308 IV. THE PRINCIPLES OF LIFE PHENOMENA 



differences involved in the usual environments of animals should be 

 insignificant, since the molecular motion is directly proportional to 

 the absolute temperature not to the ordinary one. 



Of course, organisms which may be able to utilize the molecular 

 motion of water should be very small in size, and perhaps not larger 

 than bacterial size. Animals larger than bacteria may be making use 

 for their movement of the reversible contractibility of protein molecules. 

 Muscle contraction of higher animals must also depend upon this pro- 

 perty of protein molecules. Actin molecules, the main protein com- 

 ponent of muscles, can exhibit even in vitro the reversible contraction 

 as already pointed out. 



The remarkable readiness with which impulses can be transmitted 

 along nerve fibres may be based upon the most suitable arrangement 

 of protein molecules for the spread of their structural changes. The 

 transmission of an impulse must be the spread of the wave of the 

 structural change, although this wave cannot be seen as a formal 

 change such as contraction. It is shown in electron micrographs that 

 protein threads are orderly arranged in nerve axon along the long 

 axis of the fibre (148). Chambers (149) injected oil drops into the 

 axoplasm of giant nerves of the squid and found that the drops assumed 

 ovoid shapes along the long axis of the nerve. After agitating the 

 axoplasm with a microneedle near the oil, the drop assumed a spherical 

 shape, suggesting the occurrence of a contraction of protein molecules. 



It has long been known that nerve tissues are especially rich in 

 lipids, which may serve as lubricating oil enabling the protein molecules 

 easily to alter the structure. It may be considered that phospholipids 

 are involved, in addition, in the energy metabolism for the impulse 

 transmission. Anaestetics or narcotics are generally lipid-soluble ; the 

 action of the drugs may depend upon their effect on the lipids, the 

 lubricating oils, rendering the spread of the structural change im- 

 possible. 



Neurotropic viruses, such as poliomyelitis virus, are said to travel 

 preferably along neural paths. According to Sanders, mouse encephalo- 

 myelitis virus travels at about 0.2 mm. per hour (150). If structural 

 changes of the protein are readily transmitted in nerves, the structural 

 change induced by a virus must likewise be easily spread in them. 

 The spread of the structural change induced by a virus may be nothing 

 but the travel of the virus itself. 



Since the structural change of protoplasm proteins should accom- 

 pany a change in polar groups, the spread of the change must be fol- 

 lowed by an electric disturbance, which is known as action current. 



On stimulation of muscles remarkable diffusion of K ion takes 

 place. In nerves, as in muscles, there is a difference between the 



