THE CIRCtTLATORY SYSTEM 



127 



of the heart of the horse is 7J^ pounds; of the ox 5j^ pounds. 

 This difference is in keeping with the character of the work 

 performed by each animal. 



The pericardium, a fibrous sac lined with a serous membrane, 

 surrounds the heart. In it is found a characteristic secretion, 

 the pericardial fluid. 



Pulmonary 

 veins 



Pulmonary £zz 



\ Posterior vena cava 

 Lejt atrium 



Fig. 40. — Base of heart of horse, with large vessels, photograph of specimen 

 hardened in situ. (Sisson, Anatomy of Domestic Animals.) 



When the heart is opened by incisions for inspection it is 

 seen to present four cavities — the right and left auricles and the 

 and the left ventricles (Fig. 41). The two cavities of one side 

 are partitioned off from those of the other by a septum of fibro- 

 muscular composition that keeps the pure and impure blood 

 separated. The right anterior part of the base of the heart is 

 formed by the right auricle. It has orifices for the anterior 



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