BY DR. BURDON-SANDERSON. 325 



column corresponds to the average arterial pressure. That 

 part of the instrument which is intended for recording the re- 

 spiratory movements, consists of a Marey's tympanum, c, and 

 a lever, F, similar to D, and of the same length, with which it 

 is connected. The tube, n, of the tympanum may be either 

 brought into communication with one arm of a glass "f" tube, 

 the stem of which is inserted in the trachea, or with a stetho- 

 meter applied to the chest. The lever of the tympanum is 

 connected with the recording lever by a vertical rod seen in 

 the drawing. In this way two tracings are obtained simul- 

 taneously, of which fig. 258 is an example. The arterial trac- 

 ing is marked A p, the respiratory R. In the latter, the begin- 

 ning of inspiration is indicated by the vertical stroke a; of 

 expiration by b; of the pause by c. The coincident points in 

 A P are indicated by similar strokes. The break is made by 

 removing both pens from the paper by the same act. In man, 

 the variations of frequency (which, of course, can alone be in- 

 vestigated) are absent in most healthy persons, although very 

 obvious in certain conditions of disease. In the rabbit they 

 are .much less marked than in the dog. They are regarded by 

 most physiologists as dependent on variations of activity of 

 the intracranial centre of the cardiac vagus: until very re- 

 cently it has been assumed, by way of explanation, that the 

 respiratory movements affect the cerebral circulation in such a 

 way that during the period of relaxation of the muscles of 

 respiration, the supply of blood to the medulla oblongata is 

 diminished, and increased during their contraction and that 

 the inhibitory nervous system of the heart is affected by these 

 changes. This explanation has always appeared unsatisfactory, 

 and could only be accepted provisionally ; for it seemed ex- 

 tremely improbable that there was any appreciable difference 

 in the supply of blood between the inspiratory and expiratory 

 periods. We now know that the respiratory variations in the 

 arterial pressure and in the frequency of the contractions of 

 the heart, are not necessarily dependent on the mechanical 

 effect of the respiratory movements on the heart, inasmuch as 

 they persist when these movements are abolished; and that 

 they have their primary source in the vasomotor and cardiac- 

 inhibitory centres, which act rlrythmically, not because they 

 are subject to any rhythmical excitation, but because they 

 have periods of waxing and waning activity which correspond 

 to those of the respiratory centre. A very little consideration 

 shows that this inference carries the admission that the cardiac- 

 inhibitory centre and the vasomotor centre act alternately, for 

 it can be seen in every tracing that the increase of arterial 

 tension determined by increased vascular tonus, alternates 

 with the retarded pulse and diminished tension produced by 

 " vagus excitation." In other words, the phase of maximum 



