STRUCTURE AND ACTION OF THE MAMMALIAN HEART 83 



anterior end to arrest bleeding from the internal mammary arteries. 

 The window thus opened discloses the heart within the pericardium ; 

 the latter may be cut open and the heart fully exposed. The systole, 

 followed by diastole, of auricles and ventricles can be watched, and 

 the hardening of the ventricles during their systole felt by applying 

 the finger to their surface. By attaching one of the ventricles and one 

 of the auricles by fine hooks and threads to light levers, the contraction 

 of these parts can be recorded separately on a drum. The effect of 

 stimulating the vagus in the neck and the action of atropine in abolish- 

 ing this effect can be demonstrated ; also the effect of stimulating the 

 accelerator fibres which pass from the inferior cervical ganglion of the 

 sympathetic to the cardiac plexuses. The same result is obtained by 

 stimulating the ganglion itself; this may be found by following the 

 cervical sympathetic downwards. In the cat the vagus and sympathetic 

 run in the same sheath in the neck, but they separate below and above as 

 the sympathetic passes out of and into its inferior and superior cervical 

 ganglia. In the dog they are completely united in a common peri- 

 neurium : in the rabbit they run separately from one another, and a 

 third nerve the depressor accompanies them in their passage down 

 the neck. 



