The experiments on cerebral anæmia have enabled us to duplicate, by an entirely different method, many of the results obtained by anatomical division or removal of parts of the central nervous system. In some respects the method of anæmia permits of greater precision than the method of division or excision, and avoids, in great measure, the disturbances due to the wound and to the hemorrhage caused by the latter method. The method of general anæmia, as Couty pointed out long ago, leaves something to be desired in the matter of exact localization, but this objection may be met, in some degree at least, by appropriate methods of investigation. It is desirable that the results obtained by the method of section should be duplicated by some other method in order to eliminate as much as possible the effects due to the irritation produced by the anatomical lesion.
Our results show, as we believe, that, of the bulbar mechanisms studied, the respiratory is the most automatic, the vasomotor in part automatic, and the cardiac like the swallowing mechanism, almost wholly dependent upon afferent impulses for the arousal and discharge of its normal activity.
The eye reflexes return during the resuscitation period in the animals in which the cerebral anæmia has not been too prolonged. The motor cortex loses its excitability during anæmia, but may regain it after the reëstablishment of the circulation. The pilomotor mechanism is disturbed during the spasms which occur at a certain stage in the resuscitation. The temperature falls during the occlusion period, but rises again, often to far above normal, in the days following the anæmia. So many disturbing factors, such as the violent muscular contractions during spasms, enter into the problem that it is impossible to say that there is an actual disturbance of the temperature regulating mechanism although we are inclined to believe that this is the case.
All the senses return, following cerebral anæmia, but sight and hearing may afterwards fail without causing the death of the animal. The mental processes may return without any apparent deficiency, if the period of anæmia has been short. After longer occlusion, apparent insanity has been seen, and in one case, apparent total loss of mental processes occurred.
The reflex excitability of the cord returns rather early in the resuscitation period. Reflexes from the anterior part of the cord first involve muscles on the same side as the stimulus, and later cross to involve muscles of the opposite side. The spinal cord sometimes falls into much the same condition as that following spinal transection, and the scratch reflex appears. Spinal transection, when these reflexes have appeared, does not produce shock. Practically all phenomena of spinal shock may be reproduced without section of the cord. We conclude, therefore, that spinal shock is due more to the cutting off of the reflex pathways through the higher centers of the nervous system than to the stimulation of inhibitory fibers by the anæmia.
Segmental reflex pathways in spinal shock and spinal spasticity in man