104 PHYSIOLOGY OF FARM ANIMALS [CH. 



attempt to rub away the source of irritation with its other leg, 

 but it is incapable of sensation and unconscious of its actions. 



As has been shown already the spinal cord contains numerous 

 centres which preside over certain necessary functions of the 

 body (e.g. defaecation, micturition, seminal ejaculation, penile 

 erection, and parturition) and admit of the appropriate reflexes 

 being cari'ied out. But in the normal uninjured animal it is also 

 the function of the cord to transmit sensory impulses to the higher 

 centres of the brain, and so to bring the consequent motor 

 impulses inider the control of the volition. 



Inhibition and Sleep. Certain efferent nerve fibres passing to 

 glands or muscles are not excitatory but inhibitory in function, 

 and if stimulated will prevent the continuance of the active 

 process which otherwise would have gone on. Thus, according 

 to Pawlow, the vagus contains fibres which are inhibitory for pan- 

 creatic secretion. The vagus also contains fibres which may be 

 inhibitory to the heart's action, and can transmit impulses 

 which produce increased resistance to transmission from auricle 

 to ventricle. Moreover, certain parts of the brain can exert 

 an inhibitory influence over centres situated posteriorly. Thus 

 Sherrington found that after cutting through the crura cerebri 

 centres for certain groups of muscles were greatly increased in 

 excitability. The centres in question are sitiiated between the 

 crura cerebri and the medulla, and normally their activity is 

 partially inhibited by the cerebral cortex. 



Bayliss has suggested that sleep is a condition of inactivity 

 (displayed by the parts of the brain associated with conscious- 

 ness) which follows on inhibition if no further excitatory stimuli 

 occur. The inhibition may be brought about by a stimulus as 

 in the case of a child who is soothed to sleep by a lullaby. After 

 all excitatory stimuli have been removed, the inhibition itself 

 disappears and sleep continues as a zero state, excitation and 

 inhibition both being absent. It is well known that hypnotic 

 sleep in man is caused by stimuli from without, and these stimuli 

 must be supposed to produce a state of inhibition in nerve centres 

 in the brain. 



The unconscious state characteristic of sleep can be brought 

 to an end by excitatory stimuli of relatively great intensity. 

 Taking advantage of this fact the deepness of sleejD may be 

 measured by the intensity of the stimuli which are necessary 



