120 



EXPERIMENTAL PHYSIOLOGY 



CHAPTER XI 



THE NERVES OF THE FROG'S HEART AND THEIR FUNCTIONS 



MAKE A DISSECTION TO SHOW THE NERVES OF THE 

 FROG'S HEART. — The nerve supply to the frog's heart is from two 

 sources, the vagus and the sympathetic. 



To find the vagus expose the heart in the usual manner, leaving, 

 however, the pericardium intact to serve as a protection while the 

 dissection is being made. Pull aside the lateral pieces of the sternum 

 and separate the muscles running from it to the floor of the mouth. 

 When this is done a muscle comes into view which is the guiding 

 mark for the vagus. This muscle is the petro-hyoid, which arises from 



the base of the skull and is 

 inserted into the posterior 

 cornu of the hyoid bone. Its 

 direction is from the joint of 

 the lower jaw, round the 

 pharynx in an almost trans- 

 verse direction to the body of 

 the hyoid. This muscle should 

 be clearly made out (fig. 98, 

 ph). It is arranged in three 

 separate small bands, in series 

 one behind the other. Crossing 

 it are two nerves : one late- 

 rally placed, the glossopharyn- 

 geal, gp, coursing from the 

 angle of the jaw over the muscle 

 to run forwards into the tissues forming the floor of the mouth ; the 

 other, the hypoglossal, h, crosses it much nearer the mid line. The 

 latter nerve is the first spinal nerve in the frog. In relation to the 

 lower border of the muscle lies the carotid artery, a. If the muscle be 

 now laid hold of, a nerve will be seen running along its lower edge and 

 partly covered by it. This is the vagus, v. A branch of this, the laryn- 

 geal, l, usually runs a separate course parallel to the main trunk, 



Fig. 98. — To Show the Course of the 

 Vagus in the Frog. 



