136 HAND-BOOK OF PHYSIOLOGY. 



(i.e., combustible matter or heat) which may be transformed by the cere- 

 bral 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 ex- 

 citement to work, when rest is needed, would be positively harmful. In 

 this respect the varying circulation of blood in the brain exactly resem- 

 bles that which occurs in all other energy transforming parts of the body; 

 e.g., glands or muscles. 



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

 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 function is, however, we do not 

 know. 



Somnambulism and Dreams. What we term sleep occurs often in 

 very different 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 differ- 

 ing degrees of sleep in different parts of the cerebro-spinal nervous sys- 

 tem. 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 con- 

 dition, 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 who have suffered from nightmare. 



In somnambulism the cerebrum is capable of exciting that train of 

 reflex nervous action which is necessary for progression, while the nerve- 

 centre of muscular sense (in the cerebellum?) is, presumably, fully 

 awake; but the sensorium is still asleep, and impressions made on it are 

 not sufficiently felt to rouse the cerebrum to a comparison of the differ- 

 ence between mere ideas or memories and sensations derived from external 

 objects. 



PHYSIOLOGY OF THE CRANIAL NERVES. 



The cranial nerves are commonly enumerated as nine pairs; but the 

 number is in reality twelve, the seventh nerve consisting as it does, of 

 two nerves, and the eighth of three. All arise (superficial origin") from 

 the base of the encephalon, in a double series which extends from the 

 under surface of the anterior cerebral lobes to the lower end of the 

 medulla oblongata. Traced into the substance of the brain and medulla, 

 the roots of the nerves are found connected with various masses of grey 

 matter, which are all connected one with another, and with the cerebral 

 hemispheres. 



