THE VASCULAR SYSTEM 



The vascular system is composed of the central forcing 

 muscular organ, the heart; a system of vessels, the arteries, 

 carrying the blood from the ventricles to the lungs and all 

 parts of the body; a system of vessels, the veins, returning 

 the blood to the atria; and numerous microscopic vessels, the 

 capillaries, connecting the termination of the arteries with 

 the origin of the veins. 



THE HEART 



The location of the heart in the cat may be seen by removing 

 the ventral thoracic wall, but for the study of the parts, the 

 heart of an ox or a sheep will be found more satisfactory. The 

 heart of the cat Kes between the lungs (Fig. 86) in the thoracic 

 cavity a little to the left of the median line. The caudal end 

 is the apex, and the cranial end is the base. The entire organ 

 is invested in a fibro-membranous sac the pericardium, which 

 when cut permits the pericardial fluid to run out. The walls of 

 this pericardial sac comprises two layers, the visceral layer or epi- 

 cardium, which closely invests the heart and a perietal layer, or 

 the pericardium proper, which forms a loose sac. All the 

 blood-vessels originate from the dorsocranial aspect of the heart. 



It is composed of a right and left half, each of which con- 

 sists of an atrium or auricle and a ventricle. The separation 

 between the two halves is apparent on the ventral surface. The 

 atria receive the blood from the veins and pass it to the ven- 

 tricles, which disperse it through the arteries. There is no 

 aperture between the atria or the ventricles. The atrio- 

 ventricular opening between the right atrium and the right 

 ventricle is guarded by the tricuspid valve which prevents the 

 blood from returning into the atrium when the systole or con- 

 traction pushes the blood into the pulmonary artery. The 



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