64 COMPARATIVE ANATOMY. 



to the posterior walls of the thorax, as they were in the Pigeon ; 

 the right lung is divided into three lobes, and the left into two. 

 The heart is mesially placed between the two lungs, and its 

 anterior surface is about equally divided between the two ven- 

 tricles. The aorta is seen curving over to the left side, and 

 from the summit of the arch the three following vessels take 

 origin : first, the right arteria innominata which soon splits 

 into a right subclavian and carotid arteries ; a little to the left 

 of this the left carotid arises directly from the arch ; and still 

 more to the left the left subclavian, but this is hidden from view 

 by the left superior vena cava. On each side of the root of the 

 neck the jugular and subclavian veins are to be seen converging; 

 and subsequently by their union they form the right and left 

 superior venae cavse. In order that these vessels might be better 

 displayed, the thymus, or hibernating gland, has been displaced, 

 and turned forwards over the front of the heart. Descending 

 the neck, in close relation with the carotid artery, is the 

 pneumogastric nerve, and a little to the outer side of the same 

 artery the cervical portion of the sympathetic may be seen ; 

 while in close proximity with the trachea is the delicate recurrent 

 laryngeal. Subsequently to giving off the arteries to the head and 

 fore limbs, the aorta passes downwards through the chest just in 

 front of the vertebral column, and gives off numerous intercostal 

 arteries in its course : eventually it pierces the diaphragm and 

 becomes the abdominal aorta. A little to the outer side of the 

 thoracic aorta a rather large vein passes upwards to the left 

 superior vena cava, receiving during it ascending course small 

 veins from the intercostal spaces ; this is the azygos vein. The 

 phrenic nerves in their course downwards from their origins in 

 the cervical portion of the spinal cord to their distribution in 

 the diaphragm, of which they are the motor nerves, pass a little 

 to the outer side of the superior vena cava, and afterwards in 

 front of the roots of the lungs ; the right phrenic nerve is in 

 close relation with the inferior vena cava. Behind, and a little 

 to the left of the last-mentioned vein, is the thoracic portion of 

 the oesophagus. Beneath the concavo-convex diaphragm, and 

 attached to it by a peritoneal process the suspensory liga- 

 ment is the five-lobed liver. This again is rounded above and 

 adapted by its convex upper surface to the concave inferior 



