THE FUNCTIONS OF THE BRAIN STEM 





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 apparatus for im- 

 mediate 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 in- 

 creasing 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 spontaneous 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." 



According to Schrader and Steiner, if care be taken not to injure the 

 optic thalami, spontaneous movements may be occasionally observed after 

 removal of the cerebral hemispheres. On the approach of winter such a 

 frog has been observed to bury itself in order to hibernate, and with spring to 

 resume activity and to feed itself by catching insects. The behaviour of 

 such decerebrate animals depends on the part taken in the initiation of 

 movement and adapted reactions by stimuli entering through the higher 

 sense-organs. Thus an ordinary bony fish after ablation of the cerebral 

 hemispheres maintains its normal equilibrium in water. It is continually 

 swimming about, stopping only when it reaches the side of the vessel or when 

 worn out by fatigue. Here, again, if the thalami and optic lobes be 

 intact the fish has been observed to show very little difference from 

 a normal animal and to possess the power of distinguishing edible 

 from non-edible material. On the other hand, in the elasmobranch 

 fishes, which depend mainly upon their olfactory apparatus as a guide to 

 movement, the removal of the cerebral hemispheres with the olfactory 

 lobes, or of the latter alone, produces complete immobility and absence of 



