FUNCTIONS OF THE BRAIN 975 



When the afferent field is still more restricted, as in the head of a dog 

 grafted on the circulation of another dog by anastomosis of the blood- 

 vessels, with precautions to avoid interruption of the blood-flow, not 

 only does the respiratory centre continue to discharge itself with a 

 regular rhythm, but cortical volitional movements persist (Guthrie, 

 Pike, and Stewart), and, so far as can be judged, sense perception, 

 emotional, and even intellectual, processes continue. In one case the 

 picture represented by the engrafted head was essentially the same as 

 that presented by the head of the ' host ' for over two hours. In a 

 transplanted head from a younger dog in which the circulation had 

 been interrupted for twenty-nine minutes, a remarkable return of 

 cerebral function was observed (Guthrie). 



Conditioned or Conditional Reflexes. This is perhaps the place to 

 refer briefly to certain phases of brain function which have been illus- 

 trated by the work of Pawlow, already alluded to in Chapter VI, on 

 the so-called psychical secretions. This study has led him to formulate 

 some novel views of the higher nervous activities. He has shown, for 

 instance, that if food is presented to an animal, and at the same time 

 the skin of the foot is stimulated electrically, the psychical secretion 

 of saliva, which the mere sight or smell of the food brings about, be- 

 comes, when the experiment has been repeated for a sufficient number 

 of times, associated with the electrical stimulus. It is then no longer 

 necessary to offer the food in order to elicit the secretion ; all that is re- 

 quired is to stimulate the foot. Even the nature of the stimulus may 

 be altered, cutting or burning the foot being substituted for the electri- 

 cal excitation ; the secretion is still obtained. Pawlow has termed such 

 reactions conditioned or conditional reflexes, because, unlike the 

 ordinary spinal and bulbar reflexes which are always obtainable and 

 are therefore spoken of as unconditioned reflexes, the conditioned re- 

 flexes depend upon the establishment of certain conditions, such as the 

 associated influence of some stimulation of the sense organs or of psy- 

 chical excitation. He concludes from his extensive studies on this 

 subject that an important function of the central nervous system, and 

 especially of the cerebrum, in addition to its power of integrating or 

 correlating nerve impulses, is that of dispersing impulses, producing new 

 types of activity and new reflexes. 



Localization of Function in the Central Nervous System. Let 

 us now consider a little more closely the real meaning of this 

 localization of function. Scattered all over the grey matter of 

 the primitive neural axis, and, as we have seen, over ithe grey mantle 

 of the brain as well, are numerous ' centres ' which seem to be 

 related in a special way to special mechanisms, sensory, secretory, 

 or motor. The question may fitly be asked whether those centres 

 are really distinct from each other in quality of structure or action, 

 or whether they owe their peculiar properties solely to differences 

 in situation and anatomical connection. It is clear at the outset 

 that the nature of the work in which a centre is engaged must be 

 largely determined by its connections. The kind of activity which 

 goes on in the vaso-motor centre in the bulb, for example, may 

 in no essential respect differ from that which goes on in the respira- 

 tory centre. The calibre of the bloodvessels will alter in response 

 to a change of activity in the one because it is anatomically con- 



