42 CEREBRAL FORCES. [i. 



of the nerves, as when salt is sprinkled on them ; and by this 

 experiment all probability of a mere mechanical excitement by 

 the impression, (as in filling the heart with warm fluids, or with 

 air,) is taken away. Since, also, external sensation is not neces- 

 sary to the ordinary motion of the heart, there must be natural 

 hindrances to transmission, in virtue of which certain external 

 impressions are retained in these and analogous nerves, so that 

 they cannot pass upwards to the brain (see § 55 — 61). It is diffi- 

 cult to say, in what these hindrances consist. The external im- 

 pression on the heart is really there, since every motion of the 

 heart is excited by it. It is also in the stomach after taking food, 

 since the peristaltic motion of the latter is renewed by it. What 

 prevents the propagation of all these impressions to the brain ? 

 There is nothing to be found in the nerves adapted to this end, 

 except certain formations found scattered on the motor nerves, 

 termed ganglia (14), and the point of insertion of the smaller 

 fibrils in the larger trunks, where also a sort of ganglion is 

 formed. At these points, the direct course of the fibrils is 

 interrupted, and here the external impression traversing them 

 can be deflected from its course, and its transmission to the brain 

 prevented (13, 14); the more especially, as the outer thick coat 

 of the ganglion acts in some degree as a muscle, and, by a slight 

 compression, can hinder the transmission (Monro) . But is it not 

 probable, that an external impression on motor nerves of this 

 kind, is expressly intended, when it reaches the ganglia, to be 

 deflected to the trunk or branch of another nerve, or to another 

 fibril of the same nerve interwoven in the ganglion ? For thereby 

 it would cause a reflected or retrogressive action in the fibril, 

 as if an impression were excited in it, and sent from above 

 downwards, or, as if sent from the brain ; when thus deflected, 

 it puts certain parts into movement, just as an impression really 

 transmitted along the nerves from above downwards, and so 

 imitates the latter by this reflected course (31, 121, 122, 137). 

 If, however, this conjecture be groundless, still the fact remains, 

 that external impressions on certain nerves, not received directly, 

 excite movements without reaching the brain, and without being 

 felt. 



49. V. Amongst the natural impediments to external sensa- 

 tions, those also may be classed (according to 46, iii) which 

 prevent the external impression from developing in its proper 



