CIRCULATION OF BLOOD 75 



stoppage of the heart in diastole, depending upon the 

 strength of the stimulation. How the action of the vagus 

 on the cardiac muscle is brought about is not known. 



Pathological-anatomical changes have been observed in the 

 cardiac muscle (atrophy and degeneration) after section of vagus. 



The centre for the cardiac inhibitory nerves lies in the 

 medulla oblongata. Its activity is increased by lack of 

 oxygen and increase of carbon dioxide in the blood, and by 

 increased blood pressure. It can also be stimulated in- 

 directly by stimulation carried to it by centripetal nerves 

 from the cerebral hemispheres. 



The extent of its activity depends also upon psychical influences 

 (palpitation of the heart). Reflex cardiac inhibition takes place 

 if, e.g. in a frog, the sensory nerves of the abdomen are stimulated 

 by tapping the abdomen (Goltz's tapping experiment). 



Atropin and curare in large doses destroy the action of the 

 vagus upon the heart; muscarin and nicotin stimulate the vagus 

 endings in the heart. The action of muscarin is neutralized by 

 atropin, that of curare by nicotin. Digitalin stimulates both the 

 vagus endings in the heart and the centre in the medulla. 



(^) The cardiac accelerating nerves are the nervi accele- 

 rantes which pass from the first thoracic and the cervical 

 ganglia of the sympathetic to the cardiac plexus. Stimula- 

 tion causes increase in the frequency and force of the heart- 

 beat. 



It is supposed that their centre lies in the medulla 

 oblongata, and that this is also tonic. If the vagi are cut, 

 electric stimulation of the medulla causes acceleration of the 

 pulse. 



2. Innervation of the blood vessels. The muscles of the 

 vessels (smooth muscles) are most developed in the walls of 

 the arteries, less in the veins. The walls of the capillaries 

 are also supposed to be contractile, but this is independent 

 of the nervous system. The nervous elements for the muscles 

 of the vessels lie partly in the walls of the vessels themselves 

 (ganglionic cells, nerve plexus) and partly enter the walls 

 as vaso-motor nerves. There are nerves which constrict 

 and nerves which dilate the blood vessels. 



