MAMMALIA— PHYSIOLOGY 479 



cord to a spinal nerve and thence to the proper muscle. As many 

 of the fibers cross to the opposite side, we find that a lesion of the 

 brain on the right side will cause paralysis on the left side of the body. 



Reflex Action. — In a simple reflex, we never find as few as two 

 neurones involved. The impression from the sensory cell passes 

 into the spinal cord and from the spinal sensory cell across (by two 

 or three intermediate neurons) to the spinal motor cell. Thence it 

 passes to the proper muscles. Sensation and reaction are almost 

 immediate and the proper movements have been made without 

 awakening consciousness in the cerebrum until later. 



A Triumph of Coordination. — A celebrated pianist has actually 

 been ascertained by Sir James Paget to have played 5,995 notes in 

 four minutes and three seconds. Paget estimated that there were 

 at the rate of 96 transmissions of force from the ends of the nerve 

 fibers along their course to the brain, in each of the same seconds 

 during which there were 72 transmissions going out from the brain 

 along other nerve fibers to the muscles. The notes were played in 

 due time and place, and with the sentiment of the music. (J. A. 

 Med. A.) 



Fatigue in the Nerve Cell — In some important studies made a 

 number of years ago (1892) Hodge demonstrated the effects of 

 fatigue on the nerve cell. He showed that after prolonged stimula- 

 tion of a nerve fiber its cell body and nucleus tended to shrivel, and 

 degeneration of the cytoplasm became apparent. The whole 

 structure might be compared to a shriveled apple. Later studies 

 by DoUey ^^ confirm Hodge's conclusion. There is only slight evi- 

 dence of increased CO2 in the nerve fiber after stimulation. 



Regeneration in the Nervous System. — Since there is no neu- 

 rilemma in the spinal cord and in the brain there is no complete 

 regeneration. The nerves of the body will readily regenerate, 

 provided the nucleus of the nerve cell is uninjured. 



Myelination. — Recently the entire process of myelination of 

 nerve fibers ^^ has been seen by Speidel (J. of Exp. Zool. in press) in 

 the transparent tail fin of living frog tadpoles. Individual nerve 

 sprouts and sheath cells were watched for long periods (ten days to 



12 Hodge, C. F. 1892. Jour, of Morphol., vol. 7; Jour, of Physiol., 1894, vol. 17. 

 Dolley, D. H. 1917. Jour. Comp. Neurol., vol. 27, pp. 299-324. 



13 Nerve cells and sheath cells may readily be cultivated in vitro, and the growth of 

 nerve fibers observed, but the formation of the myelin sheath has never been obtained 

 in artificial media. 



