480 HANDBOOK OF PHYSIOLOGY. 



consists of fine fibres which pass into the gray matter at different levels; 

 it probably consists of connecting fibres to connect the gray matter of 

 different levels. These fibres have not a long course; (7) Anterior 

 ground fibres (A. G. F.) are vertical fibres which probably connect the 

 anterior cornua at different levels. Some fibres pass to the anterior 

 commissure and connect with the anterior cornu of the opposite side; 

 (8) Antero-lateral ascending tract is a tract which degenerates upwards. 

 It is a sensory tract, and is connected with the posterior nerve-roots of 

 the opposite side. 



Functions of the Spinal Nerve-Roots. The anterior spinal 

 nerve-roots are efferent or motor: the posterior are afferent or sensory. 

 The fact is proved in various ways. Division of the anterior roots of 

 one or more nerves is followed by complete loss of motion in the parts 

 supplied by the fibres of such roots; but the sensation of the same parts 

 remains perfect. Division of the posterior roots destroys the sensibility 

 of the parts supplied by their fibres, while the power of motion continues 

 unimpaired. Moreover, irritation of the ends of the distal portions of 

 the divided anterior roots of a nerve excites muscular movements; irri- 

 tation of the ends of the proximal portions, which are still in connection 

 with the cord, is followed by no appreciable effect. Irritation of the 

 distal portions of the divided posterior roots, on the other hand, pro- 

 duces no muscular movements and no manifestations of pain; for, as 

 already stated, sensory nerves convey impressions only towards the nerv- 

 ous centres: but irritation of the proximal portions of these elicits signs 

 of intense suffering. Occasionally, under this last irritation, muscular 

 movements also ensue; but these are either voluntary, or the result of 

 the irritation being reflected from the sensory to the motor fibres. 

 Occasionally, too, irritation of the distal ends of divided anterior roots 

 elicits signs of pain, as well as producing muscular movements: the pain 

 thus excited is probably the result either of cramp or of so-called recur- 

 rent sensibility. 



Recurrent Sensibility. If the anterior root of a spinal nerve be 

 divided, and the peripheral end be irritated, not only movements of the 

 muscles supplied by the nerve take place, but also of other muscles, in- 

 dicative of pain. If the main trunk of the nerve (after the coalescence 

 of the roots beyond the ganglion) be divided, and the anterior root be 

 irritated as before, the general signs of pain still remain, although the 

 contraction of the muscles does not occur. The signs of pain disappear 

 when the posterior root is divided. From these experiments it is be- 

 }ieve$ that the stimulus passes down the anterior root to the mixed nerve, 

 and returns to the central nervous system through the posterior root by 

 means of certain sensory fibres from the posterior root, which loop back 

 into the anterior root before continuing their course into the mixed 

 nerve-trunk. 



