THE NERVOUS SYSTEM 449 



The Stepping Reflex. — When in the dog the spinal cord has 

 been severed in the hinder part of the cervical region, and the 

 1 shock * from the transection has passed off, reflex walking is 

 observable. The walking movement includes alternate flexing 

 and straightening of the limb. The forward movement of the 

 hind-leg in taking a step is produced by flexion at the hip, and 

 to prevent the foot brushing against the ground as the leg swings 

 forward flexion occurs at the stifle and hock, so as to somewhat 

 raise the foot. The limb is then straightened again, so that the 

 foot may reach the ground and bear the weight of the body. 

 In order to prevent the limb doubling up under this burden, the 

 extensor muscles which support the patella joint and hock from 

 bending have to contract with sufficient power. Stiffened by 

 the contraction of these muscles, the limb serves as a prop to 

 carry the body. While the foot rests on the ground the body 

 moves forward, so that in due course the hip advances in front 

 of a vertical drawn upward from the foot. The extended hind- 

 limb at this time is sloped somewhat backward as well as down- 

 ward. When this posture is reached, the extensor muscles are 

 thrown into further action, and give the limb a push off from 

 the ground, propelling the body forward. The hind-limb thus 

 makes its contribution to the forward progression of the body ; 

 in galloping in the normal dog this extensor thrust is very marked. 



In this reflex spinal stepping, we may study first the flexion 

 of the limb which occurs in the forward movement of the step. 

 Flexion similar but more pronounced can be easily excited in 

 the spinal dog* by exciting the skin of the foot electrically. 

 Though the flexion occurs at hip, patella joint, and hock together, 

 it will be simpler to confine our examination to the flexion at 

 one of these joints only, for what occurs in the muscles of each 

 of the three joints is, so far as concerns us now, the same. The 

 chief muscles which flex the stifle are the semitendinosus and 

 biceps of the back of the thigh. The electric stimulation of the 

 skin of the foot is found to throw these muscles into contraction, 

 and, with them, also the psoas muscles (flexors of the hip) and 

 the tibialis anticus, etc. (flexors of the hock). But this is only 

 part of what happens. At the same time as the flexors of the 

 stifle contract, the extensor muscles of the stifle, the vasti and 

 crureus, are relaxed. The same stimulus which excites the motor 

 neurones of the flexors to discharge motor impulses into those 

 muscles, causes the motor neurones of the antagonistic muscles, 

 the extensors of the knee, to cease discharging impulses, and 

 keeps them prevented from discharging impulses. The stimulus 



* ' Spinal dog ' is the term used to indicate that the reflexes are entirely- 

 spinal , owing to the brain having been destroyed, or the cord having been 

 cut off from the brain. 



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