444 PHYSIOLOGY 



' decerebrate rigidity.' Though respiratory movements continue 

 normally, the whole musculature is in a cataleptic condition, the 

 elbows and knees being extended and resisting passive flexion ; the 

 tail is stiff and straight, the neck and head retracted. This condition 

 seems to depend on an over- activity of the reflex tonic functions of the 

 lower centres. That it is reflex is shown by the fact that the rigidity 

 is at once abolished in a limb on dividing the appropriate posterior 

 roots. The position of the limbs may be also modified by sensory 

 stimuli. A similar condition of increased tonus is observed in the frog. 

 The apparatus for emotional expression is still intact though some- 

 what modified, and an impression which would give rise to pain in the 

 intact animal may cause vocalisation in an animal in whom the brain 

 above the mesencephalon has been destroyed. 



THE BRAIN STEM AS A WHOLE (INCLUDING THE THALAM- 



ENCEPHALON, OR OPTIC THALAMI) 



The introduction of the head ganglia of the brain stem, viz. the 

 optic thalami, completes in the lower animals at all events the appa- 

 ratus for immediate response to stimulus. The powers of such an 

 apparatus may be studied by examining the behaviour of an animal 

 in whom the cerebral hemispheres have been destroyed. The result 

 of this operation varies according to the type of animal chosen, though 

 all types present certain common features. When a frog's cerebral 

 hemispheres have been excised, a casual observer would not at first 

 notice anything abnormal about the animal. It sits up in its usual 

 position, and on stimulation may be made to jump away, guiding 

 itself by sight, so that it avoids any obstacles in its path. Movements 

 of swallowing and breathing are normally carried out. The animal, 

 thrown on to its back, immediately turns over again. If put into 

 water, it swims about until it comes to a floating piece of wood or 

 any support, when it crawls out of the water and sits still. If it be 

 placed on a board and the board be inclined, it begins to crawl slowly 

 up it, and by gradually increasing the inclination may be made to 

 crawl up one side and down the other. But a striking difference 

 between it and a normal frog is the almost entire absence of sponta- 

 neous motion that is to say, motion not reflexly provoked by changes 

 immediately taking place in its environment. All psychical phenomena 

 seem to be absent. It feels no hunger and shows no fear, and will 

 suffer a fly to crawl over its nose without snapping at it. " In a word, 

 it is an extremely complex machine, whose actions, so far as they go, 

 tend to self-preservation ; but still a machine in this sense, that it 

 seems to contain no incalculable element. By applying the right 

 sensory stimulus to it we are almost as certain of getting a fixed 

 response as an organist is when he pulls out a certain stop." 



