314 AN INTRODUCTION TO ZOOLOGY 



In making a practical examination of a heart care should be taken 

 to choose a specimen that has not been damaged in removal from 

 the body, and one that has the bases of the main arteries and veins 

 left as long as possible. If a sheep's heart be chosen, it is often 

 advisable to remove the greater part of the fat attached to it before 

 starting dissection. The following description is based mainly on 

 the heart of the sheep. 



The mammalian heart is a somewhat conical structure lying 

 almost in the mid -ventral line of the thorax with its broader end, the 

 basis cordis or base, directed anteriorly and to the right, and the 



DA 



SV 



^&. *AJT . ^mffl;// 



RV 



LV 



A 



FIG. 106. Ventral view of sheep's heart with atria collapsed. 



A., apex ; D.A., dorsal aorta ; F., fatty tissue ; I., innominate artery ; L.A., left auricle ; L.V., 

 left ventricle ; P., pulmonary artery ; R. A., right auricle ; R.V., right ventricle ; S.V., sinus longi- 

 tudinalis ventralis. 



bluntly-pointed apex cordis pointing posteriorly and to the left. 

 It consists of four entirely separate chambers, a right and left 

 ventricle and a right and left atrium, and when empty the main part 

 of its bulk consists of the two large fleshy ventricles with parts of 

 the atria appearing as flaps upon them, and separated by the 

 deep coronary sulcus. In the body the heart is enclosed in a 

 double-walled sac, the pericardium, whose inner layer is tightly 

 attached to the wall of the heart, and whose loose outer layer 

 is generally removed in taking the heart from the body. The line 

 where the inner layer is reflected to form the outer will be seen 



