820 THE CENTRAL NERVOUS SYSTEM 



bellar 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. 918) 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. 924). 



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. 850), 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 dis- 

 tribution 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 sometimes be identified by 

 the alterations in the chromatin (p. 831) 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 spinal artery, and cause softening 

 mainly of the grey matter of the lumbar portion of the cord. When 

 the abdominal aorta of a rabbit is temporarily compressed (for about 

 an hour) below the origin of the renal arteries, the grey matter of the 

 corresponding portion of the cord is so seriously injured that it and 

 the fibres that arise from it degenerate, while the fibres whose cells of 



