THE VISCERAL AND GUSTATORY APPARATUS 235 



rate is regulated through the cardiac nerves. The caliber of 

 the smaller blood-vessels and hence the amount of blood which 

 can pass through them is regulated by vasomotor nerves. Both 

 the heart and the muscular walls of the vessels have a double 

 innervation. The heart has an accelerator nerve and an in- 

 hibitory nerve; the smaller arteries have vasodilator and vaso- 

 constrictor nerves. The amount of blood pumped by the heart 

 at any time will depend upon the equilibrium existing between 

 its accelerator and its inhibitory fibers and upon the resist- 

 ance offered by the peripheral vessels; that flowing through 

 any particular system of blood-vessels will be affected also by 

 the equilibrium between the vasodilator and the vasoconstrictor 

 nerves of these vessels. 



There are sympathetic ganglia within the heart. Its extrinsic 

 nerve supply includes afferent fibers to the brain and efferent 

 fibers of two sorts, viz., the accelerator and inhibitory fibers 

 already mentioned. The afferent fibers are represented in a 

 small sympathetic nerve, the nerve of Cyon, which is also called 

 the depressor nerve. They arise from the walls of the ventricles 

 of the heart and join the vagus trunk, through which they enter 

 the medulla oblongata. Stimulation of this nerve produces a fall 

 of arterial pressure by dilating the vessels throughout the body, 

 especially in the viscera. It appears to act to reduce the labor 

 of the heart when intraventricular pressure becomes excessive. 



The medulla oblongata contains a center whose stimulation 

 causes inhibition of the heart-beat. These efferent fibers go out 

 as preganglionic fibers of the vagus nerve and terminate in the 

 cardiac sympathetic plexus (Fig. 108), where their postganglionic 

 neurons are located. There is also a center in the medulla ob- 

 longata (which has not been precisely localized) whose stimula- 

 tion causes acceleration of the heart-beat. These accelerator 

 nerve-fibers do not leave the brain through the vagus, but appar- 

 ently they descend through the spinal cord to the lower cervical 

 region and pass out into the sympathetic nervous system at this 

 level. The centers of vasomotor control of various regions of 

 the body are indicated in Fig. 111. 



Organs of Respiration. Oxygen is supplied to the tissues of 

 the body in a great variety of ways in different animals. In 

 some of the simpler animals, as in plants generally, oxygen is 



