CENTRAL NERVOUS SYSTEM. 251 



of the cortex in the region of the frontal lobes marked " eve " (Fig. 99) was 

 followed by movements of the eye. Schafer ' has shown that very precise 

 movements of the eye also follow the stimulation of the temporal and 

 various parts of the occipital cortex. Since the efferent fibres which control 

 the muscles concerned start from the cell-groups forming the nuclei of the 

 third, fourth, and sixth cranial nerves, it would appear most probable that in 

 both parts of the cortex there are located cells the axones of which pass to 

 those groups and are capable of exciting them. An alternative hypothesis 

 — namely, that the cortical impulse always travels first to the cortical cells in 

 the frontal lobe and thence, by way of them, to the efferent cell-groups — was 

 at one time considered, for the latent period of contraction of the eye muscles 

 was less by several hundredths of a second when the stimulus was applied in 

 the frontal region than when applied elsewhere. The experiments of Schafer 

 show, however, that when the occipital and frontal lobes are separated from 

 one another by a section severing all the association-fibres the reactions can 

 still be obtained by stimulation in the former locality — showing that the con- 

 nections of the two cortical areas with the cell-groups controlling the muscles 

 of the eye are independent of each other. 



This instance of the direct control of the same efferent cell-groups from dif- 

 ferent areas of the cortex is analogous to the control of efferent cell-groups in 

 the spinal cord, either by impulses coming down from the cerebrum or by 

 those entering the cord through the dorsal roots, and the instance here cited 

 is typical of a general arrangement. 



Cortical Control Crossed. — Where the stimulation of the cerebral cortex 

 causes a response on one side only, that response is on the side opposite to 

 the stimulated hemisphere. It sometimes happens, however, that two groups 

 of symmetrically placed muscles both respond to the stimulus applied to one 

 hemisphere only ; but these cases — the conjugate movements of the eyes, 

 movements of the jaw muscles or those of the larynx — usually depend on the 

 response of muscles which are naturally contracted together. 



This last reaction must be determined by the arrangement of the fibres in 

 the cord, since in lower mammals (dog and rabbit, for example) it is not 

 seriously disturbed by the removal of one hemisphere. 



Here should be added the very important observations of Sherrington 2 

 already mentioned on p. 224, which show that a stimulus which applied to 

 the cortex will cause one set of muscles, the flexors of the arm, lor example. 

 to contract, causes at the same time a relaxation of the antagonistic muscles, 

 thus rendering co-ordination possible in the movements of the limb. 



Course of Impulses Leaving- the Cortex. — In the higher mammal-, as 

 well as in man, it is by way of the pyramidal fibres that impulses travel from 

 the cortex to the efferent cell-groups of the cord. The pyramidal tracts by 

 definition form, in part of their course, the bundles of fibres lying on the ven- 

 tral aspect of the bulb, caudad to the pons, ventrad to the trapezium, and 



1 Proceedings <>_( tin- Royal Society, 1888, vol. xliii. 



2 Sherrington : Journal of Physiology, 1 897—1898, vol. xxii. 



