CHAP, iv.] THE VASCULAR MECHANISM. 371 



is inadequate to secure an adequate discharge into the arteries, 

 the residual blood in the ventricles is increased. If the augmentor 

 mechanism be brought to bear on such a weakened and dilated 

 ventricle, it may induce a fruitless expenditure of energy; the 

 beat though increased is still inadequate to secure the needed 

 discharge of the contents, while the fibre is exhausted by the 

 increased metabolism. And the final result of such an effort may 

 be the cessation of the beat. 



192. Turning now to the minute arteries and the peripheral 

 resistance which they regulate, we may call to mind the existence 

 of the two kinds of mechanism, the vaso-constrictor mechanism, 

 which, owing to the maintenance by the central nervous system of 

 a tonic influence, can be worked both in a positive constrictor, and 

 in a negative dilator direction, and the vaso-dilator mechanism, 

 which, so far as we know, exerts its influence in one direction only, 

 viz. to dilate the blood vessels. The latter, dilator mechanism seems, 

 as we have seen, to be used in special instances only, as seen in the 

 cases of the chorda tympani and nervi erigentes ; the use of the 

 former, constrictor mechanism appears to be more general. Thus 

 the relaxation of the cutaneous arteries of the head and neck, which 

 is the essential feature in blushing, seems due to mere loss of tone, 

 to the removal of constrictor influences previously exerted through 

 the vaso-constrictor fibres of the cervical sympathetic. Though 

 probably dilator fibres pass directly along the roots of the cervical 

 and of certain cranial nerves to the nerves of the head and neck, 

 we have no evidence that these come into play in blushing ; as we 

 have seen, blushing may be imitated by mere section of the 

 cervical sympathetic. So also the ' glow ' and redness of the skin 

 of the whole body, i.e. general dilation of the cutaneous arteries, 

 which is produced by external warmth, is probably another in- 

 stance of diminished activity of tonic constrictor influences; though 

 the result, that the dilation produced by warming an animal in an 

 oven is greater than that produced by section of nerves, seems to 

 point to the dilator fibres for the cutaneous vessels which, as we 

 have seen, probably exist in the sciatic and brachial plexuses and 

 possibly in all the spinal nerves, also taking part in the action. 

 A similar loss of constrictor action in the cutaneous vessels may 

 be the result of certain emotions, whether going so far as actual 

 blushing of the body, or merely producing a ' glow.' The warm 

 and flushed condition of the skin, which follows the drinking of 

 alcoholic fluids, is probably, in a similar manner the result of an 

 inhibition of that part of the vaso-motor centre which governs the 

 cutaneous arteries. The effect of cold on the o.ther hand, and of 

 certain emotions, or of emotions under certain conditions, is to 

 increase the constrictor action on the cutaneous vessels, and the 

 skin grows pale. It may be worth while to point out, that in 

 both the above cases, while both the cold and the warmth produce 

 their effects, chiefly at all events through the central nervous 



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