TO Dec, 1908.] 



Elements of A mmal Physiology. 



721 



If may be regarded as the organ of memory and of skilled movements. 

 In all the regions of the central nervous system previously described the 

 neuron paths, along which the impulses run, are inborn and are probably 

 not .affected in the animal's lifetime. A dog without a forebrain will eat, 

 sleep, bark, growl and bite, but it will never associate its sensations. If 

 fed by a particular man and from a particular vessel it will never link up 

 the sight of the man or the vessel with the act of eating but will struggle 

 violently with its keeper until the food is thrust into its jaws. A human 

 being in a state of complete idiocy is in much the same condition. Into 

 the forebrain there enter relay fibres from all the afferents which give 

 sensations and particularly those of sight, smell, hearing and pain. From 

 the brain there emerge fibres which never pass out of the central nervous 

 system but proceed, firstly, to the grey matter throughout the whole cord 

 which controls muscular movements, and, secondly, but to a much lesser 

 degree, to the various visceral centres. The action of the cerebrum mav be 



m 17 



^ 10 



Fig. 61. Vertical longitudinal section through brain of horse. — i, left fore- 

 brain or cerebral hemisphere ; 2, cerebellum ; 3, medulla oblongata ; 4, spinal cord ; 

 5, pons; 7, olfactory '• nerve"; 8, optic "nerve" cut short; 14, corpora quadri- 

 gemina ; 15, thalamus; 16, pineal gland; 17, bridge (cut across) connecting the two 

 cerebral hemispheres; iq, pituitary body. (After Hagemann.) 



looked on largelv as an interference with the reflex mechanisms of the rest 

 of the C.N.S. ; some of the fibres start or augment, a great many can 

 check reflexes ; other fibres can set muscles acting in a particular combina- 

 tion and sequence that would never occur in any reflex at all. These last 

 mentioned, which have to do with skilled movements, show increasing 

 development the higher the animal is in the evolutionary scale. When the 

 heart's action is increased by fear, when the saliva and gastric juice are 

 .■^ecreted on merelv seeing preparations being made for a meal, we have 

 two of the manv instances of the action of the forebrain on visceral 

 leflexes. 



We mav regard the nerve paths (or the synapses — see Chapter III.) in 

 the forebrain as being sensitised so that the passage of a particular group 

 of impulses produces an actual physical change and allows subsequent 

 impulses to pass more readily along these paths. This is the physical 

 basis of memorv whereby past experiences are registered and to it we may 

 ascribe also the fact that " an act repeated becomes a habit " &c. An 

 animal possessing a cerebrum can guide its conduct by its own previcjs 

 eirors and successes, whilst human actions can be regulated not only by 

 these but also bv the experiences of others proper Iv communicated, by 

 true reasoning, and by ideals. But bereft of its forebrain an animal is a 

 pure automaton whose responses to stimuli are predestined and predictable. 



