LABORATORY EXPERIMENTS ON THE NERVOUS SYSTEM 675 



LABORATORY EXPERIMENTS ON THE NERVOUS SYSTEM. 



1. Irritability of the Neurone. Repeat the tests outlined in Experi- 

 ment 3 under Laboratory Experiments in Muscle. Apply these tests both 

 to nerve trunk and to any ganglion, for example, the ganglion on the 

 posterior root of the tenth spinal nerve of the frog, or the superior cervical 

 or the celiac ganglion of the cat. 



2. Conductivity of the Neurone. a. Conductivity can be demonstrated 

 quantitatively by the method of Experiment 19, under Muscle Nerve 

 Physiology, using the muscle as an index of nerve activity. 



b. A better method is to measure the latency of the action current of an 

 isolated sciatic nerve. Connect one end of the nerve by means of non- 

 polarizable electrodes with a delicate galvanometer. Stimulate the 

 nerve at the far end, then at the near end. 



c. That conduction is in either direction can also be demonstrated on 

 branched motor nerves. Dissect out the sartorius muscle of the frog with 

 its motor nerve, which has two branches. Split the muscle between the 

 branches. Pick up one end and stimulate its nerve branch. Both muscles 

 will contract. The nerve impulse developed at the point of stimulation 

 also travels away from its muscle to the point of branching of the axone, 

 then down the second nerve branch to stimulate the second division of the 

 muscle. Branched axones are involved, not separate neurones. This is 

 the type of reaction in a pseudo reflex. 



3. Differentiation Polarity in Neurones. Neurones have a polarity 

 because of two facts, first, their anatomical relations, and second, their 

 physiological contacts. Afferent or sensory nerves cannot be stimulated 

 normally except at the sense organs. The nerve impulses developed must 

 pass in the centripetal direction, affecting such other neurones as are in 

 physiological contact, until a motor organ is reached. 



Differentiations can be demonstrated by certain drugs, for example, 

 paint a nerve trunk with nicotine. No effect follows. Paint a ganglion, 

 the nicotine stimulates, then poisons, the cell bodies of the ganglion. Or, 

 paint the accelerator nerve trunks to the heart with o.i per cent, adrenalin, 

 no obvious change occurs. Paint the heart or perfuse the heart with 

 adrenalin to bring the hormone in contact with the cardiac motor nerve 

 endings. The endings are specifically stimulated and the heart beats 

 faster and stronger. 



4. The Afferent and Efferent Functions of the Spinal Nerve Roots. 

 Skilfully dissect and expose the two roots of the eighth, ninth, and tenth 

 spinal nerves of the right side of a large frog. Reserve the left side for a 

 second try. Insert silk threads under each root using care not to stretch 

 or injure the delicate nerves. The posterior root is the smaller and its 

 orange-colored ganglion aids in identifying it. 



