206 



EXPERIMENTAL PHYSIOLOGY 



the depth of respiration, the increase in depth to a certain extent 

 compensating for the diminution in rate. 



4. Stimulation of the central end of the vagus. — What we may 

 regard as the most typical result of stimulation of the central end, 

 both vagi being cut, is that recorded in fig. 163. In this case the 

 tetanisation was very weak and caused an acceleration in rate from 32 

 respirations per min. to 36 per min., and the amplitude of the record 

 diminished from 5 cm. to 4 cm. The diminution in extent is seen to 

 be brought about by a less complete relaxation, as well as by a less 

 extensive contraction. This result is much better obtained if the 

 anaesthesia be not too deep. If the strength of stimulation be increased 

 the effects are found to vary considerably. A common result is 

 that reproduced in fig. 164, where it is seen that there is a gradually 

 increasing tendency to standstill with the diaphragm neither relaxed 

 nor contracted, but in a state of mid-contraction. In some cases 

 standstill is produced in an inspiratory phase, inspiratory tetanus ; in 

 others, again, in an expiratory phase, expiratory tetanus. 



RECORD OF THE RESPIRATORY MOVEMENTS IN MAN 



Various instruments have been devised for recording the move- 

 ments of the thorax in man. Of these the two following should be 

 examined : — 



Experiment 1.— Take a tracing with Marey's pneumograph (fig. 165). The 

 instrument consists of a thin flat iron plate, /, with two stout bars of brass at 

 either end. Attached to one bar is a tambour, h, which moves a lever, b. To 



Fig. 165. — Marey's Pneumograph. (McKendrick.) 



the other is fixed a vertical bar with a horizontal screw, g, which fits into the 

 upper part of the lever b. By the band e e d the apparatus is tied firmly on 

 to the chest. Then with each inspiration the plate / is bent, the vertical bar 



