THE CENTRAL NERVOUS SYSTEM 745 



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 

 cerebellar tract large fibres ; the large pyramidal cells (giant cells 

 or cells of Betz), in what we shall afterwards have to distinguish 

 as the ' motor area ' (p. 844) of the cerebral cortex, are the cells 

 of origin of fibres of the pyramidal tract subserving the volitional 

 movements of the limbs and trunk. The pyramidal cells of the 

 ' face area ' are comparatively small. In general, an efferent or motor 

 nerve-cell is larger the longer its axon is e.g., the largest of all the 

 pyramidal cells in the ' motor ' region are found in the portion known 

 as the ' leg area,' from which the pyramidal fibres have to pass all 

 the way down the cord to the segments from which the spinal nerves 

 going to the lower limbs arise. 



The recent work of Brodman and of Campbell has shown that the 

 cerebral cortex may be histologically differentiated into regions 

 which correspond to a great extent to the various functional regions 

 mapped out by physiological methods (p. 851). 



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 the various 

 areas of grey matter on the surface of the brain, 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. 776), 

 which is the last of the spinal tracts to become myelinated' 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. 

 Conversely, the cells in which a tract of nerve-fibres arises may some- 

 times be identified by the alterations in the chromatin (p. 756) and 

 other changes which occur in them after section of their axons. 

 Particularly 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 arteries, stick in the branches of the anterior 



