SOME FEATUKES OF THE CIRCULATION. 241 



by disordered nutrition (as, for instance, by imperfect coronary circulation, 

 such as seems to accompany diseases of the aortic valves leading to regur- 

 gitation from the aorta into the ventricle, in which cases sudden death i& 

 not uncommon), which is able just to accomplish each beat, but no more, 

 which has but a scanty saving store of energy, under some strain or other 

 untoward influence, misses a leap, falls, and is no more able to rise. Doubt- 

 less in such cases could adequate artificial aid be promptly applied in time, 

 could the fallen heart be stirred even to a single good beat, the favorable 

 reaction of that beat might bring a successor, and so once more start the 

 series ; but such a period of grace, of potential recovery, is a brief one. 

 Even a coarse skeletal muscle, when cut off from the circulation, soon loses 

 its irritability beyond all recovery, and the heart cut off from its own influ- 

 ence on itself runs down so rapidly that the period of possible recovery is 

 measured chiefly by seconds. 



178. Turning now to the minute arteries and the peripheral resist- 

 ance 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 direc- 

 tion, and the vaso-dilator mechanism, which, as far as we know, exerts its 

 influence in one direction only, viz., to dilate the bloodvessels. 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 sec- 

 tion of the cervical sympathetic. So also the " glow " and redness of 

 the skin of the whole body i. e., dilatation generally 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 dilatation 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 

 effect of cold, on the other hand, and of certain emotions, or of emotions 

 under certain conditions, is to increase the constrictor action on the cutane- 

 ous 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 warmth produce their 

 effect chiefly at all events through the central nervous system, and very 

 slightly, if at all, by direct action on the skin, their action on the central 

 nervous system is not simply a general augmentation or inhibition of the 

 whole vasomotor centre. On the contrary, the cold, while it constricts the 

 cutaneous vessels, so acts on the vasomotor centre as to inhibit that portion 

 of the vasomotor centre which governs the abdominal splanchnic area ; 

 while less blood is carried to the colder skin, by the opening up of the 

 splanchnic area more blood is turned on to the warmer regions of the body,, 

 and the rise of blood-pressure which the constriction of the cutaneous vessels 

 16 



