548 Dynamic Theory. 



the office of the ganglia on the roots of the spinal nerves, is to " cut 

 off sensation." (Carpenter.) By this it is meant that these ganglia 

 dissipate the stimulus of sensation. Thus, if one fibre of a bundle is 

 the vehicle of a stimulus from a touch upon a part of the skin, it car- 

 ries its stimulus uncommunicated till it reaches the ganglion on the pos- 

 terior root. There it is communicated to the vesicular matter of the 

 ganglion, and by it scattered to many, perhaps all, the fibres of the bun- 

 dle as they proceed from the ganglion into the spinal cord. The same 

 principle, says Carpenter, seems to apply to the motor fibres, because, 

 when such fibres from the cerebro-spinal system pass through the sym- 

 pathetic ganglia, they do not transmit motor impulses so rapidly or so 

 surely as in the cases in which they are not so connected. Evidently 

 the motor stimulus in emerging from the ganglion passes into so many 

 fibres that its power over any one is reduced ; but its influence is com- 

 municated wherever these subdivisions of it go, and tends to modify 

 the action of other stimulations with which it mingles in any cell or 

 ganglion. These vesicular bodies therefore elaborate the stimuli by 

 these processes of dispersion and collection. 



The movements involved in eating and assimilating food, are neces- 

 sarily partly animal and partly vegetative functions, with essential co- 

 operation between the two classes of actions. The muscular movements 

 of placing the food in the mouth, and wagging the jaws in mastication, 

 are under the control of the will, but mastication is attended with the 

 involuntary flow of saliva, which is liberated from the salivary glands, 

 and is an automatic resultant or accompaniment growing out of and 

 consequent upon the voluntary animal movement of the jaws. But this 

 flow of saliva may take place without the actual mastication of food, 

 the thought of doing so being sufficient to produce the necessary stim- 

 ulus. The propulsion of food through the pharynx and down the 

 esophagus, is purely automatic, and is reflex from the stimulus excited 

 by the presence of the food itself. It is not possible to perform the 

 operation of swallowing unless there is something to swallow, the direct 

 stimulus of a body in contact with the parts being essential. The 

 movement cannot be initiated by thinking about it, nor by an exercise 

 of the will, although the pharynx is connected with the cranio spinal 

 nerve centers by means of the glosso pharyngeal nerve, and the esopha- 

 gus is connected with the same by means of the vagus. These, with 

 other nerve connections, however, serve to establish co-operation in the 

 successive contraction of the parts concerned in the action of swallow- 

 ing, which is therefore purely reflex. The passage of the stimulus 

 through the cranial nerve centers may or may not arouse consciousness 

 of the action going on, but at all events consciousness is not essential 

 to the performance of the action. 



