PRACTICAL EXERCISES 199 



(o'y per cent.), calcium chloride (0*025 per cent.), and potassium 

 chloride (0*03 per cent.) (a modified Ringer's solution). A longer 

 series of rhythmical contractions will be obtained than in either (i) 

 or (3). That this is not due to the potassium chloride acting alone 

 can be shown by immersing a strip in a solution of potassium chloride 

 (about o'9 per cent.) isotonic with the sodium chloride solution used 

 in (i). No contractions will be caused. 



13. The Action of the Mammalian Heart. Inject under the skin of a 

 dog (preferably a small one) i c.c. of a 2 per cent, solution of morphine t 

 hydrochlorate for every kilo of body-weight. As soon as the morphine 

 has taken effect (in 15 to 30 minutes, but better after an hour), fasten 

 the animal ba.ck down on a holder (as in Fig. 135, p. 295), pushing the 

 mouth-pin behind the canine teeth and screwing the nut home.* In 

 the meantime select a tracheal cannula f of suitable size, and get ready 

 instruments for dissection one or two pairs of artery-forceps, a pair 

 of artery-clamps (bulldog pattern), two or three glass cannulae of 



'B' 



Pig 94. Arrangement for recording the Beginning and End of Stimulation. C. 

 Pohl's commutator without c'r oss- wires ; B, battery in circuit of primary coil P; 

 B', battery in circuit of electro-magnetic signal T; K, simple key in primary 

 circuit; S, secondary coil. When the bridge of the commutator is tilted into 

 the position shown in the figure, the primary circuit is closed and the circuit of 

 the signal broken. 



various sizes for bloodvessels, ten strong waxed ligatures, sponges, 

 hot water, a towel or two, and a pair of bellows to be connected with 

 the tracheal cannula when the chest is opened. Arrange an induction- 



* A simple but efficient and convenient holder for a dog may be easily 

 constructed as follows: Take a board of the length required (2^ to 5 feet, 

 according to the size of the dog) . At one end fasten two short upright wooden 

 pins, with a clear space of 4 to 6 inches between them. These are- pierced 

 from side to side with four or five holes at different heights. An iron pin passes 

 behind the canine teeth of the animal through two corresponding holes in the 

 uprights, and the muzzle is tied over this by a cord which secures the head. 

 For a large dog an upper pair of holes is used, for a small dog a lower pair. 

 The feet are fastened by cords to staples inserted into the sides of the board, 

 the fore-legs being drawn tailwards for all operations on the neck or head, 

 headwards for operations on the thorax. A rabbit-holder can be made in 

 exactly the same way. 



f A tracheal cannula is easily made by heating a piece of glass tubing, 

 about 6 inches long, a short distance from one end, and drawing it out slightly 

 so as to form a ' neck.' The tubing is then bent about its middle to an obtuse 

 angle, and the end next the neck is ground obliquely on a stone. The diameter 

 of the cannula should be about the same as that of the trachea, into which it 

 is to be inserted by its oblique end. 



