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CHAPTEE VIII 



SOME EXPEBIMENTS TO DETEEMINE THE FUNCTIONS OF NEKVES 



In examining the functions of a nerve we have two main methods 

 which we may employ, section and stimulation. 



By section we can observe the loss of function resulting from the 

 loss of impulses normally passing along it. 



By stimulation we can observe the converse effect, i.e. the production 

 of some functional activity, such as a muscular movement, secretion 

 of a gland, &c, when impulses are sent along the nerve. 



The two methods of experimentation illustrate the two great 

 physiological attributes of a nerve, viz. conductivity and excitability. 

 Section teaches us the function of a nerve by observation of the 

 results of interrupting its conductivity at some spot. Stimulation 

 makes use of its property of excitability to give us knowledge of the 

 results of impulses travelling along the particular nerve. 



We have already made use of a nerve's excitability in our experi- 

 ments upon muscle, and in our study of the variations of function of 

 a nerve under different experimental conditions motor nerves are 

 generally employed, because the muscular response enables us to readily 

 determine small changes in the nerve's activity. 



As conductors nerves carry impulses either to or from the central 

 nervous system, i.e. they are afferent or efferent in function. At their 

 entrance into the cord, the spinal nerves divide into two roots, 

 anterior and posterior, the former carrying only efferent, the latter only 

 afferent impulses. Show this upon the frog by the following 

 experiment : — 



Experiment 1. — Decapitate a frog and cut away the upper third of the 

 vertebral column. With a fine pointed pair of scissors cut away the lamina; 

 of the remaining vertebrae, taking great care to avoid injuring the spinal cord. 

 This brings to view the cord in its lower part with the large roots of the 

 nerves which form the sciatic plexus. To see the anterior roots, the posterior, 

 which cover them up, must be displaced, (i.) Lift up the largest posterior 

 root with a seeker and pass a silk thread under it. Tie this close to the 

 cord and cut through the root between the ligature and the cord. Note that 

 on tying and on section irregular movements of the limbs are caused which 



