790 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



readily proved by the entire failure to evoke reflex movements after 

 destruction of the cord. Thus, while reflex movements are produced 

 with the greatest readiness in frogs in whom the cerebrum has been 

 removed, if the spinal cord be then disorganized by passing an instrument 

 down the vertebral canal all reflex movements are then impossible, even 

 although the nerves possess their power of conductivity and the muscles 

 their power of contraction. 



To appreciate the functions of the spinal cord as a collection of 

 centres for producing reflex action we have only to recall the statements 

 made on the nervous system as met with in the lowest articulata, in 

 which we have a collection of nervous matter, ganglionic in its nature 

 and comparable to the medulla spinalis of vertebrates, with afferent 

 fibres running to and efferent fibres running from these ganglia. Such a 

 type of nervous system is seen in the star-fish. If an- irritation be ap- 

 plied to an extremity of the limb of a star-fish a sentient impression is 

 conducted along the sensory nerve to the ganglionic centres, and a motor 

 impulse goes out along the motor nerve and contraction of the muscles 

 supplying the body results. So, if a decapitated centipede be placed 

 upon the ground it begins to make forward locomotive efforts as soon as 

 the impression is made upon the sentient extremities of the nerves dis- 

 tributed to its feet ; this impression is conveyed to the spinal ganglia, 

 and motor impulses are sent out along each one of the legs and loco- 

 motion results. If it comes in contact with an obstacle, however, as 

 high as itself it will mount over it, but if higher it will butt against it 

 its decapitated extremity until all nervous force is exhausted, when it 

 becomes quiet. Still more striking phenomena are present when, after 

 decapitation, the remainder of the body be cut in two ; if then the 

 halves of the body be placed upon the ground locomotive efforts will 

 continue in each, but they will not be harmonious. All these movements 

 depend upon physical excitation, and in some instances they require to 

 be excited by the elements in which the animal naturally moves. Thus, 

 if we take a decapitated water-beetle and place it upon the floor no 

 motion results, but place it in water and it begins to move with vigor. 

 The above are examples of reflex action, and result from excitation of 

 sentient surfaces and the conduction of that irritation to a nervous' 

 ganglion, and the reflection of that stimulation through a motor nerve. 



It has been mentioned that the impulses reaching the cord through 

 a single sensorj^ nerve may spread to the adjacent receptive ganglia, 

 and so lead to the transmission of motor impulses through a number 

 of different motor nerves. Ordinarily the degree of reflex action is in 

 proportion to the stimulus. 



Under certain conditions the irritability of the spinal cord may be 

 so modified that a gentle stimulus may produce excessive stimulation of 



