638 HANDBOOK OF PHYSIOLOGY. 



physiological conditions. Blood is supplied to the brain for two partly dis- 

 tinct purposes. (1.) It is supplied for mere nutrition's sake. (2.) It is neces- 

 sary for bringing supplies of potential or active energy (i.e., combustible matter 

 or heat) which may be transformed by the cerebral corpuscles into the various 

 manifestations of nerve-force. During sleep blood is requisite for only the first 

 of these purposes ; and its supply in greater quantity would be not only 

 useless, but by supplying an excitement to work, when rest is needed, would be 

 positively harmful. In this respect the varying circulation of blood in the 

 brain exactly resembles that which occurs in all other energy -transform ing 

 parts of the body ; e.g. , glands or muscles. 



At the same time, it is necessary to remember that the normal anaemia of 

 the brain which accompanies sleep is probably a result, and not a cause of the 

 quiescence of the cerebral functions. What the immediate cause of this 

 periodical partial abrogation of functions is, however, we do not know. 



Somnambulism and Dreams. What we term sleep occurs often in very differ- 

 ent degrees in different parts of the nervous system ; and in some parts the 

 expression cannot be used in the ordinary sense. 



The phenomena of dreams and somnambulism are examples of differing 

 degrees of sleep in different parts of the cerebro- spinal nervous system. In the 

 former case the cerebrum is still partially active ; but the mind-products of its 

 action are no longer corrected by the reception, on the part of the sleeping 

 sensorium, of impressions of objects belonging to the outer world ; neither can 

 the cerebrum, in this half -awake condition, act on the centres of reflex action 

 of the voluntary muscles, so as to cause the latter to contract a fact within 

 the painful experience of all \vho have suffered from nightmare. 



In somnambulism the cerebrum is capable of exciting that train of reflex 

 nervous action w r hich is necessary for progression, while the nerve-centre of 

 muscular sense (in the cerebellum?) is, presumably, fully awake ; but the sen- 

 sorium is still asleep, and impressions made on it are not sufficiently felt to 

 rouse the cerebrum to a comparison of the difference between mere ideas or 

 memories and sensations derived from external objects. 



The centres for muscular co-ordinations. In asserting that the co- 

 ordination of complicated muscular movements is connected with the 

 middle parts of the brain below the cerebrum and above the bulb, we 

 were stating a fact deduced from experiments upon animals. It is diffi- 

 cult to understand the exact way in which these parts of the brain are 

 concerned. It appears, however, that co-ordinated movements such as 

 standing, walking, and the maintenance of the equilibrium generally, 

 require to be guided and governed by afferent impulses, which tell of 

 the condition of the body and of its relations to its environment (" its 

 position in space"). The afferent impulses ^IQ firstly visual and tactile 

 sensations, secondly sensations by which we appreciate the condition of 

 our muscles (muscular sense), and thirdly, as appears from experiments 

 on pigeons and other animals, sensations produced by the pressure, in 

 different directions, of the fluid in the semicircular canals of the in- 

 ternal ear. 



Experiments show that when the horizontal semicircular canal is 



