90 



VETERINARY PHYSIOLOGY 



(fig. 89). When the reflex arc at the level of the 3rd and 

 4th lumbar nerves is interfered with, the knee jerk is 

 diminished or is absent, and when the activity of the arc is 

 increased, by the removal of the influence of the cerebrum, 

 the jerk is increased. The fact that the latent period is very 

 much shorter than that of most reflex actions has been used 

 as an argument against the reaction being a true reflex. But 

 the reflex arc is certainly necessary for its production, and, 

 in spite of the short latent period, it may be regarded as a 

 true reflex from the quadriceps extensor muscle involving 



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Fig. 39. — The Neuro-rouscular Mechanism concerned in the Knee Jerk, and 

 the time of the knee jerk (A.J.) compared with the time of a reflex 

 action (A.R.). 



only one or two synapses and set up by the sudden develop- 

 ment of tension in the muscle which is being maintained in 

 a state of tonus (fig. 89). 



Tonus is most manifest in muscles concerned with the 

 maintenance of posture, and it might be called a postural 

 reflex response. 



When, as the result of disease or injury, a part of 

 the spinal cord has been severed from the brain, a marked 

 increase in the spinal reflexes is manifest. This is particu- 

 larly well seen when the downcoming fibres from the 

 cerebrum are interrupted (p. 194). In this case the 

 condition is similar to that of the decerebrated dog or cat. 



