Isolating the Cerebro-medullary Circulation. 115 



When the experimenter is ready to institute artificial respiration, 

 the thorax is opened by sawing through the entire length of the 

 sternum. This gives rise to very little hemorrhage and, if the 

 sterno-cleido-mastoid muscles are detached from their origin, 

 allows free access to the vessels at the base of the heart. The 

 two pairs of sternal vessels are now double ligated and cut between 

 the ligatures. A full exposure of the superior vena cava is thus 

 provided. By carefully separating the loose tissue over it the 

 nerves which run alongside may be avoided and a ligature passed 

 around the vessel just above the entrance of the azygos vein. 

 The large brachio-cephalic artery which lies just behind and to 

 the left of the vena cava is exposed by blunt dissection at its point 

 of origin from the arch of the aorta, and a ligature passed about it. 

 The second large arterial branch, the left subclavian, best ap- 

 proached through the left side of the mediastinum, is similarly 

 treated. Ligation of the subclavian arteries requires the greatest 

 care, for it is essential to have the ligatures so placed as to 

 occlude the thyroid axis, superior intercostal and suprascapular 

 arteries, which might, through their anastomosing branches, form 

 a communication with the circulation of the trunk. The same 

 care in the ligation of the corresponding veins insures against the 

 passage of venous blood from the trunk into the cerebral circuit. 



The right common carotid artery which has already been freed 

 is ligated and severed at its upper end, and, by the method of 

 vessel anastomosis described by Crile, its central end is attached 

 to the central end of a carotid artery of the donor, preferably a 

 larger dog, and the isolation of the cerebral circulation of the 

 recipient is next performed. 



By means of the ligatures that have been placed about them the 

 large vessels are raised and clamped. First the left subclavian and 

 then the branchio-cephalic arteries are clamped close to the aorta. 

 Simultaneously the blood is allowed to pass through the trans- 

 fusion anastomosis. The exposed left external jugular vein is then 

 opened close to the subclavian branch to allow venous outflow 

 from both directions and the superior vena cava is raised and 

 clamped. When it is desired to have a complete circulation be- 

 tween the donor and the cerebral circulation of the recipient, a 

 second anastomosis is made between the central end of the right 



