LEPUS CUNICULUS 315 



in the at rial region around the bases of the main blood-vessels. 

 Between the two layers is the pericardia! space, rilled in life with a 

 lymph-like pericardial fluid. 



On the ventral, more rounded surface of the heart a shallow groove 

 containing small blood-vessels, the sulcus longitudinalis ventralis, 

 runs from a point on the base to the left of the middle line obliquely 

 across to the right side, a short distance above the apex. A some- 

 what similar but shallower groove also containing blood-vessels, 

 the sulcus longitudinalis dorsalis, is present on the dorsal, flatter 



L.V 



A 



FIG. 107. Dorsal view of sheep's heart with atria collapsed. 



A., apex ; C.S., coronary sulcus ; D.A., dorsal aorta ; F., fatty tissue ; L.A., left auricle ; 

 L.V., left ventricle ; P.A., pulmonary arteries ; P.C., pre-caval veins ; Po., post-caval veins ; P.V., 

 pulmonary vein ; R.A., right auricle ; R.V., right ventricle ; S.D., sinus longitudinalis dorsalis. 



side of the heart. These two external grooves mark the position of 

 an internal partition separating right and left ventricles. The 

 atria are marked off from the ventricles by a very deep cleft, the 

 coronary sulcus, that runs completely round the heart save where it 

 is interrupted by the main blood-vessels. 



The two atria when inflated appear on the external dorsal 

 surface almost as one large thin- walled sac, but they are nevertheless 

 completely separated internally by the septum atriorum. Each 

 atrium consists of a hollow sac with a thin wall, and bears at its 

 outer corner a thicker appendage, the auricle, internally marked by a 



