658 A MANUAL OF PHYSIOLOGY 



I. Structure of the Central Nervous System. 



In unravelling the complex structure of the central nervous 

 system, we avail ourselves of information derived (i) from 

 its gross anatomy ; (2) from its microscopical anatomy ; 

 (3) from its development ; (4) from what we may call, 

 although the term is open to the criticism of cross-division, 

 its physiological and pathological anatomy. 



Certain tracts of white or grey matter are differentiated from each 

 other by the size of their fibres or cells. For example, the postero- 

 median column of the spinal cord has small fibres, the direct cere- 

 bellar tract large fibres ; the pyramidal cells in what we shall after- 

 wards have to distinguish as the Meg area' (p. 743) of the cerebral 

 cortex are large ; those of the ' face area ' are comparatively small. 



The study of development enables us not only to determine the 

 homology, the morphological rank, of the various parts of the brain 

 and cord, but also, by comparison of animals of different grades of 

 organization, sometimes to decide the probable function and physio- 

 logical importance of a strand of nerve-fibres or a column of nerve- 

 cells. It is of special value in helping us to differentiate and to 

 trace the various tracts or paths into which the white matter of the 

 central nervous system may be divided. For the medullary sheath 

 is not developed at the same time in all the tracts, and a strand of 

 nerve-fibres in which it is wanting e.g., the pyramidal tract (p. 687) 

 at birth in man and such animals as are unable to stand and run when 

 they are born is readily distinguished under the microscope. 



Then, again and this is what we propose to include under the 

 fourth head experimental physiology and clinical and pathological 

 observation throw light not only on the functions, but also on the 

 structure, of the central nervous system. For instance, complete or 

 partial section, or destruction by disease, of the white fibres of the 

 cord or brain, or of the nerve roots, or removal of portions of the 

 grey matter, is followed by degeneration in definite tracts. And 

 since, as we have already seen, degeneration of a nerve-fibre is caused 

 when it is cut off from the cell of which it is a process, the amount 

 and distribution of such degeneration teaches us the extent and 

 position of the central connections of the given tract. And, particu- 

 larly in young animals, removal of a peripheral organ an eye or a 

 limb -or section of its nerves, may be followed by atrophy of 

 portions of the central nervous system immediately related to it. 



' Softening' of a definite portion of the white or grey matter may 

 also in certain cases be caused by depriving it of its blood-supply by 

 the injection of artificial emboli, and the resulting degenerations may 

 then be studied. For instance, fine particles like lycopodium spores 

 are injected into the abdominal aorta between the origins of the 

 renal and inferior mesenteric arteries. They are prevented by 

 clamps from entering these vessels, and, passing through the lumbar 



