118 CIRCULATION 



motor changes are often called Traube-Heriny waves. They 

 indicate alterations in the tonus of the bloodvessels, and are 

 caused by irradiations of impulses from other centres to the 

 vasomotor centre. Vasomotor reflexes arise by impulses which 

 come either from the bloodvessels or from the end organs of 

 sensory nerves. In the latter case the dilatation affects not 

 only the portion from which the impulses come, but also parts 

 functionally related. Thus stimulation of the tongue causes 

 dilatation of the vessels of the submaxillary gland. The fact 

 that stimulation of the same nerve gives rise sometimes to 

 a reflex dilatation instead of the more usual constriction, 

 has given rise to the conception of special pressor and depressor 

 fibers. The former constrict, the latter dilate. The cardiac 

 depressor nerve is a good example. Its fibers form afferent 

 paths from the heart and from the root of the aorta. If cut, 

 and the peripheral end is stimulated, no result follows. If, 

 however, the central end is stimulated, a fall of blood pressure 

 occurs and also perhaps a slowing of the heart beat. The 

 latter effect is due to a reflex stimulation of the cardio-inhibi- 

 tory centre. The fall in blood pressure takes place 'because 

 the nerve when stimulated inhibits the tonic activity of the 

 vasoconstrictor centre. In this way the nerve plays an impor- 

 tant regulatory role, for if the blood pressure rises above normal 

 limits the endings of the nerve will be stimulated. Each heart 

 beat, in fact, sends up the depressor nerve a nerve impulse 

 which can be detected by a string galvanometer. It forms 

 part of an important compensatory blood-pressure mechanism. 



The chemical regulation of the caliber of bloodvessels has 

 recently been emphasized. Acids, in slight concentration, 

 like lactic or carbonic, produce a dilatation. They may serve 

 as the cause of a local dilatation during functional activity, 

 and thus provide the organ with more blood. On the other 

 hand, the internal secretion of the adrenal gland and possibly 

 also of the infundibular portion of the pituitary have a reverse 

 effect. The distention of the arterioles by internal pressure 

 may act as a mechanical stimulus and lead to an increased 

 tone. 



Circulation of Lymph. Lymph is a pale, straw-colored 

 liquid found in the extravascular spaces (and lymph vessels 

 of the body), bathing every tissue element. It contains a 



