6oo 



THE NERVOUS SYSTEM 



appears, however, that the tendon reflexes are not exactly what their name 

 implies. The interval between the tap and the contraction is said to be too 

 short for the production of a true reflex action. It is suggested that the con- 

 traction is caused by local stimulation of the muscle, but that this would not 

 occur unless the muscle had previously been stimulated by the tension applied, 

 and placed in a condition of excessive irritability. It is probable that the 



condition on which it depends is a reflex change 

 in the spinal irritability acting on the muscle or 

 exaggerated muscular tone, which is admitted 

 to be a reflex phenomenon in the spinal cord. 

 Conduction in the Spinal Cord. With 

 the differentiation of the central nerve axis in 

 vertebrates the conduction in the spinal cord 

 becomes of increasing importance, reaching its 

 maximum in man. It is evident that the 

 cord is the path by which all nerve impulses 

 arising in the trunk or in the arms and legs 

 must reach the brain, or vice versa. Impulses 

 of peripheral origin can and do produce re- 

 flexes, but they can arouse sensations and be 

 perceived only after they have been conducted 

 to the cerebral cortex. Motor impulses arising 

 in the brain can reach the motor cells of the 

 anterior columns of the cord only through the 

 cord as a conducting path. The continuity of 

 the cord, therefore, while not necessary for the 

 execution of reflexes, is absolutely necessary 

 for the higher co-ordinations of the reflexes 

 and for the excitation and controlling influence 

 of the brain. 



Illustrations of this are furnished by various 



Columns of the Cord. (Edinger.) examples of paralysis, but by none better than 



by the common paraplegia, or loss of sensation 



and voluntary motion in the lower part of the body, in consequence of 

 destructive disease or injury of a section including the whole thickness of 

 the spinal cord. Such lesions destroy the communication between the brain 

 and all parts of the spinal cord below the seat of injury, and consequently 

 cut off from their connection with the brain the various organs supplied 

 with nerves issuing from those parts of the cord. 



It is not probable that the conduction of motor and of sensory impulses is 

 effected under ordinary circumstances, to so great an extent as was formerly 

 supposed, through the gray substance of the cord, i.e., from cell to cell 

 through the short filaments lying wholly within the gray substance. But 



FIG. 371. Diagram to Show 

 the Manner in which the Fibers 

 of the Posterior Nerve Roots 

 Enter and Ascend the Posterior 



