THE BLOODVESSELS 93 



circulation to avoid once more referring to the heart, and it is 

 impossible to avoid being struck by the close similarity in the 

 nervous arrangement for the care, management, and control of 

 these two communicating systems — heart and bloodvessels. The 

 nerve fibres which constrict the bloodvessels, and those which 

 cause a more forcible contraction of the heart, are of the non- 

 medullated variety. They excite muscular action and increase 

 wear and tear. The fibres which dilate the bloodvessels, and 

 those which slow the heart, are both medullated, muscle-restrain- 

 ing, and exciters of repair rather than of disintegration. The 

 impulses affecting heart and bloodvessels pass from the periphery 

 to the centre — viz., they pass from skin and muscle, viscera and 

 glands, and even from the heart and bloodvessels themselves, to 

 the nerve centres presiding over these two important systems. 

 They are true reflex effects, and without these impulses the 

 centres themselves are powerless to effect regulation. 



The delicacy of the balance must not be displaced. The body 

 does not consist of a set of isolated functions working in sequence, 

 three or four or a dozen may be in operation at the same moment, 

 and to each of these a full blood supply must be guaranteed for 

 so long and no longer than it is necessary, or in such reduced or 

 increased quantity as may be needed. Awake or asleep, this 

 remarkable give and take, this perfect adjustment to all require- 

 ments, is taking place from birth to death. 



There has been reason to think that a certain amount of self- 

 help has been obtained from the tissues themselves. For in- 

 stance, the lactic acid and carbon dioxide formed during mus- 

 cular activity may cause dilatation of the vessels and an increased 

 blood supply. This chemical help has been for some time recog- 

 nised, but it is only within recent years that we have learnt 

 that the body is capable of elaborating substances which, when 

 introduced into the blood, greatly affect the calibre of the blood- 

 vessels and the rhythm and tone of the heart itself. 



The Chemical Stimulus in Blood Supply. — The study of the 

 so-called ductless and functionless glands of the anatomist has 

 yielded one of the romances of physiology. The adrenals and 

 pituitary bodies — tissues so insignificant in size that nothing 

 more than a glance was rarely given them during dissection — 

 are known to furnish to the blood-stream chemical substances of 

 the utmost importance to the circulation. 



An extract of the medulla of the adrenals, when introduced 

 into the circulation, increases both the rhythm and tone of the 

 cardiac muscle (see p. 59), and strongly stimulates the contrac- 

 tion of the arteries, excepting those of the heart, with a conse- 

 quent great rise in blood-pressure. There is evidence that the 

 internal secretion of the adrenals finds its way into the blood, 



