CHAP, vii.] THE CAT'S OEGANS OF CIRCULATION. 201 



bag, the fibres of which, interlace in all directions, adheres posteriorly 

 to the anterior surface of the central tendon of the diaphragm, 

 while anteriorly, it is attached to the outer coats of the great vessels 

 which proceed to and from the heart, and is prolonged for some 

 distance along their course. The internal, or serous, lining of the 

 pericardium is (after lining the fibrous part forwards to its attach- 

 ments) reflected back upon the outer surfaces of the great vessels 

 just referred to, and closely invests the immediate outer surface of 

 the heart, which is thus (like the roots of the great vessels) 

 invested both by two layers of serous, and by one of fibrous 

 membrane. 



The external surface of the heart indicates its composition, since 

 two longitudinal furrows indicate the separation of the heart into a 

 right and left half, while a transverse furrow indicates the separation 

 of each half of the heart into an anterior and a posterior portion. The 

 anterior portion of each half is an auricle, and the posterior portion 

 of each half is a ventricle. The auricular part of the heart (the 

 two dorsal portions) is thin- walled and soft, and each portion bears 

 a small, appendage something like an animal's ear whence the 

 name "auricle." The two auricular appendages (one for each 

 auricle) appear conspicuously on the anterior part of the heart. (See 

 Fig. 104, RA and LA.) The ventricular part of the heart (the two 

 ventricular portions) is thick-walled, firm and slightly conical from 

 before backwards. 



9. GREAT BLOOD-VESSELS are connected with the anterior part 

 of the heart as follows : The anterior part of the heart is con- 

 spicuously distinguished by two large arterial trunks proceeding 

 from it contiguously, whereof the smaller and more dorsally situated 

 arches backwards and to the left over the right-hand branch of the 

 two branches into which the larger and more anteriorly situated 

 trunk divides. This more yentrally situated trunk proceeds 

 obviously from the right ventricle, it is the pulmonary artery, and 

 its two branches just referred to go respectively to the right and 

 left lung. The deeper or more dorsal trunk is the aorta, and a 

 little further examination will show that it proceeds from the left 

 ventricle. If now the heart be turned round, a large vein will 

 be seen proceeding forwards to it on what is naturally the right 

 side of the descending continuation of the arch of the great aorta. 

 This vein is the vena cava inferior, and opposite to it proceeding 

 backwards to the heart from the front part of the body is 

 another vein, the vena cava superior, which passes backwards on 

 the ventral side (the heart being in its natural position,) of the 

 right branch of the pulmonary artery. These two great veins 

 proceed to the right auricle. Other smaller veins (two on each 

 side) are seen proceeding more or less horizontally inwards to the 

 heart. These are the pulmonary veins, and they go to the left 

 auricle. 



10. The HEART is divided internally into four cavities corre- 

 sponding with the four-fold division already noticed. 



