216 Ley ton and Sherrington 



85 seconds in right hemisphere. In this animal closure of the right carotid 

 alone diminished the motor responses in right hemisphere practically to 

 extinction, though less speedily than did closure of both vessels, and so 

 similarly did closure of left carotid those of left motor region. But in the 

 second animal closure of one carotid alone did not abolish the motor 

 response in either hemisphere, although the closure of both carotids 

 together extinguished all motor responses from both hemispheres in 

 120 seconds. The responses in this case became re-elicitable on each side 

 in 90 seconds after freeing the carotids. In the other case the closing of 

 the carotids extinguished cortical excitability only after 4 minutes. No 

 convulsions were evoked by any of the occlusions, but transient increase 

 of exitability of the cortex was noted in one instance: cf. L. Hill (21). 



IX. Functional Grouping of Pyramidal-tract Fibres in 



Crura and Pons. 



The size of the pyramidal tract in the anthropoids is large enough to 

 offer a better chance than in animals which are smaller or in which it is 

 less developed for testing by faradisation the degree to which the various 

 fibre groups from the various fields of the motor cortex lie separate or 

 commingled in the tract at various levels. In the largest of our orangs 

 we removed the whole brain in front of a transection through the posterior 

 part of the anterior colliculi, and examined by faradisation the cross- 

 section of the crusta. As so exposed, the pyramidal-tract fibre bundles 

 run of course perpendicular to the plane of the transection. With fibres 

 thus exposed the unipolar method of faradisation gives better opportunity 

 than does the bipolar for minutely localised stimulation. With the former 

 method the current lines converge in a direction more nearly parallel with 

 the lengfthwise direction of the fibres it is devised to excite. Examined 

 by unipolar faradisation, the results obtained from the orang's crusta were 

 as follows : — The most lateral third of the cross-section gave no detected 

 responses at all, neither did the most mesial fourth. The intermediate 

 portion gave responses which, taken in sequence from its lateral edge to its 

 mesial, were in the following order : toes, ankle and knee, hip, trunk, arm, 

 face, and tongue (fig. 30, A). There was very great overlapping of the 

 areas yielding these results ; thus it was easy to obtain from some points 

 concurrent movement in leg, trunk and arm, or again of arm, face, and 

 tongue. 



The severity of the operation necessary for exposing such a cross- 

 section did not allow a repetition of the observations at a lower level in 

 the same animal. But in the largest of our gorillas we removed the whole 

 brain in front of a transection through the highest part of the pons, and 

 examined by faradisation with the unipolar electrode the cross-section of 

 the pyramidal tract at that level. The results obtained in both right and 

 left pyramidal tracts were similar, confirming each other. They Avere that. 



