CHAPTER XCIII 

 THE AFFERENT PATHS OF SENSORY IMPULSES - 



The insulation of conduction in the nerve fiber, and the fact that im- 

 pulses can pass from neuron to neuron in one direction only, makes the 

 arrangement of neurons in the nervous system a matter of great sig- 

 nificance. We have already seen that the recognition of the quality and 

 location of a stimulus depends upon the connection of each receptor in 

 the skin with a definitely corresponding part of the brain. The ana- 

 tomical arrangement of the paths conducting afferent impulses from 

 receptors for each quality of sensation and from the different parts of 

 the body is of importance in elucidating the sensory phenomena of 

 disease. 



The Segmental Distribution of Afferent Nerves 



The vertebrate embryo develops as a segmented organism, at least 

 so far as its nervous, muscular, and skeletal structures are concerned. 

 The primitive segmentation of the vertebrate embryo is preserved in the 

 arrangement of the spinal nerves of man. Each dorsal root of the spinal 

 nerves contains afferent fibers coming from a definite segmental area 

 of the skin. This may be shown by cutting all of the dorsal roots except 

 one and then determining what parts of the skin retain their sensitivity. 

 The areas innervated by adjoining roots overlap considerably, so that 

 most parts of the skin receive a double innervation. Consequently 

 damage to a single dorsal root does not produce a considerable loss of 

 sensation. The areas supplied by each dorsal root agree closely with 

 the segmental areas to which visceral pain is referred, with the exception 

 that the latter do not overlap (Fig. 214). The areas of referred pain 

 appear to represent the central portion of the segment innervated by a 

 single dorsal root in which the overlapping is less considerable. 



The position of the skin areas do not correspond to the level of the 

 dorsal roots which innervate them because of the downward slope of the 

 spinal nerves. The skin areas, particularly of the lumbar and sacral 

 roots, are somewhat below the corresponding segments of the spinal 

 cord. In the limbs the segmental arrangement becomes obscure, until 

 considered with respect to the segmental origin of the limb buds in the 

 embryo. The areas innervated by the different roots contributing to the 

 brachial and lumbar plexuses are nevertheless quite distinct. 



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