988 ANIMAL PHYSIOLOGY. 
caused by several events combined, in which, however, the ten- 
sion of the valves must take a prominent share. The work of 
the heart is dependent on the quantity of blood it ejects and 
the pressure against which it acts. The pulse is an elevation 
of the arterial wall, occurring with each heart-beat, in conse- 
quence of the passage of a wave over the general blood-stream. 
There is a distention of the entire arterial system in every di- 
rection. The pulse travels with extreme velocity as compared 
with the blood-current. The heart-beat varies in force, fre- 
quency, duration, etc., and with age, sex, posture, and numer- 
ous other circumstances. 
The whole of the circulatory system is regulated by the cen- 
tral nervous system through nerves. There is in the medulla 
oblongata a small collection of nerve-cells making up the 
cardio-inhibitory center. This center, with varying degrees of 
constancy, depending on the group of animals and the needs 
of the organism, sends forth impulses (which modify the beat 
of the heart in force and frequency) through the vagi nerves. 
There are nerves of the sympathetic system with a center in 
the cervical spinal cord, and possibly another in the medulla, 
which are capable of originating either an acceleration of the 
heart-rhythm or an increase of the force of the beat, or both 
together, known as accelerators or augmentors. In the verte- 
brates thus far examined the vagus is in reality a vago-sympa- 
thetic nerve, containing inhibitory fibers proper, and sympa- 
thetic, accelerator, or motor fibers. 
The inhibitory fibers can arrest, slow, or weaken the cardiac 
beat; the sympathetic accelerate it or augment its force. 
When both are stimulated together, the inhibitory prevail. 
These nerves, as also the accelerators, exercise a profound 
influence upon the nutrition of the heart, and effect its electri- 
cal condition when stimulated, and we may believe when influ- 
enced by their own centers. 
The inhibitory fibers tend to preserve and restore cardiac 
energy; the sympathetic, whether in the vagus or as the aug- 
mentors, the reverse. The vagus nerve (and probably the de- 
pressor) acts as an afferent, cardiac sensory nerve reporting on 
the intra-cardiac pressure, etc., and so enabling the vaso- 
motor and cardio-inhibitory centers, which are, it would seem, 
capable of related and harmonious action to act for the general 
good. . 
The arterioles must be conceived as undergoing very fre- 
quent changes of caliber. They are governed by the vaso- 
