88 HAND-BOOK OF PHYSIOLOGY. 



would be impossible. A most important part of the process by which 

 these acquired reflex actions come to be performed automatically consists 

 in what is termed association. If two acts be at first performed voluntarily 

 in succession, and this succession is often repeated, the performance of 

 the first is at once followed mechanically by the second. Instances of 

 this "force of habit" must be within the daily experience of every one. 



Of course it is only such actions as have become entirely reflex that 

 can be performed during complete unconsciousness, as in sleep. Oases 

 of somnambulism are of course familiar to every one, and authentic 

 instances are on record of persons writing and even playing the piano 

 during sleep. 



4. Automatism. To nerve centres, it is said, belongs the property 

 of originating nerve-impulses, as well as of receiving them and conducting 

 and reflecting them. 



The term automatism is employed to indicate the origination of nervous 

 impulses in nerve-centres, and their conduction therefrom, independently 

 of previous reception of a stimulus from another part. It is impossible, 

 in the present state of our knowledge, to say definitely what actions in 

 the body are really in this sense automatic. An example of automatic 

 nerve-action has been already referred to, i.e., that of the respiratory 

 centre, but the apparently best examples of automatism are found, 

 however, in the case of the cerebrum, which will be presently considered. 



5. and 6. Augmentation and Inhibition. Nerve cells not only 

 receive and reflect nerve impulses, and also in some cases even originate 

 such impulses, but they are also capable of increasing the impulse, and 

 the result is what is called augmentation; and when a nerve centre is in 

 action its action is also capable of being increased or diminished (inhibi- 

 tion) by afferent impulses. This is the case in whatever way the centre 

 has caused the action, whether of itself or by means of previous afferent 

 impulses. The action, by which a centre is capable of being inhibited or 

 exalted, has been well shown in the case of the vaso-motor centre, before 

 described (p. 155, Vol. I.). This power, which can be exerted from the 

 periphery, is very important in regulating the action even of partially 

 automatic centres such as the respiratory centre. 



CEKEBKO-SPINAL NEKVOUS SYSTEM. 



The physiology of the cerebro- spinal nervous system includes that of 

 the Spinal Cord, Medulla Oblongata, and Brain, of the several Nerves 

 given off from each, and of the Ganglia on those nerves. 



Membranes of the Brain and Spinal Cord. The Brain and 

 Spinal Cord are enveloped in three membranes (1) the Dura Mater, 

 (2) the Arachnoid, (3) the Pia Mater. 



(1.) The Dura Mater, or external covering, is a tough membrane com- 



