THE BACK. 193 



wMch here separate to accommodate the central organ of 

 circulation. Internally it presents a smooth surface, for it 

 is lined by a serous membrane, serous pericardium, which 

 becomes reflected from the base of the heart in a downward 

 direction over that organ, which it invests as far as the 

 apex, and thus forms a closed sac, pericardial sac, which 

 sometimes contains a little serous fluid, and allows the 

 frictionless play of the heart on the inner surface of the 

 fibrous pericardium, whose function is to restrain its 

 excessive action. 



If we have opened the chest on the right side we shall 

 now be able to see the large veins which bring the blood 

 from the various parts of the system to the right or pulmo- 

 nary division of the heart. 



Anterior vena cava commences at the junction of the 

 jugulars between the two first ribs, and runs directly back- 

 wards to open into the supero-anterior part of the right 

 side of the heart. It receives branches corresponding to 

 the arteries given off by the arteriae innominatae, — axillary, 

 vertebral, dorsal, internal thoracic, and several others, 

 including 



Vena azygos, a vein which commencing underneath the 

 loins from the most anteriorly placed lumbar veins, runs 

 in a forward direction underneath the spine with posterior 

 aorta and the thoracic duct (being situated most to the 

 right side). About oioposite the sixth dorsal vertebra it 

 dips downwards to open into the anterior vena cava just at 

 its termination, and during its course receives the thirteen 

 posterior intercostal veins and the subcostal branch from 

 the dorsal ; just before its termination it receives the bron- 

 chial veins. In consequence of its situation the left inter- 

 costal veins are longer than the right, but we have observed 

 an accessory vena azygos on the left side of the spine of the 

 ass. The anterior vena cava at its termination for about 

 three inches presents red muscular fibres, so that it seems 

 to be a continuation of the wall of the auricle ; thus, in 

 the heart removed from the body, it may be distinguished 

 from the 



Posterior vena cava, which, on emerging into the thorax 

 through foramen dextrum enclosed in a special double fold 

 of pleura (with the right phrenic nerve), runs direct to the 

 postero-superior part of the right side of the heart, receiving 

 no vessels in its course. The large arteries may be seen 



13 



