112 GANGLIA. [BOOK i. 



determine the complexity and character of the movements which 

 are affected. In other words, the central nerve-cells concerned in 

 reflex actions are to be regarded as constituting a sort of molecular 

 machinery, the character of the resulting movements being deter- 

 mined by the nature of the machinery set going and its condition 

 at the time being, the character and amount of the afferent 

 impulses determining exactly what parts of and how far the central 

 machinery is thrown into action. 



Actions of Sporadic Ganglia. Seeing that in the spinal cord 

 the nerve-cells undoubtedly are the central structures concerned in 

 the production of reflex action, it is only natural to infer that the 

 nerve-cells of the sporadic ganglia possess similar functions. Yet 

 the evidence of this is at present of very limited extent. With 

 regard to the ganglia on the posterior roots of the spinal nerves, all 

 the evidence goes to shew that these possess no power whatever of 

 reflex action. Of the larger ganglia visible to the naked eye, such 

 as the ciliary, otic, &c., we have indications of reflex action in 

 one only, viz. the submaxillary, and these indications are, as we 

 shall see in treating of the salivary glands, disputed. We have no 

 exact proof that the ganglia of the sympathetic chain, or of 

 the larger sympathetic plexuses, are capable of executing reflex 

 actions. 



In fact, in searching for reflex actions in ganglia, we are 

 reduced to the small microscopic groups of cells buried in the 

 midst of the tissues to which they belong, such as the ganglia of 

 the heart, of the intestine, the bladder, &c. When a quiescent 

 frog's heart is stimulated by touching its surface, a beat takes 

 place. This beat is, as we shall see, a complex, co-ordinated move- 

 ment, very similar to a reflex action brought about by means of 

 the spinal cord; and in its production it is probable that the 

 cardiac ganglia are in some way concerned. When a quiescent 

 intestine is touched or otherwise stimulated, peristaltic action is 

 set up. Here again the ganglia present in the intestinal walls 

 may be supposed to play a part ; but this movement is much more 

 simple than the beat of the heart, and as regards it, and more 

 especially as regards the similar peristaltic action of the ureter, it 

 becomes difficult to distinguish between a movement governed by 

 ganglia, and one produced by direct stimulation of the muscular 

 fibres. We have seen that the great distinction between a reflex 

 action and a movement caused by direct stimulation of a nerve or 

 of a muscle lies in the greater complexity of the former ; and we 

 may readily imagine, that by continued simplification of the central 

 nervous machinery, the two might in the end become so much 

 alike as to be almost indistinguishable. 



In the vertebrate animal then the chief seat of reflex action 

 is the spinal cord and brain. We say 'and brain' because, as 

 we shall see later on, the brain, in addition to its automatism, 

 is as busy a field of reflex action as the spinal cord. 



