RESPIRATORY & CIRCULATORY SYSTEMS 171 



sudden that the rider is amazed to see his horse in this 

 condition. *' Prevention is better than cure " even when 

 the latter is possible, but pulmonary apoplexy is a very 

 fatal disease, and the symptoms extremely urgent. 



Pleurisy 



This consists of an inflammatory state of the pleural 

 membranes, i.e. the coverings of the lungs and the lining 

 of the chest walls. As stated in the preceding paragraphs 

 it usually forms part of pneumonia. Sometimes it is the 

 result of a wound in the chest wall. It is a painful 

 disease, and commonly fatal, except in the case of 

 mild attacks. It is often accompanied by effusion, 

 and if this is severe the probabihty is that the animal 

 will die. The removal of fluid from the chest of a horse 

 is not, as a rule, a successful operation, and it is desirable 

 to avoid this whenever possible. 



The Heart 



The heart is enclosed in a sac — the pericardium — and 

 this organ, Hkewise its investing membrane, is liable to 

 suffer from various diseases, some of an organic nature, 

 others purely functional. The cavities of the heart are 

 lined by a very delicate membrane known as the endo- 

 cardium, and this membrane assists in forming the valves 

 of the heart. The functions of the heart are those of 

 distributing blood throughout the body for purification 

 at the lungs and thence back to the heart. Large blood- 

 vessels spring from the base of this organ, which help to 

 suspend it from the roof of the chest, with the apex 

 inclined towards the left side. Heart affections occur 

 much more frequently in the horse than may be suspected, 

 doubtless owing to the severe strain which is necessarily 

 put upon this organ under certain conditions. A sound 

 heart represents a measure of the animal's vigour, and it 

 is indispensable for the due maintenance of proper health. 



