere. The crossed representation in the case of hearing is less complete, partly 

 because every sound wave reaches both ears, but also because the crossing of 

 the central auditory pathway seems to be incomplete. 



The grouping of the afferent fibers in the peripheral nerves differs from that 

 in the spinal cord. In each of the spinal nerves several varieties of sensory fibers 

 are freely mingled. In the cutaneous branches are found conductors of thermal, 

 tactile, and painful sensibility; while the deeper nerves contain fibers for pain 

 and sensations of pressure-touch as well as for muscle, joint, and tendon sensi- 

 bility. Because of the intermingling of the various kinds of fibers a lesion of a 

 spinal nerve results in a loss of all modalities of sensation in the area supplied 

 exclusively by that nerve. 



But in the spinal cord a regrouping of the afferent impulse occurs, such that 

 all of a given modality travel in a path by themselves. All those of touch and 

 pressure, whether originally conveyed by the superficial or deep nerves, find 

 their way into a common path in the cord. In the same way all painful impulses, 

 whether arising in the skin or deeper parts, follow a special course through the 

 cord. Another intramedullary path conveys impulses from the muscles, joints, 

 and tendons. These various lines of conduction within the cord are so distinct 

 from each other that a localized spinal lesion may interrupt one without affecting 

 the others. A striking illustration of this is the loss of sensibility to pain and 

 temperature over part of the body surface without any impairment of tactile 

 sensibility as a result of a disease of the spinal cord, known as syringomyelia. 

 While we shall here confine our attention to the afferent channels leading 

 directly toward the cerebral cortex, it should not be forgotten that these are in 

 communication with the reflex apparatus of all levels of the spinal cord and brain 

 stem. 



The Spinal Path for Sensations of Touch and Pressure. Tactile impulses 

 which reach the central nervous system by way of the spinal nerves are relayed 

 to the cerebral cortex by a series of at least three units. 



Neuron I. The first neuron of this conduction system has its cell body, 

 which typically is unipolar, located in the spinal ganglion; and its axon divides 

 in the manner of a T or Y into a central and a peripheral branch. The per- 

 ipheral branch runs through the corresponding spinal nerve to the skin, or in 

 the case of those fibers subserving the tactile functions of deep sensibility, to the 

 underlying tissues. The central branch from the stem process of the spinal 



