630 HANDBOOK OF PHYSIOLOGY. 



taken as examples by which to illustrate the common modes of actk>k of 

 all nerve-centres. Indeed, complex as the sympathetic system, taken as 

 a whole, is, it presents in each of its parts a simplicity not to be found 

 in the cerebro-spinal system: for each ganglion with afferent and efferent 

 nerves forms a simple nervous system, and might serve for the illustra- 

 tion of all the nervous actions with which the cerebrum is unconnected. 



The parts principally supplied with sympathetic nerves are usually 

 capable of none but involuntary movements, and when the cerebrum 

 acts on them at all, it is only through the strong excitement or depress- 

 ing influence of some passion, or through some voluntary movement with 

 which the actions of the involuntary part are commonly associated. 

 The heart, stomach, and intestines are examples of these statements; for 

 the heart and stomach, though supplied in large measure from the 

 pneumogastric nerves, yet probably derive through them few filaments 

 except such as have arisen from their ganglia, and are therefore of the 

 nature of sympathetic fibres. 



The parts which are supplied with motor power by the sympathetic 

 nerve continue to move, though more feebly than before, when they are 

 separated from their natural connections with the rest of the sympathetic 

 system, and wholly removed from the body. Thus, the heart, after it 

 is taken from the body, continues to beat in Mammalia for one or two 

 minutes, in reptiles and Amphibia for hours; and the peristaltic motions 

 of the intestine continue under the same circumstances. Hence the 

 motions of the parts supplied with nerves from the sympathetic are 

 shown to be, in a measure, independent of the brain and spinal cord; 

 this independent maintenance of their action being, without doubt, due 

 to the fact that they contain, in their own substance, the apparatus of 

 ganglia and nerve-fibres by which their motions are immediately gov- 

 erned. 



It seems to be a general rule, at least in animals that have both cere- 

 bro-spinal and sympathetic nerves much developed, that the involuntary 

 movements excited by stimuli conveyed through ganglia are orderly and 

 like natural movements, while those excited through nerves without 

 ganglia are convulsive and disorderly; and the probability is that, in the 

 natural state, it is through the same ganglia that natural stimuli, im- 

 pressing centripetal nerves, are reflected through centrifugal nerves to 

 the involuntary muscles. As the muscles of respiration are maintained 

 in uniform rhythmic action chiefly by the reflecting and combining power 

 of the medulla oblongata, so are those of the heart, stomach, ^and intes- 

 tines, by their several ganglia. And as with the ganglia of the sympa- 

 thetic and their nerves, so with the medulla oblongata and its nerves 

 distributed to the respiratory muscles if these nerves of the medulla 

 oblongata itself be directly stimulated, the movements that follow are 

 convulsive and disorderly; but if the medulla be stimulated through a 



