THE NERVOUS SYSTEM 445 



It is only where judgment is required or instructions to be issued 

 that the higher centres take any share in locomotion. 



There are movements which may be excited in the limbs after 

 death in both horse and ox, but particularly the latter, which 

 remind one very strongly of the reflex frog. If immediately 

 after a horse is destroyed an attempt be made to open the 

 abdomen the animal kicks. Some minutes must be allowed to 

 elapse before the irritability of the cord disappears. In the ox 

 the period is longer, and even after decapitation the apparently 

 purposeful movements are very remarkable. For these reasons 

 we believe the cord plays in these animals a more independent 

 part in locomotion than is generally considered. How largely 

 locomotion is reflex may be indicated by the walking of the chick 

 out of the egg. A volitional act requires some experience and 

 training ; a reflex act is innate, and may be complete at birth. 



Impulse Paths. — When an impulse enters the cord — and it can 

 only gain entrance by the dorsal spinal roots — it may be dealt 

 with locally by a single spinal segment. It may be distributed 

 by several local segments, or it may pass the entire length of the 

 cord and be dealt with by the cerebellum or cerebrum (Fig. 139, 

 A and B). The strength of the entering stimulus may deter- 

 mine whether one or more segments of the cord is involved, 

 as we have seen in the strychnine experiments on the reflex 

 frog. Such a spreading of impulses is termed irradiation, and 

 at one time it was believed that this could only occur in a 

 forward direction. Sherrington showed that it could also 

 occur down the cord, though it remains limited to certain 

 lines. We may here trace the path of an impulse, selecting 

 any spinal reflex act. What is true for this is true for all 

 impulses passing to the cord by afferent nerves, and the principle 

 is equally true for those, like sight, hearing, etc., which do not 

 communicate with the cord, but pass direct to the brain. We 

 have seen that a reflex arc (Fig. 132) consists of a chain of 

 nerve-cells, each complete link being called a ' neurone,' and the 

 neurones following each other end to end like the links of a chain. 

 Further, that a complete neurone is made up of a nerve-cell body 

 (perikaryon*) with its processes, some of these, the dendrites, 

 being the receiving, another the axon, being the transmitting 

 process. We also know that the axon terminates by arborising 

 around the dendrites of the next link. The passage of an impulse 

 in a nerve-arc must lie during part of its course, within the neurone 

 (in all cases travelling from dendrite to axon, though not neces- 

 sarily traversing the perikaryon, which may be short-circuited). 

 Having arrived at the end of one neurone, the impulse has then 

 to cross the space existing between it and its neighbouring 



* See footnote, p. 433. 



