24 TOPOGRAPHICAL ANATOMY OF THE 



is obscured by the prolongation of atrial muscular fibres on to the 

 terminal part of the vein. 



The collateral tributaries of the cranial vena cava are the internal 

 thoracic, vertebral, deep cervical and costo-cervical veins from both 

 sides of the body. It frequently happens that the left vertebral, left 

 deep cervical, and left costo-cervical veins join together before reaching 

 the vena cava. The vena azygos may or may not be a tributary of the 

 vena cava. 



V. AZYGOS. — The azygous^ vein begins in the abdomen and drains a 

 considerable proportion of the wall of the chest. At the present 

 moment no attempt need be made to do more than examine its terminal 

 part. The vein either joins the cranial vena cava just as this is enter- 

 ing the heart, or it opens into the right atrium itself. 



V. CAVA CAUDALis. — The caudal vena cava begins within the 

 abdomen on a level with the fifth lumbar vertebra, and enters the 

 thorax by the foramen venas cavse of the diaphragm, where it receives 

 two or three large phrenic veins (vv. phrenicse) that drain the greater 

 part of the diaphragm. The thoracic part of the caval vein, which is all 

 that can be examined at present, lies in the cleft between the main 

 mass of the right lung and its intermediate lobe, in the edge of the 

 caval fold of the right pleura. The vein pierces the pericardium and 

 opens into the right atrium of the heart. 



The right phrenic nerve, as has been already noted, lies lateral and 

 ventral to the vena cava. 



Dissection. — In order to give more room for the dissection of the 

 heart, &c., the lungs may now be removed. 



The heart (Cor). — The heart is a hollow muscular organ of conical 

 shape, with a base that is dorsal in position and an apex that is ventral. 

 The long axis of the organ is mainly vertical, but has a degree of 

 inclination that brings its ventral end towards the diaphragm and the 

 left side of the body. This obliquity causes about three-fifths of the 

 total bulk of the heart to be on the left side of the median plane of the 

 body. 



The heart of the horse lies under cover of the third, fourth, fifth 

 and sixth ribs ; the cranial border generally lying opposite the second 

 intercostal space, while the caudal border extends more or less beyond 

 the sixth rib. The position of the base of the heart may be indicated 

 as coinciding with a horizontal plane taken on a level with the junction 



' a priv. + ^vy6s (zygos) [Gr.], a yoke ; unpaired. 



