viii CONDUCTIVITY AND EXCITABILITY OF NERVE 77 



too, the dissimilatory and assimilatory processes may be affected 

 in different degree by cooling. 



However this may be, it is certain that within a given range 

 (which is lower for cold-blooded and higher for warm-blooded 

 animals), increase of temperature produces increase of reflex 

 excitability. The next point is to determine this range experi- 

 mentally. Excessive rise and fall of temperature are alike 

 injurious to all excitable tissues. At high temperatures heat- 

 rigor (paralysis) overtakes the protoplasm ; excitability is of course 

 affected at a somewhat lower limit. If a frog is kept for some 

 time at a temperature of 30 38 C., it falls into a state of 

 apparent death. The heart still beats, but the animal gives no 

 reaction even the strongest stimuli have no perceptible effect, 

 and localised muscular contractions are alone discharged. If the 

 frog is then placed in cold water for a short time, it soon recovers 

 all its central functions. Eeflex movements of the throat- 

 muscles appear first, then spontaneous respiratory movements, and 

 finally the reflex excitability of the spinal cord is also restored. 

 Later still the other centres of the medulla oblongata, and last 

 of all the cerebrum, resume their activity, along with the power 

 of voluntary movement. This sequence recalls the effects of 

 increasing venosity of the blood. That the gases contained in 

 the blood must be quantitatively and qualitatively normal, 

 has long been recognised in warm-blooded animals as a neces- 

 sary condition for the normal functioning of certain parts of 

 the central nervous system, more especially the " respiratory 

 centre," and if the gas exchanges of the blood are interrupted, i.e. 

 if an animal is suffocated, a succession of striking excitatory 

 effects arising in the central organs makes its appearance. These 

 involve not merely the respiratory centre, vaso-motor centre, etc., 

 but the whole of the central nervous system, which falls into a state 

 of exaggerated excitation. The same occurs (in warm-blooded 

 animals) when the blood-supply is entirely cut off. Blood and 

 blood of normal composition, more especially with regard to its 

 contained gases is absolutely indispensable to the preservation 

 of the functions of the nervous centres ; but it is indispensable in 

 very different degrees to warm and cold-blooded animals. If we 

 ligature the heart of a frog, or place the animal in a medium devoid 

 of oxygen, it long retains the power of voluntary movement- 

 leaps, swims, feels, etc, Interruption of the circulation does not at 



