224 CLINICAL APPLIED ANATOMY. 



In forming an estimate of the amount of wasting of the pos- 

 terior roots outside the cord, it is well to bear in mind that they 

 are normally twice or three times the size of the anterior. The 

 degeneration can be traced distally as far as the posterior root 

 ganglion, and there ceases. The changes in the ganglion are 

 often insignificant, whilst the changes in the sensory nerves 

 beyond the ganglion are variable, and tend to be most marked at 

 the periphery. In the process of development the cells of the 

 posterior ganglia send out processes which bifurcate, one branch 

 growing into the cord as a posterior root fibre, whilst the other 

 becomes connected with skin, muscle, joint, or tendon as part of 

 the peripheral sensory nerve. A lesion of the sensory nerve 

 trunks, or the posterior roots of the spinal cord, may cut off all 

 impulses from the structures mentioned. Impulses from the 

 joints, tendons, and muscles are mainly concerned in produc- 

 ing the sense of position, and deprivation of these impulses 

 may induce ataxy with muscular atony and loss of sense of 

 position, without the necessary existence of any skin anaesthesia 

 at all. 



The ataxy which in some instances accompanies the peripheral 

 neuritis of alcoholism, influenza, and diphtheria, is probably due 

 to lesions of the peripheral sensory nerves. 



When cutaneous anaesthesia is present in tabes it is of the 

 segmental type. Often there is an anaesthetic zone corresponding 

 to the fourth and fifth dorsal segments. Extending higher it 

 may involve the dorsal segmental areas corresponding to the 

 inner side of the arm, but is rarely found to extend on the 

 thorax above the third rib, for the skin above this is supplied 

 by descending cervical nerves which are rarely implicated. 

 Segmental anaesthesia, usually restricted to analgesia, is often 

 found in the areas of the lumbo-sacral roots, particularly the 

 first sacral which supplies the sole of the foot, and the fifth 

 lumbar which supplies the peroneal region of the leg. The dis- 

 sociation of sensation which occurs in various forms and various 

 degrees, is attributed to unequal affection of the various fibres 

 which go to make up the compound nerves, and intra-medullary 



