THE NERVOUS SYSTEM 443 



on the skin of the abdomen will evoke a kick. The excitability 

 of the cord lasts for a few minutes after death ; in the ox it lasts 

 longer, but no comparative observations have as yet been made. 



Locomotion is often essentially a reflex act. The exact group- 

 ing of muscles, and the regulation of the degree and rapidity of 

 their contraction, would appear at first sight to need the super- 

 vision of the highest centres in the brain ; but this is not the 

 case. A pigeon will fly after decapitation ; a brainless dog can 

 walk, and a headless cat, under appropriate stimulation, flexes 

 and extends its legs alternately. If a horse thought of every 

 step he had to take, he would soon be worn out and blunder. 

 That the higher centres also come into play is shown by the 

 judgment which the animal exercises when jumping — viz., the 

 proper distance at which to take off, the amount of muscular 

 contraction required to lift the body, and the needful height to 

 which it should be raised. Locomotion, however, is not purely 

 a reflex act, as it is carried out with the knowledge and consent 

 of the animal, but it functions as such. A true reflex act is 

 involuntary, and carried out without the knowledge of the 

 animal. 



By a Co-ordinate Movement is meant one in which the con- 

 traction of various related groups of muscles is so adjusted that 

 the extent of their contraction, and everything necessary for a 

 perfect movement, is present and faithfully carried out. Co- 

 ordination of movement may occur without the assistance of 

 the brain. In the spinal cord, therefore, not only reflex but 

 co-ordinate movements are generated. The crossed or diagonal 

 movements of locomotion in quadrupeds are of this nature, and 

 are carried out by the spinal cord. Movements which are 

 irregular and purposeless, or in any way fail to co-ordinate, are 

 termed inco-ordinate. We shall see later that the co-ordination 

 of muscles is a complex reflex mechanism. All reflex actions are 

 co-ordinate. 



The Reflex Frog. — It is usual to illustrate reflex action by 

 reference to the decerebrated frog. In this animal the spinal 

 cord is capable of carrying out the most complex reflex acts, far 

 higher in character than is exhibited by animals with a greater 

 nervous development. If a decerebrated frog be placed in water, 

 it swims ; if it be stroked, it croaks ; if stimulated, it springs ; if 

 placed on its back, it recovers its normal position ; if acid be 

 applied to the right thigh, the left foot will be employed to wipe 

 it off ; or if this be held, the right leg is flexed. Still more re- 

 markable is the fact that if a decerebrated frog be placed on a 

 board which is gradually brought from the horizontal to the 

 vertical position, the animal gradually crawls up and, when the 

 board is vertical, sits at the top. If the board be lowered to the 



