194 PHYSIOLOGY FOR DENTAL STUDENTS. 



blood pressure and in the velocity of the circulation of the blood, 

 suggests that normally the vasomotor and cardiac nerves play 

 an important role in the proper distribution of blood in the 

 various parts of the body. It may be supposed that normally the 

 nerves of the vascular system function to control the blood flow 

 through the various organs according to their respective needs. 

 Whenever the work of an organ is increased, the blood flow like- 

 wise is augmented in the part, while in the rest of the body the 

 blood flow is diminished to a greater or less extent. The blood 

 supply is continually changing according to the call of the vari- 

 ous tissues for blood ; now the muscles, now the digestive organs, 

 now the brain demand more blood, and this is supplied in the 

 proper amount by the nervous system commanding some arte- 

 rioles to dilate and others to constrict. 



Haemorrhage. The action of the vasomotor mechanism is 

 beautifully shown in the case of haemorrhage. As blood is with- 

 drawn, the vasomotor nerves are stimulated by the falling pres- 

 sure in the brain. This brings about a more powerful tonic con- 

 striction of the vessels through the action of vasoconstrictor 

 nerves, the vascular area becomes smaller and smaller in size, 

 and less blood is required to maintain the blood pressure. Be- 

 cause of this mechanism a relatively large amount of blood can 

 be lost without affecting the general blood pressure (Fig. 26, 

 No. V). 



The Regulation of the Blood Supply by Chemical Stimuli. 

 The calibre of the blood vessels may be influenced by other means 

 than through their nervous mechanism. Acids in very small 

 concentrations cause a vascular dilation. For example, lactic 

 acid and carbonic acid, both of which are formed during muscu- 

 lar work, may produce a local dilatation of the blood vessels, the 

 phenomenon thus constituting an automatic mechanism for deliv- 

 ering more blood to a part when it is needed. On the other hand, 

 the secretion of the adrenal and of a portion of the pituitary 

 gland (see p. 131) produces a constriction of the vessels and 

 thus tends to maintain the normal blood pressure. Recently it 

 has been shown that during periods of excitement and sensory 

 pain the amount of the adrenal secretions may be increased and 



