CENTRAL NERVOUS SYSTEM. 



687 



the first place by being less co-ordinated, in the second by continuing only so 

 long as the stimulus lasts, and in the third place by giving rise to less intense 

 electrical changes connected with the passing impulse. 



These facts, taken together, lead to the conclusion that when the cortex is 

 stimulated the impulses concerned in producing the muscular contractions 

 traverse cell-bodies at the point of stimulation, and are transmitted thence 

 through the underlying fibres. We shall see later that this direct course 

 probably does not represent the sole pathway for these impulses. 



Secondary Degeneration. The course of these impulses is next inferred 

 from the relation between the removal of different parts of the cortex and 

 the consequent secondary degenerations throughout the length of the central 

 nervous system. When the part of the cortex removed is taken from the 

 motor area, then the degeneration occurs in the internal capsule and in the 



FIG. 187. Schema of the projection fibres within the brain (Starr) ; lateral view of the internal cap- 

 sule : A, tract from the frontal gyri to the pons nuclei, and so to the cerebellum ; B, motor tract ; C, sen- 

 sory tract for touch (separated from B for the sake of clearness in the schema) ; >, visual tract ; E, audi- 

 tory tract ; F, O, H, superior, middle, and inferior cerebellar peduncles ; J, fibres between the auditory 

 nucleus and the inferior quadrigeminal body ; K, motor decussation in the bulb ; Vt, fourth ventricle. 

 The numerals refer to the cranial nerves. The sensory radiations are seen to be massed toward the 

 occipital end of the hemisphere. 



callosum. The path of the fibres forming outgrowths of the cortical cells 

 can be followed thence through the crusta and pyramids to the spinal cord. 

 After removal of the motor region of one cerebral hemisphere the degen- 



