726 A MANUAL OF PHYSIOLOGY 



impulses from the stomach. But clinical testimony is by no means 

 unanimous on this point, and experiments on animals show that 

 other factors are involved in these sensations. 



The spinal accessory nerve, soon after the junction of its bulbar 

 and spinal portions, divides into two branches, an internal and an 

 external. The external branch passes out to supply the trapeziiis 

 and sterno-mastoid muscles with motor fibres. The internal branch 

 passes bodily into the vagus. 



The twelfth or hypoglossal nerve is exclusively an efferent nerve. 

 Its nucleus of origin is an elongated collection of large nerve-cells 

 lying in the bulb close to the median line and parallel to it. It 

 contains the motor supply of the intrinsic and extrinsic muscles of 

 the tongue and of the thyro- and genio-hyoid. Paralysis of it causes 

 deficient movement of the corresponding half of the tongue. When 

 the tongue is put out, it deviates towards the paralyzed side, being 

 pushed over by the unparalyzed genio-hyoglossus of the opposite 

 side, which is thrown into action in protruding the tongue. 



The Functions of the Brain. 



The paths by which the various parts of the central nervous 

 system are connected with each other and with the periphery 

 have been already described, and we have completed the 

 examination of the functions of the spinal cord and medulla 

 oblongata. The events that take place in the upper part of 

 the central nervous stem and in the cortex of the cerebellum 

 and cerebrum now claim our attention. 



From very early times the brain has been popularly believed to be 

 the seat of all that we mean by consciousness sensation, ideation, 

 emotion, volition. And he who loves to trace the roots of things 

 back into the past may see, if he choose, running through the whole 

 texture of the older speculations a belief that the brain does not act 

 as a whole, but is divided into mechanisms, each with its special 

 work a foreshadowing, often in grotesque outlines, of the doctrine 

 of localization so widely held to-day. But until comparatively recent 

 times, cerebral physiology remained a kind of scientific terra incog- 

 nita ; and no notable additions were made for a thousand years to 

 the doctrines of Galen. Even to-day the utmost limit of our know- 

 ledge is reached when in certain cases we have connected a particular 

 movement or sensation with a more or less sharply defined anatomical 

 area. How the cerebral processes that lead to sensations and 

 movements, to emotions and intellectual acts, arise and die out; 

 what molecular changes are associated with them ; above all, how 

 the molecular changes are translated into consciousness how, for 

 example, it is that a series of nerve-impulses flickering across the 

 labyrinth of the occipital cortex should light up there a visual 

 sensation these are questions to which we can as yet give no 



