Chap, iv.] THE VASCULAR MECHANISM. 303 



future efforts ; it saves the heart at the expense of the rest of the 

 economy. The heart, so far as we know, cannot in the working 

 of the living economy be brought to a final arrest by the simple 

 action of the vagus. The effect of the augmentor action on the 

 other hand is to increase the expenditure of energy ; it saves the 

 rest of the economy at the expense of the heart. And probably 

 in some cases augmentor action may bring about the cessation of 

 the heart beat. Disordered cardiac nutrition shews itself fre- 

 quently in a dilated condition of the ventricles ; the systole 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. 



§ 170. 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 mechan- 

 ism 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 previ- 

 ously 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 instance of diminished activity of tonic con- 

 strictor 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 



