758 MOVEMENTS OE CORTICAL ORIGIK [BOOK m. 



clear, but which is probably determined by the respective rela- 

 tions of the several parts of the motor region to the functional 

 activity of the other parts of the cortex. In the shifting from 

 the one order to the other, the several constituent fibres, as we 

 have said, describe a somewhat peculiar course ; and when we 

 remember, that the order shewn in Fig. 125 is only the order 

 obtaining at one particular level of the capsule, and that from 

 the dorsal beginnings of the capsule in the corona radiata to its 

 ventral end in the pes, the capsule is continually changing in 

 form, and its fibres therefore are continually shifting their relations 

 to each other, the whole course of the several fibres of the tract 

 from their origin in the cortex until they are gathered up into 

 the central portion of the pes (Fig. 128 Py^) must be a very 

 complicated one. 



When the area of one hemisphere is stimulated, the move- 

 ment which results is in most cases seen on the other side of 

 the body, and on that other side alone. Thus when the area 

 for the fore limb is stimulated on the left hemisphere it is the 

 right fore limb which is moved. This is in accordance with 

 what we have learnt of the pyramidal tract and its ulti- 

 mate entire decussation befope it reaches the motor nerves, 

 the decussation either occurring massively as in the case 

 of the crossed pyramidal tract, or in a more scattered man- 

 ner along the upper part of the spinal cord in the case of 

 the direct pyramidal tract ; and, as we have seen, there is a 

 similar decussation for such part of the pyramidal tract as is 

 connected with the cranial nerves above the decussation of the 

 pyramids. Except in the case of certain areas for movements 

 naturally bilateral of which we shall speak presently, the move- 

 ment is normally on the crossed side, and on the crossed side 

 only. Under abnormal conditions however the limb of the 

 other side, that is of the same side as the hemisphere stimulated, 

 may move also. But such an abnormal movement of the same 

 side has not the same characters as the proper movement of the 

 crossed limb. Instead of being an orderly coordinate movement, 

 it is a more simple, either tetanic or perhaps tonic, or rhythmic, 

 clonic, contraction of the muscles. Obviously its mechanism is 

 of a different nature from that by which the proper movement 

 of the crossed limb is effected ; but it is important to bear in 

 mind that a movement of the uncrossed limb may take place ; 

 and further that, the abnormal conditions continuing, similar 

 movements of an uncoordinated character may spread to the 

 hind limb and other parts of the crossed side, though the stimu- 

 lation be still confined to the arm area, then to other parts of 

 the uncrossed side, until as we have said the whole body is 

 thrown into epileptiform convulsions. This feature must not 

 be forgotten. In fact it may be fairly insisted upon that while 

 we may speak of a particular coordinate movement as being the 



