I.] DISSECTION OF A RABBIT AND OF A DOG. 19 



the depressor receives a branch direct from the 

 pneumogastric ; occasionally this is its sole 

 origin. 



19. Remove the first rib and the remains of the 

 sternum, avoiding any injury to the tissues 

 below. Observe the thymus, a fatty looking 

 body covering the roots of the great vessels. 

 It may be torn away. 



20. Trace out on each side the junction of the 

 external jugular and subclavian veins to 

 form respectively the right and left venae 

 cavae superiores : near th'e junction ends the 

 internal jugular vein, this brings blood from 

 the brain and may be traced from the foramen 

 jugulare (cp. E. 21) down the neck laterally of 

 the common carotid and vagus. 



21. Observe the right vena cava superior passing 

 straight down to join the right auricle ; the left 

 vena cava superior passing obliquely downwards 

 underneath the left auricle to join the right 

 auricle; and the inferior vena cava passing 

 upwards from the diaphragm to join the right 

 auricle. 



22. Trace up one phrenic nerve. It makes its way 

 out of the thorax by the side of the superior 

 vena cava, and then passes beneath it. Place a 

 double ligature round the vein and divide be- 

 tween the ligatures. Follow up the phrenic to 

 its origin from the 4th and 5th (and also from 

 the 6th and 7th) cervical nerves. 



22 



