yo2 THE CENTRAL NERVOUS SYSTEM 



The eleventh or spinal-accessory nerve contains only efferent fibres. 

 The cells of origin of its spinal portion lie in the lateral horn of the 

 cord, from about the level of the first to the fifth or sixth cervical 

 nerves. The bulbar portion, sometimes called the bulbar accessory, 

 arises from the lower two-thirds of the dorsal accessory-vagus nucleus, 

 from about the level of the first cervical nerve up to the level of the 

 tip of the calamus scriptorius. The accessory portion of the nucleus 

 lies behind and to the side of i.e., dorso-lateral to the central canal; 

 the upper or vagus portion is more laterally placed in the floor of the 

 fourth ventricle. Soon after the junction of its bulbar and spinal 

 portions, the nerve divides into two branches, an internal and an 

 external. The external branch, containing the spinal fibres, passes out 

 to supply the trapezius and sterno-mastoid muscles with motor fibres. 

 The internal branch, containing the bulbar fibres, 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 ex- 

 tending throughout approximately the lower two-thirds of 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 un- 

 paralyzed genio-hyoglossus of the opposite side, which is thrown into 

 action in protruding the tongue. 



SECTION X. FUNCTIONS OF THE CENTRAL NERVOUS SYSTEM 



(2) 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 fore- 

 shadowing, 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 incognita ; and no notable 

 additions were made for a thousand years to the doctrines of Galen. 

 Even to-day the utmost limit of our knowledge 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 cere- 

 bral processes that lead to sensations and movements, to emotions and 

 intellectual acts, arise and die out; what molecular changes are asso- 

 ciated with them; above all, how the molecular changes are translated 

 into consciousness how, for example, it is that a series of nerve- 

 impulses from the optic radiation flickering across the labyrinth of the 





