CH. XLII.] HEMISECTION OF THE CORD 619 



sufficient time was allowed to elapse (roughly about a year) for all traces of the 

 degeneration due to this lesion to have disappeared. The cord is then left, as it 

 were, like a cleaned slate, on which once more a new degeneration can be written 

 without fear of confusion with a previous one. The second degeneration produced 

 by such an operation as hemisection would then affect the intra-spinal fibres only, 

 all the long tracts from brain to cord having been wiped out by the first operation. 

 The complete topography of all these fibres, which are very numerous, has not yet 

 been worked out. The degenerated fibres are scattered throughout the white 

 matter, but are most numerous at the margins of the cord. This is especially true 

 for the longer fibres, and some of them appear to be very long indeed. In the case 

 of the longer fibres there is no evidence of decussation ; in the case of the shorter 

 fibres there is some but not very conclusive evidence that they in part cross to the 

 other side. 



Complete transverse section of the spinal cord leads to : 



1. Loss of motion of the parts supplied by the nerves below the 

 section on both sides of the body. The paralysis is not confined to 

 the voluntary muscles, but includes the muscular fibres of the 

 blood-vessels and viscera. Hence there is fall of blood-pressure, 

 paralysis of sphincters, etc. 



2. Loss of sensation in the same regions. 



3. Degeneration, ascending and descending, on both sides of the 

 cord. 



Hemisection. If the operation performed is not a complete cut- 

 ting of the spinal cord across transversely, but a cutting of half the 

 cord across, it is termed hemisection, or semi-section. 



This operation leads to : 



1. Loss of motion of the parts supplied by the nerves below the 

 section on the same side of the body as the injury. 



2. Loss of sensation in the same region. The loss of sensation is 

 not a very prominent symptom, and is limited to the sense of localisa- 

 tion and the muscular sense. The animal can still feel sensations of 

 pain and of heat and cold. 



3. Degeneration, ascending and descending, nearly entirely con- 

 fined to the same side of the cord as the injury. The most important 

 of these are shown in the photo-micrographs (fig. 452) on the opposite 

 page, the small text beneath which should be carefully studied. 



Differences in different regions of the spinal cord. The outline of the grey 

 matter and the relative proportion of the white matter varies in different regions of 

 the spinal cord, and it is, therefore, possible to tell approximately from what region 

 any given transverse section of the spinal cord has been taken. The white matter 

 increases in amount from below upwards. The amount of grey matter varies ; it is 

 greatest in the cervical and lumbar enlargements, viz. , at and about the 5th lumbar 

 and 6th cervical nerve, and least in the thoracic region. The greatest development 

 of grey matter corresponds with greatest number of nerve-fibres passing from the 

 cord. 



In the cervical enlargement the grey matter occupies a large proportion of the 

 section, the grey commissure is short and thick, the anterior horn is blunt, whilst 

 the posterior is somewhat tapering. The anterior and posterior roots run some 

 distance through the white matter before they reach the periphery. At the extreme 

 upper part of the cervical region, the end of the posterior horn is swollen out by 



