VASOMOTOR ACTIONS. 211 



ventricle (and so of the heart as a whole) is in a measure governed by the 

 auricle. 



An interesting combination of direct mechanical effects and indirect ner- 

 vous effects is seen in the relation of the heart's beat to blood-pressure. When 

 the blood -pressure is high, not only is the resistance to the ventricular systole 

 increased, but other things being equal, more blood flows (in the mamma- 

 lian heart) through the coronary artery. Both these events would increase 

 the activity of the heart, and we might expect that the increase would be 

 manifest in the rate of the rhythm as well as in the force of the individual 

 beats. As a matter of fact, however, we do not find this. On the contrary 

 the relation of heart-beat to pressure may be put almost in the form of a 

 law, that the " rate of the beat is in inverse ratio to the arterial pressure," 

 a rise of pressure being accompanied by a diminution, and fall of pressure 

 with an increase, of the pulse-rate. This, however, only holds good if the 

 vagi be intact. If these be previously divided, then in whatever way the 

 blood-pressure be raised whether by injecting blood or clamping the aorta 

 or increasing the peripheral resistance, through that action of the vasomotor 

 nerves which we shall have to describe directly or in whatever way it be 

 lowered, no such clear and decided inverse relation between blood-pressure 

 and pulse-rate is observed. It is inferred, therefore, that increased blood- 

 pressure causes a slowing of the pulse, when the vagi are intact, because the 

 cardio-inhibitory centre in the medulla is stimulated by the high pressure, 

 either directly by the pressure obtaining in the bloodvessels of the medulla, 

 or in some indirect manner, and the heart in consequence to a certain extent 

 inhibited. 



CHANGES IN THE CALIBRE OF THE MINUTE ARTERIES. VASOMOTOR 



ACTIONS. 



149. All arteries contain plain muscular fibres, for the most part cir- 

 cularly disposed, and most abundant in, or sometimes almost entirely con- 

 iined to, the middle coat. Moreover, as the arteries become smaller the 

 muscular element, as a rule, becomes more and more prominent as compared 

 with the other elements, until, in the minute arteries, the middle coat consists 

 almost entirely of a series of plain muscular fibres wrapped around the in- 

 ternal coat. Nerve fibres, of whose nature and course we shall presently 

 speak, are distributed largely to the arteries and appear to end chiefly in 

 fine plexuses around the muscular fibre, but their exact terminations have 

 not as yet been clearly made out. By mechanical, electrical, or other 

 stimulation, this muscular coat may, in the living artery, be made to con- 

 tract. During this contraction, which has the slow character belonging to 

 the contractions of all plain muscles, the calibre of the vessel is diminished. 

 The veins also, as we have seen, possess muscular elements, but these vary 

 in amount and distribution very much more in the veins than in the arteries. 

 Most veins, however, are contractile, and may vary in calibre according to 

 the condition of their muscular elements. Veins are also supplied with 

 nerves. It will be of advantage, however, to consider separately the little 

 we know concerning the changes in the veins, and to confine ourselves at 

 present to the changes in the arteries. 



If the web of a frog's foot be watched under the microscope, any indi- 

 vidual small artery will be found to vary considerably in calibre from time 

 to time, being sometimes narrowed and sometimes dilated ; and these changes 

 may take place without any obvious changes either in the heart-beat or in 

 the general circulation ; they are clearly changes of the artery itself. Dur- 

 ing the narrowing, which is obviously due to a contraction of the muscular 



