PKACTICAL PHYSIOLOGY 



the lower border of the petro-hyoid muscle. It is a small nerve, and 

 not easily seen. Having traced the nerve so far, cut away the lower jaw 

 and as much of the larynx as can safely be removed. Next cut away 

 the mucous membrane which covers the base of the skull and upper 

 vertebrae. You will thus expose on either side a broad muscle, the 



levator scapulae inferior. This 

 muscle arises from the skull 

 round the jugular foramen, 

 and is inserted in the scapula. 

 Unpin the frog, and hold the 

 skull in the left hand, so that, 

 while the skull is horizontal, 

 the body hangs vertically. Cut 

 through the levator muscle, 

 and under the upper part of 

 this muscle observe the 

 vagus ganglion and the vagus 

 and glosso-pharyngeal nerves. 

 Trace the sympathetic nerve, 

 which is marked by black 

 pigment, along the upper 

 vertebrae to its junction with 

 the vagus ganglion. The 

 cardiac sympathetic fibres arise from the 3rd spinal nerve, and pro- 

 bably have their cell stations in the third sympathetic ganglion. 

 Pass a fine thread (by means of a sewing needle) under the sym- 



Fio. 66. Diagram of the origin of the vago-sympa- 

 thetic nerve (V.-S.). L.A.S. = levator anguli scapulae 

 muscle. Ao. =aorta. 1, 2, 3, 4, = first to fourth spinal 

 nerves. Sym. = sympathetic nerve. G.P. = Glosso- 

 pharyngeal nerve. G.= vagus ganglion. (Gaskell.) 



FIG. 67. Contraction of the frog's heart. The effect of weak stimulation of the 

 vago-sympathetic nerve. The white line marks the duration of excitation. Note 

 the latent time, the acceleration and increased tone and the after-effect. The curve 

 should be read from left to right. (Pembrey and Phillips.) 



pathetic at the level of the large brachial (2nd spinal) nerve. Tie 

 it, and divide the nerve below the ligature. Pass a thread under 

 the glosso-pharyngeal and vagus nerves, but do not tie it. 



Place the frog again on the board, and record the heart by the 

 suspension method (slow rate of drum). With the interrupted current 



