THE KESPIEATOEY SENSE. 159 



upper extremities were constricted, the respiratory efforts always ceased when 

 the vessels were freed. 



The object of these experiments was to study the effects of cutting off the 

 supply of oxygenated blood from different parts. It may be assumed that 

 the respiratory nervous centre is in the medulla oblongata, and an attempt 

 was made to devise some means of cutting off the arterial supply from 

 this part. Animals respire when all of the encephalic centres have been de- 

 stroyed except the medulla oblongata, so that it is improbable that cutting 

 off the supply of blood from the brain would affect the muscles of respiration, 

 provided that artificial respiration were efficiently maintained. Blood may 

 be supplied to the medulla oblongata by the internal carotids, which are con- 

 nected with the circle of Willis, by the vertebral arteries, which unite to form 

 the basilar artery, and perhaps by other vessels ; but it is certain that if all 

 the arteries given off from the arch of the aorta be tied, the medulla must be 

 deprived of oxygenated blood. 



In one experiment, the innominate artery and the left subclavian artery 

 were constricted, and the animal made respiratory efforts in two minutes and 

 eight seconds, notwithstanding that artificial respiration was kept up. 



In another experiment, the same vessels were constricted, and the animal 

 made respiratory efforts in two minutes and five seconds. 



In a third experiment, both subclavian arteries and both carotids were 

 constricted, and the animal made respiratory efforts in two minutes and seven 

 seconds. Both vertebral arteries and both carotids were constricted, and the 

 animal made no respiratory efforts for five minutes ; but respiratory efforts 

 were made in one minute and thirty-five seconds after both subclavians had 

 been constricted in addition to the vertebrals and carotids. 



It seems from these experiments, that in order to induce respiratory 

 efforts in an animal under the influence of ether and with the lungs supplied 

 with air by artificial respiration, either the innominate artery and the left 

 subclavian artery, or both subclavians, both carotids and both vertebral arte- 

 ries, must be tied. In other words, according to the view taken of the cause 

 of these respiratory efforts, the supply of blood to the medulla oblongata can 

 not be cut off completely except by tying all the vessels given off from the 

 arch of the aorta. 



These observations, taken in connection with the experiments of 1861, 

 lead to the conclusion that the sense of want of air, under certain conditions, 

 is due to a want of circulation of oxygenated blood in the medulla oblongata. 

 This view has been advanced by some writers, but it has lacked the positive 

 experimental proof afforded by the experiments of 1877. 



If the sense of want of air be regarded as due, under certain conditions, 

 to a deficiency of oxygen in the medulla oblongata which can hardly be 

 doubted it becomes an important question to determine whether the normal 

 respiratory movements be actually reflex in their character or whether they 

 be due to direct excitation of the nerve-cells in the respiratory centre. 



It is difficult to account for the phenomena observed in experiments in 

 which the pneumogastrics are divided or stimulated, without assuming that 



