176 A MANUAL OF PHYSIOLOGY 



(cephalic) end of the sympathetic on very fine and well- insulated 

 electrodes, and stimulate (Fig. 52, p. 143). (To insulate electrodes 

 the points may be covered with melted paraffin. When the paraffin 

 has cooled, a narrow groove, just sufficient to lay bare the wires on 

 the upper side, is made in it, and the nerve is laid in this groove.; 



Experiments 7, n (i) and n (2; will be rendered more exact 

 by connecting a second electro-magnetic signal with a Pohl's com- 

 mutator without cross-wires (Fig. 66), in such a way that the circuit 

 is interrupted at the instant when stimulation begins. 



12. The Action of the Mammalian Heart. Inject under the 

 skin of a dog (preferably a small one) i cc. of a 2 per cent, solution 

 of morphia hydrochlorate for every kilogramme of body-weight. As 

 soon as the morphia has taken effect (in 15-30 minutes), fasten the 

 animal back down on a holder (as in Fig. 100), pushing the mouth- 



FIG. 66. ARRANGEMENT FOR RECORDING THE BEGINNING AND END OF 

 STIMULATION. 



C, Pohl's commutator without cross-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. 



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

 meantime select a tracheal cannulaf of suitable size, and get ready 



* 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 passing behind the canine teeth of the animal through two 

 corresponding holes in the uprights, and tied over the muzzle by a cord 

 arranged in a figure of eight, 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 opera- 

 tions on the thorax. A rabbit-holder can be mace in exactly the same way. 



t 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. 



