CHAP, vii.] THE CAT'S GROANS OF CIRCULATION. 203 



directly continuous with the walls of the arteries and veins which 

 open into it. The muscular fibres, however, of which the heart 

 is composed are very numerous, forming a thick mass, and they are 

 of the striated kind and of a deep red colour. Individually, never- 

 theless, they are smaller than the ordinary striated fibres. They are 

 also devoid of sarcolemma, and they often branch and anastomose. 



The fibres are arranged in layers disposed in various spiral curves 

 around the cavities of the heart. The muscular walls of the ventricles 

 are thicker than those of the auricles, and that of the left ventricle 

 is again thicker than that of the right ventricle. Blood-vessels, 

 nerves and lymphatics, with fat and areolar tissue, enter into the com- 

 position of the heart, and fibrous andfibro-cartilaginous structures play 

 important parts in its composition in connection with certain valves. 



These substances are so arranged that (as already remarked) a 

 complete partition is formed between the right and left sides of the 

 heart ; a septum entirely separating the cavities of the two auricles 

 and the two ventricles respectively. Thus there is no direct 

 communication between the ventricles, or between the auricles. 



The cavity of the right auricle however communicates not only 

 with the interior of the venaa cavaB which open into it, but also 

 with that of the right ventricle, this latter opening being called the 

 right auriculo- ventricular aperture. Besides this communication 

 with the right auricle, the right ventricle only opens into the 

 pulmonary artery. Similarly the cavity of the left ventricle com- 

 municates not only with the interior of the pulmonary veins which 

 open into it, but also with that of the left ventricle, this latter 

 opening being called the left auriculo- ventricular aperture. Besides 

 this communication with the left auricle, the left ventricle only 

 opens into the aorta. 



11. The COURSE OF THE P,LOOD THROUGH THE HEART is as 

 follows : Venous blood is brought back by two large veins (the 

 vence cavce) from all parts of the body to the right auricle, whence it 

 proceeds to the right ventricle. From the right ventricle it is 

 carried to the lungs by the pulmonary artery and brought back 

 from the lungs by the pulmonary veins to the left auricle ; from 

 thence it proceeds to the left ventricle which distributes it, by 

 the aorta, over every part of the body. 



Thus the blood, in its course to and from the heart, performs two 

 different circuits. On the one hand, the whole of the blood passes 

 from the right ventricle, through the lungs, and back to the left 

 auricle, thus performing what is called the pulmonary circulation. 

 On the other hand, the whole of the blood passes from the left 

 ventricle all over the body, and thence back to the right auricle, 

 thus performing what is called the systemic circulation. 



It is while passing through the lungs that the blood undergoes 

 that process of gaseous interchange which makes it acquire a bright 

 scarlet colour the colour of normal arterial blood. On its way to 

 the lungs it is of a deep purple tint the colour of normal venous 

 blood. Thus it comes about that the vessels which carry the blood 



