88 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



opposite to that which the normal impulse takes. Further, it was found 

 that if the sciatic nerve was excited, a negative variation current could be 

 detected in the anterior as well as the posterior roots. Normally the irritation 

 only passes up the posterior roots and down the anterior, for normally the 

 sensory fibres of the posterior roots are excited only at the peripheral end and 

 the motor fibres of the anterior roots only at the central end. The experiment 

 showed both sensory and motor fibres to be capable of conducting in both 

 directions. 



There is no doubt but that nerve-protoplasm can conduct in both directions, 

 although normally the nerve is stimulated only at one end and therefore con- 

 ducts in only one direction. This question is of considerable importance, not 

 only with reference to the possibility of the central nervous system being 

 influenced by stimuli passing from the muscles, for instance, back along the 

 motor nerves, but more especially with reference to the spread of impulses 

 through the central nervous system, a problem which will be considered later 

 with others of a similar character. 



Rate of Conduction. The activity of the conduction process varies 

 greatly in different tissues. The nerves of warm-blooded animals conduct more 

 rapidly than those of cold ; in a given animal the nerve-fibres conduct more 

 rapidly than muscle-fibres ; striated muscle conducts more rapidly than smooth 

 muscle ; and even within a single cell different portions may transmit the ex- 

 citing process at different rates ; thus the myoid substance of the contractile fibres 

 of one of the rhizopods conducts more rapidly than the less highly differen- 

 tiated protoplasm of the cell. In general, it may be said that, " the power to 

 conduct increases with increase of mobility and sensitiveness to external irri- 

 tants, a fact which reveals itself in the protozoa, by a comparison of the slowly 

 moving rhizopods with the lively flagellata and ciliata." l A study of different 

 classes of muscle-tissue supports this view. 



(a) Hate of Conduction in Muscles. The conduction process is invisible, 

 hence we estimate its strength and rate by its effects. It is most readily fol- 

 lowed in such mechanisms as muscle, where the conducting medium itself 

 undergoes a change of form as the exciting influence passes along it. 



Hate of Transmission of Wave of Contraction. If a muscle be excited to 

 action by an irritant applied to one end, it does not contract at once as a whole, 

 but the change of form starts at the point which is irritated and spreads thence 

 the length of the fibres. At the same time that the muscle shortens it 

 thickens, and under certain conditions the swelling of the muscle can be seen 

 to travel from the end which is excited to the further extremity. In the case 

 of normal, active, striated muscle, the rate at which the change of form spreads 

 over the muscle is far too rapid to be followed by the eye, and hence the 

 muscle appears to act as a whole. By suitable recording mechanisms, evidence 

 can be obtained of the rate at which the exciting influence and contraction pro- 

 cess pass along the fibre. Thus two levers can be so placed as to rest on the 

 two extremities of a muscle, at the same time that the free ends of the levers 

 1 Biedermann : Elektrophysiologie, 1895, Bd. i. S. 124. 



