NERVE 89 



and the cat sinks down on its forequarters. This is due, not 

 merely to stimuli coming from the joints of the neck, but 

 also to stimuli coming from the labyrinth of the internal ear 

 (p. 121). 



In ducks it has been found that by placing the head and 

 neck in various positions — e.g. in that taken up in diving — 

 a postural reflex inhibition of the movements of breathing is 

 induced. 



Fatigue of the Reflex Arc. — Nerve fibres do not manifest 

 fatigue (p. 6 9), but reflex arcs readily do so, apparently through 

 a change in the synapses. These synapses are also much 

 more susceptible to the influence of poisons — e.g. deficiency 

 of oxygen or the action of such drugs as chloroform — than 

 are nerve fibres. As a reflex response decreases from fatigue 

 it becomes more and more easily replaced by other reflexes. 

 But a very brief cessation of the stimulus, and, still more, 

 the brief substitution of another reflex, rapidly restores the 

 activity of the action. 



Visceral Reflexes. 



Reflex actions in connection with various visceral muscles 

 are also connected with the spinal cord. Many of these are 

 complex reflexes involving inhibition of certain muscles and 

 increased action of others, some visceral, some skeletal. The 

 best marked of these are the reflex acts of micturition 

 (p. 581), defaecation (p. 335), erection, and ejaculation (p. 623). 

 The lumbar enlargement is the part of the cord involved. 



As has been shown by the study of postural reflexes, the 

 nervous arcs in the cord exercise a constant reflex tonic 

 action, due to the constant inflow of incoming impressions. 

 When this tonic action is interfered with by any condition 

 which interferes with the integrity of the reflex arc, the 

 response from the muscle may be diminished. This is 

 exemplified in the contraction of the quadriceps extensor 

 femoris which occurs when the ligamentum patellae is struck 

 sharply, causing a kick at the knee joint — the knee jerk 



