352 TEXT-BOOK OF PHYSIOLOGY 



impulses from (i) the cerebrum in consequence of emotional states of 

 a joyous character (see page 326), and (2) from different regions of 

 the body transmitted by afferent nerves, the terminals of which are 

 stimulated by physiologic processes. In either instance the nerve impulses 

 inhibit the normal activity of the cardio-inhibitor center, causing it to 

 discharge a less number of nerve impulses through its efferent fibers to the 

 heart, thus permitting the heart to increase in activity under the 

 accelerating influence of the cardio-accelerator center. 



Though it is difficult to point out any particular region of the body 

 from which nerve impulses, developed by physiologic processes, are trans- 

 mitted by afferent nerves to this center and inhibit or decrease its activity, 

 nevertheless the results of physiologic experimentation and clinic observa- 

 tions lead to the inference that this is the case under normal conditions. 

 Thus a partial or complete inhibition or a condition of decreased activity 

 of the cardio-inhibitor center as shown by an increase in the rate of the 

 heart, follows stimulation of the central end of many afferent nerves; 

 thus, stimulation of the central end of the sciatic nerve with weak induced 

 electric currents caused not only an increase in rate but an increase in 

 blood pressure of 37 mm. Hg. ; stimulation of the saphenous nerve, the 

 brachial plexus, etc., has a similar effect. Pathologic processes and opera- 

 tive procedures in different regions of the body whereby the terminals 

 of afferent nerves are stimulated, frequently lead to an acceleration of 

 the rate which may be regarded as an expression of an inhibition more or 

 less complete of the cardio-inhibitor center. Whether there is a con- 

 comitant rise of pressure or not will- depend on the presence or absence 

 of other factors connected with the pathologic process or operative 

 procedure. 



The effect on the heart rate that follows a partial or complete inhibition 

 of the cardio-inhibitor center is shown experimentally by the division 

 of its efferent fibers contained in the trunk of the vagus, thus eliminating 

 its influence on the heart entirely; thus division of one vagus in the dog 

 was followed by an increase in the rate of the heart-beat from 1 1 1 to 144 

 and a rise in the blood pressure from 130 to 140 mm. Hg. in the course of 

 a few seconds ; division of the second vagus led to a further increase of the 

 rate of 36 and a further rise of the blood pressure of 30 mm. Hg. (Dawson). 

 The administration of atropin has a similar effect (see page 327). Any 

 diminution in the activity of the cardio-inhibitor apparatus in any part 

 of its course is followed by an increase in the rate and perhaps in the 

 force of the heart-beat. As to whether there will be an attendant rise of 

 pressure will depend on the presence of factors connected with the causes 

 inducing the inhibition of the center. 



2. An increase in the peripheral resistance due to an increased contrac- 

 tion of the arteriole muscles, will by diminishing the outflow of blood 

 lead to an increase in the volume of the blood in the arteries and a rise of 

 pressure even though the rate and force of the heart remain the same. 



Increased contraction of the arteriole muscles may be brought about 

 (i) by an increase in the activity of the general vaso-motor (constrictor) 

 center and (2) by the direct action of drugs, e.g., adrenalin. The vaso- 

 motor center may be excited to increased activity (i) by an increase in the 

 percentage of carbon dioxid in the blood and (2) by the arrival of nerve 



