14 DISEASES OF THE HORSE. 
In disease, the pulse may become slower or more rapid than in 
health. Slowing of the pulse may be caused by old age, great ex- 
haustion, or excessive cold. It may be due to depression of the cen- 
tral nervous system, as in dumminess, or be the result of the admin- 
istration of drugs, such as digitalis or strophantus. A rapid pulse 
is almost always found in fever, and the more severe the infection 
and the weaker the heart the more rapid is the pulse. Under these 
conditions, the beats may rise to 80, 90, or even 120 per minute. 
When the pulse is above 100 per minute the outlook for recovery is 
not promising, and especially if this symptom accompanies high 
temperature or occurs late in an infectious disease. In nearly all of 
the diseases of the heart and in anemia the pulse becomes rapid. 
The pulse is irregular in diseases of the heart, and especially where 
the valves are affected. The irregularity may consist in varying inter- 
vals between the beats or the dropping of one or more beats at regu- 
lar or irregular intervals. The latter condition sometimes occurs in 
chronic diseases of the brain. The pulse is said to be weak, or soft, 
when the beats are indistinct, because little blood is forced through 
the artery by each contraction of the heart. This condition occurs 
when there is a constriction of the vessels leading from the heart and 
it occurs in certain infectious and febrile diseases, and is an indication 
of heart weakness. 
In examining the heart itself it is necessary to recall that it lies in 
the anterior portion of the chest slightly to the left of the median line 
and that it extends from the third to the sixth rib. It extends almost 
to the breastbone, and a little more than half of the distance between 
the breastbone and the backbone. In contracting, it rotates slightly 
on its axis, so that the point of the heart, which lies below, is pressed 
against the left chest wall at a place immediately above the point of 
the elbow. The heart has in it four chambers—two in the left and 
two in the right side. The upper chamber of the left side (left auri- 
*cle) receives the blood as it comes from the lungs, passes it to the 
lower chamber of the left side (left ventricle), and from here it is sent 
with great force (for this chamber has very strong, thick walls) 
through the aorta and its branches (the arteries) to all parts of the 
body. The blood returns through the veins to the upper chamber of 
the right side (right auricle), passes then to the lower chamber of the 
right side (right ventricle), and from this chamber is forced into the 
lungs to be oxidized. The openings between the chambers of each 
side and into the aorta are guarded by valves. 
If the horse is not too fat, one may feel the impact of the apex of 
the heart against the chest wall with each contraction of the heart by 
placing the hand on the left side back of the fifth rib and above the 
point of the elbow. The thinner and the better bred the horse is the 
more distinctly this impact is felt. If the animal is excited, or if he 
