The Heart. 51 



one, and is exactly represented in Fig. 261 of Chauveau's 

 ' Anatomy,' first edition. 



The semi-lunar or sigmoid valves, which guard the 

 entrance into the aorta and pulmonary artery, are composed 

 of fibrous and elastic material, and possess at the centre of 

 each segment a small hard bod} 7 , corpus Arantii, which is 

 particularly marked in the aorta. 



Owing to the arrangement of the muscular fibres of the 

 heart, we find that the auricles and ventricles are capable 

 of acting quite independently of each other : the two 

 auricles contract, and then the two ventricles. The con- 

 traction of either auricle or ventricle is spoken of as its 

 systole, whilst the dilatation is described as its diastole. 

 We observe that when the two auricles are in a condition 

 of diastole the two ventricles are in one of systole. Where 

 death is caused from bleeding, we find that the two ven- 

 tricles cease beating before the auricles, and the auricular 

 portion of the right auricle acts longer than the left 

 auricle. 



A Cardiac Revolution or Cardiac Cycle, is the term used to 

 describe the changes which occur in the heart during the 

 time which elapses between one contraction or dilatation 

 of the auricle and the one which immediately succeeds it. 

 We observe that the auricles are filling with blood, which 

 pours into them by the vena: cavae and pulmonary veins, 

 owing to the fact that the pressure within the auricles is 

 lower than that in the bloodvessels, and also because the 

 efforts of inspiration favour an aspiratory effect, due to the 

 dilatation of the walls of the heart producing a sucking 

 action in the auricles, and so helping to fill them. The 

 auricles being full of blood the cavities contract, the 

 mouths of the vessels opening into them also contract 

 owing to the circular muscular fibres in their coats, and 

 so produce a slight stagnation of blood in these veins and 

 a distinct regurgitation in the anterior cava, which can 

 be readily seen at the root of the neck of most horses. 

 Systole of the auricle follows, the blood being driven into 

 the ventricles which have been partly filling during the 



4^—2 



