PRACTICAL EXERCISES 



181 



With the vagi, Experiment 7 may be performed. It must be 

 remembered that the activity of the two vagi is unequal in the 

 tortoise, the right being the more active. 



6. Dissection of the Vagus and Cardiac Sympathetic Nerves in 

 the Frog. (i) Put the tissues in the region of the neck on the 

 stretch by passing into 

 the gullet a narrow test- 

 tube or a thick glass rod 

 moistened with water, 

 and by pinning apart 

 the anterior limbs. Ex- 

 pose the heart by cut- 

 ting through the pectoral 

 girdle in the way de- 

 scribed in 2 (p. 177). On 

 clearing away a little 

 connective tissue and 

 muscle with a seeker, 

 three large nerves will 

 come into view. The 

 upper is the glosso- 

 pharyngeal, the lower 

 the hypoglossal ; the 

 vagus crosses diagonally 

 between them (Fig. 79). 

 Above the vagus trunk, 

 running parallel to it, 

 and separated from it 

 by a thin muscle and a 

 bloodvessel (the carotid 

 artery), lies its laryngeal 

 branch. The vagus 

 should be traced up to 

 the ganglion situated on 

 it near its exit from the 

 skull. 



(2 ) Then cut away the 

 lower jaw, dividing and 

 reflecting the membrane 

 covering the roof of the 

 mouth. At the j unction 

 of the skull and the 

 backbone will be seen 

 on each side the levator 

 anguli scapulre muscle 

 (Fig. 80). Remove this 

 muscle carefully with 

 fine forceps. Clear away 

 a little connective tissue 

 lying just over the 

 upper cervical vertebrae, 

 and the sympathetic 

 chain, with its ganglia, will be seen. Pass a fine silk thread beneath 

 the sympathetic about the level of the large brachial nerve, by 

 means of a sewing-needle which has been slightly bent in a flame 

 and fastened in a handle. Tie the ligature, divide the sympathetic 



FIG. 78. ARRANGEMENT FOR RECORDING AURICU- 

 LAR AND VENTRICULAR CONTRACTIONS (AND 

 STUDYING THE INFLUENCE OF TEMPERATURE 

 ON THE HEART). 



C, clamp holding the heart at the auriculo- 

 ventricular groove ; P, pulley round which a 

 thread attached to the apex of the ventricle passes 

 to the lever L' ; L, lever connected with auricle. 

 (The rest of the arrangement is for studying the 

 influence of temperature on the heart and its 

 nerves, G being a vessel filled with physiological 

 salt solution in which the heart is immersed ; R, an 

 inflow tube from a reservoir containing salt solu- 

 tion at the temperature required ; O', an outflow 

 tube by which G may be emptied into the beaker 

 B' ; O, a tube passing to the beaker B to prevent 

 overflow from G ; T, a thermometer.) 



