268 ANIMAL PHYSIOLOGY. 
inhibited by gentle stimulation of almost any portion of the 
animal is wonderful. Again, in some animals the vagus arrests 
| | | —-——- Brain above Medulla. 
T 
Cardio-inhibitory Center 
in Medulla Oblongata. 
R. Vagus. 
Heart. 
——— Afferent Nerve. 
Outlying Area with its 
Nerves. 
Fig. 240.—Diagram of the inhibitory mechanism of the heart. The arrows indicate in all 
cases the path the nervous impulses take. I. Path of afferent impulses from the heart 
itself. II. Path from parts of the brain above (or anterior to) the vaso-motor center. A 
similar one might, of course, be mapped out along the spinal cord. III. Path from some 
peripheral region. The downward arrows indicate the course of efferent impulses, which 
» probably usually pass by both vagi. 
the heart for only a brief period, when it breaks away into its 
usual (but increased) action. 
In the fish, menobranchus, and probably other animals, the 
irritability of some subdivision of the heart is lost during the 
vagus inhibition—i. e., it does not respond to a mechanical 
stimulus. 
There is usually acertain order in which the heart recom- 
mences after inhibition (viz., sinus, auricles, ventricles) ; but 
there are variations in this, also, for different animals: It is 
