280 CIRCULATION OF THE BLOOD. 



crossed the lung, the nerve finds its way directly to the sinus 

 venosus, but is so surrounded with gray-looking connective 

 tissue, that in small frogs it is difficult to trace it. As it 

 enters the heart it is closely applied to the superior vena cava 

 and to the wall of the sinus. 



74. Method. A frog, having been slightly curarized or 

 rendered motionless by section of the medulla, is fixed in the 

 prone position. The sternum is then divided in the middle 

 line, and the two halves of the wall of the chest drawn to 

 either side, so as to expose the pericardium and lungs, while a 

 stout glass rod is passed down the oesophagus. The following 

 objects (see fig. 237) are then seen : 1. The two aortic, parting 

 from each other in the middle line, ascend outwards and up- 

 wards close to the cartilaginous tips of the posterior horns of 

 the hyoid bone. 2. From each of these horns muscular fibres 

 are seen to stretch backwards and upwards, towards the 

 occipital region ; these are the petrohyoid muscles already 

 mentioned, which originate from the petrous bone, and are 

 inserted into the cartilaginous processes just referred to. The 

 lower of these nearly parallel bundles of fibres, is the guide 

 to the vagus nerve, which always lies along its lower edge. 3. 

 Following the muscles backwards, they are seen to be crossed 

 by a white nervous cord (the hypoglossal nerve), which ascends 

 upwards and inwards towards the muscles of the tongue. 

 Nearer the middle line, lying somewhat further from the 

 surface, but following the same general direction, another 

 nerve is seen, the glosso-pharyngeal. 4. Crossing upwards to 

 the larynx, over the tip of the inferior horn of the hyoid, the 

 laryngeal nerve is seen. This is the only nerve which is likely 

 to be mistaken for the vagus ; it must therefore be traced back 

 for a short distance from the cartilage and divided. It is 

 convenient also to get rid of the hypoglossus. 



The vagus, with the muscular slip which accompanies it, can 

 now be readily placed on or between the electrodes. On 

 opening the key, the heart usuall}' stops in diastole, with its 

 cavities full of blood, the arrest not being preceded by any 

 previous slowing. If, however, Helmholtz's arrangement of 

 the induction apparatus is used, and the secondary coil is 

 placed at a sufficient distance, a degree of excitation may be 

 attained which, while it falls short of stopping the heart, is 

 enough to diminish its frequency. With reference to this 

 effect, it is to be noticed that, although it is mainly due to 

 mere lengthening of the diastolic intervals, it is also accompa- 

 nied with an impairment of the vigor of the ventricular 

 systole; so that if the heart is connected with a manometer 

 (N'T 63), the manometer rises less during the period of slow- 

 ing than it did before. Another interesting and important 



