142 DEMONSTRATIONS. 



follows that of the heart at an interval of about eight-hundredths of a second. 

 The duration of the ventricular impulse is about three-tenths of a second. 



b. The sphygmograph having been adjusted so as to record the radial pulse, 

 a receiving tympanum on the carotid is connected with a recording tympanum 

 attached to the frame of the sphygmograph, so that its lever writes on the 

 same surface as that of the sphygmograph. The interval of time between the 

 impulse of the carotid and that of the radial is about the same as that between 

 the carotid and the heart. 



2. The StethOgraph. The changes of form of the thorax in respira- 

 tion are investigated by the measurement of the diameters of the chest. The 

 most important diameters are the antero-posterior (from upper end of sternum 

 to third dorsal spine, 150 millims. and from lower end of sternum to eighth 

 spine, 200 millims. ) ; the transverse (at the eighth rib, about 230 millims. ). 

 These measurements refer to an adult male, as taken during the respiratory 

 pause. The first of these diameters increases about a millimeter, the second 

 about two millimeters, and the third about two and a half in ordinary tranquil 

 inspiration. These measurements, when recorded by the stethograph, yield 

 the " respiratory curve. " 



VI. The Heart of the Frog. 



1. Rhythmical Motions of the Ventricle ; Influences 

 thereon of Temperature and other External Conditions. 



2. The Cardiac Vagus, and the Intracardiac Inhibitory 

 Centre. 



The experiments relating to these subjects are described in the Practical 

 Exercises. Such of them only as can be seen at a distance are shown. 



VII. Electromotive Phenomena of Mtisde. 



The most important instrument used is a Thomson's Reflecting Galvanometer 

 of high resistance, the terminals of which are connected by insulated copper 

 wires with non-polarizable electrodes. These are in contact by their clay 

 plugs with the two surfaces to be compared. 



To the needle of the galvanometer a light concave mirror is attached, on 

 which a beam of light falls and is focussed, after reflection, on a divided 

 screen. Thus the smallest deflection of the needle (by which any electrical 

 difference between the two contacts is indicated) can be exactly measured. 

 By means of a suitable shunt, either the whole, a tenth, or other decimal 

 fraction of any current flowing through the circuit can be led through the 

 galvanometer. 



i. Electromotive Phenomena of Muscle. The gastrocnemius 



muscle of the frog is used. One of the electrodes is in contact with the convex 

 surface of the muscle near its upper end, the other with the expansion of the 



