CHAP. IIL] AUTOMATIC ACTIONS. 109 



matic movements ; T, r e cannot speak of them as being simply excited 

 by the presence of food in the canal, any more than we can say that 

 the beat of the heart is caused by the presence of blood in its 

 cavities. When absent they may be set agoing, and when present 

 may be stopped without any change in the contents of the canal. 

 They may, of course, be influenced by the contents, just as the beat 

 of the heart is, influenced by the quantity of blood in its cavities. 

 Throughout the intestines are found the nerve plexus of Auerbach 

 and that of Meissner ; to each or both of these the automatism of the 

 peristaltic movements has been referred. Yet in the ureter, whose 

 peristaltic waves of contraction closely resemble that of the intes- 

 tine, automatism is evident in the middle third of its length even 

 when completely isolated; in which region (in the rabbit at 

 least), according to Engelmann, ganglia, and indeed nerve-cells, are 

 entirely absent. 



Thus, while in the spinal cord there is doubt whether purely 

 automatic, as stringently distinguished from reflex, actions take 

 place, in the case of the sporadic ganglia the uncertainty is whether 

 the clearly automatic movements of the organs with which the 

 ganglia are associated are due to the nerve-cells of the ganglia, or 

 to the muscular tissue itself. 



Reflex Actions. The spinal cord offers the best and most 

 numerous examples of reflex action. In fact, reflex action may be 

 said to be, par excellence, the function of the spinal cord ; and the 

 grey matter of the spinal cord may be broadly considered as a 

 multitude of reflex centres. We have here to consider the cord 

 merely in its general aspects ; and must postpone the special con- 

 sideration of the particular forms of reflex action which it exhibits, 

 as they come before us in various connections, or until we have to 

 deal with it as part of the great central nervous machinery. 



In its simplest form a reflex action is as follows. All the ma- 

 chinery it demands is (a) a sentient surface (external or internal), 

 connected by (6) a sensory, or to adopt the more general and 

 better term afferent nerve, with (c) a central nerve-cell or group 

 of connected nerve-cells, which is in relation by means of (d) a 

 motor, or efferent, nerve, or nerves, with (e) a muscle, or muscles, 

 or some other irritable tissue-elements, capable of responding by 

 some change in their condition, to the advent of efferent impulses. 

 The afferent impulses started in a, passing along 6, reach the 

 centre c, are there transmuted into efferent impulses, which, 

 passing along d, finally reach e, and there produce a cognisable 

 effect. The essence of a reflex action consists in the transmutation, 

 by means of the irritable protoplasm of a nerve-cell, of afferent into 

 efferent impulses. As an approach to a knowledge of the nature 

 of that transmutation, we jnay lay down the following propositions. 



The number, intensity, character and distribution of the efferent 

 impulses are determined chiefly by the events which take place in the 



