SCOTT, STUDY OF CHANGES IN MUSTELUS CANIS 59 



the ventricle and connected by a thread to a lever recording on a slowly 

 moving drum. With its dorsal surface downward the fish rested on an 

 inclined support in a tank of sea-water so that the head and gills were 

 under water. No sea-water entered the pericardial cavity. After the 

 normal heart beat had been recorded for a few minutes, a stream of 

 fresh water w^as turned into the tank, and a record was made of the 

 changes in the heart beat for 80 minutes. During the period that the 

 fish was immersed in sea-w^ater, the heart was beating at the rate of 50 

 per minute. The rate changed to 59 per minute during the second minute 

 after the fresh water had been turned on, and then gradually fell as fol- 

 lows : 6th minute, 45 beats per minute; 12th minute, 28 beats; 16th min- 

 ute, 13 beats; 18th minute, 11 beats, 22nd minute, 9 beats; 30th minute, 

 10 beats; 35th minute, 6 beats; 44th minute, 14 beats; 51st minute, 12 

 beats; 58th minute, 13 beats; 70th minute, 15 beats; 75th minute, 14 

 beats; 80th minute, 12 beats. Accompanying the early diminution in 

 heart rate, there was an increased amplitude of contraction. In fact, the 

 amplitude of the beat varied for a time inversely with the rate. The in- 

 creased amplitude and slower rate began to be marked about the 14th 

 minute after the fresh water was turned on, coinciding somewhat with 

 the time at w^hich the water was entirely fresh, being most marked be- 

 tween the 30th and 40th minutes. A diminishing respiratory rate accom- 

 panied this increased amplitude of contraction. The forcible and slow 

 heart beat gradually failed after respiration ceased. Soon after respira- 

 tion ceased, the heart beat showed great irregularity in the time taken by 

 each contraction. At the end of an hour, the amplitude of contraction 

 was about equal to that of the normal heart beat but the rate was only 

 about one-fourth as great. After this, the extent of the contraction di- 

 minished gradually, although by stimulating the heart mechanically it 

 increased for a time. About 70-80 minutes after immersion in fresh 

 water and about twenty minutes after respiration ceased, the heart beat, 

 although slow and regular, was very weak and was probably not effective 

 enough to drive the blood through the gill capillaries with sufficient 

 rapidity to maintain life. This agrees in the main with Mosso's ('90) 

 observation. 



Another related feature accompanying the change in cardiac activity 

 were the respiratory convulsions similar to those mentioned on page 57. 

 This is strongly suggestive of an associated action of the bulbar cardiac 

 and respiratory mechanisms which exists in the mammal. The gill covers 

 became greatly contracted and simultaneously the heart was slowed and 

 greatly dilated. The inhibition of the heart in diastole and the character 

 of the recovery as shown in Fig. 12 suggests that the cardiac spasm is 



