CHAP. L] SENSORY NERVES. 483 



is distributed lose the sensibility which they previously possessed. 

 During the section of the root, and when the proximal stump is 

 stimulated, sensory effects are produced. When the distal stump 

 is stimulated no movements are called forth. These facts demon- 

 strate that sensory impulses pass exclusively by the posterior root 

 from the peripheral to the central organs, and that motor impulses 

 pass exclusively by the anterior root from the central to the 

 peripheral organs. 



An exception must be made to the above general statement, 

 on account of the so-called recurrent sensibility which is witnessed 

 in conscious mammals, under certain circumstances. It often 

 happens that when the peripheral stump of the divided anterior 

 root is stimulated, signs of pain are witnessed. These are not 

 caused by the concurrent muscular contractions or cramp which 

 the stimulation occasions, for they remain if the whole trunk of 

 the nerve be divided some little way below the union of the roots 

 above the origins of the muscular branches, so that no contractions 

 take place. They disappear if the posterior root be also cut, and 

 they are not seen if the mixed nerve-trunk be divided close to the 

 union of the roots. The phenomena are probably due to the fact, 

 that bundles of sensory fibres of the posterior root after running a 

 short distance down the mixed trunk turn back and run upwards 

 in the anterior root, and by this recurrent course give rise to the 

 recurrent sensibility. 



Concerning the ganglion on the posterior root, we may say defi- 

 nitely that it is neither a centre of reflex nor of automatic action. 

 Our knowledge concerning its function is almost limited to the 

 fact that it is in some way intimately connected with the nutrition 

 of the nerve. When a mixed nerve-trunk is divided, the peripheral 

 portion degenerates from the point of section downwards towards 

 the periphery. The central portion does not so degenerate, and if 

 the length of nerve removed be not too great, the central portion 

 uniting with the degenerating peripheral portion may grow down- 

 wards, and thus regenerate the nerve. This degeneration is 

 observed when the mixed trunk is divided in any part of its 

 course from the periphery to close up to the ganglion. When the 

 posterior root is divided between the ganglion and the spinal cord, 

 the portion attached to the spinal cord degenerates, but that 

 attached to the ganglion remains intact. When the anterior root 

 is divided, the proximal portion in connection with the spinal cord 

 remains intact, but the distal portion between the section and the 

 junction with the other root degenerates; and in the mixed nerve- 

 trunk many degenerated fibres are seen, which, if they be carefully 

 traced out, are found to be motor fibres. If the posterior root be 

 divided carefully between the ganglion and the junction with the 

 anterior root, the posterior root above the section remains intact, 

 but in the mixed nerve-trunk are seen numerous degenerated 

 fibres, which when examined are found to have the distribution of 



312 



