674 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



Now, after extirpation of the superior cervical ganglion in the rabbit, 

 and consequent degeneration of the vaso-constrictor fibres to the sub- 

 maxillary gland, vaso-dilatation is still produced by stimulation of the 

 chorda tympani. 1 In this instance, then, the inhibition must be caused 

 by nervous impulses affecting directly the tissue. And we may take it 

 as probable that peripheral inhibition by autonomic nerves is always 

 brought about in this manner. 



It must be mentioned that, according to Fletcher, 2 the rat's retractor penis 

 muscle contains a terminal nervous network, which does not degenerate either 

 when the motor or the inhibitory post-ganglionic fibres to it are cut. His 

 experiments are still in progress. 



But, accepting this, we are still far from any real knowledge of the 

 processes involved in inhibition. One a priori possibility may be dis- 

 posed of. The decrease of rigidity in the inhibited muscular tissue 

 shows that inhibition is not caused by the development of a contractile 

 force, acting in a direction opposed to the normal one and overpowering 

 it. We are then brought to the conclusion that certain nerve impulses 

 — the inhibitory nerve impulses — are able to lessen or to stop the 

 chemical change in the tissue which leads to contraction. As this 

 chemical change is accompanied by a setting free of energy, it is spoken 

 of as a dis-assimilatory or katabolic change, so that inhibitory nerve 

 impulses may be said to lessen or to stop some katabolic change in 

 the tissue. Beyond this we cannot at present go safely. Gaskell 3 has 

 propounded the theory that inhibitory nerves cause assimilation or 

 anabolism, and that the anabolic process stops for the time the 

 katabolic process. And it appears to be implied that the two processes 

 are mutually antagonistic. Gaskell, however, does not enter into a 

 discussion of the relation which he takes to exist between the two 

 processes. 



It does not seem probable that, in general, anabolism interferes with 

 katabolism. The histological changes which take place in the majority 

 of glands during secretion afford good evidence that the two processes 

 go on actively at the same time. No inhibitory nerve fibres have, how- 

 ever, been shown to exist in the glandular tissues ; and, so far, the 

 question is left open, whether anabolism does not stop katabolism in the 

 tissues in which inhibition does occur. 



It is clear, if inhibition is caused by anabolism, that, as regards the 

 unstriated muscle of the arteries, anabolism goes on up to a certain 

 limit simultaneously with katabolism, for by a proper gradation of the 

 stimulus applied to dilator nerves, all degrees of decrease of tone in the 

 arteries can be obtained. 



To account for this, we might suppose that, with increasing strength 

 of stimuli, more and more molecules are involved. On this supposition 

 it follows that a given molecule can only be concerned with one process 

 at a time, and that the predominance of one process in the cell neces- 

 sarily means a diminution of the other. And, consequently, the more 

 fully contracted an artery, the less should be the assimilative processes 

 going on in it. Here we are met by the serious difficulty that in some 



1 Langley, Journ. Physiol., Cambridge and London, 1885, vol. vi. p. 87. 



2 " Proc. Phys. Soc," 1898, p. xxxvii.. Journ. Physiol., Cambridge and London, 

 vol. xxii. 



3 Ibid., 1886, vol. vii. p. 46. 



