98 EXPERIMENTAL PHYSIOLOGY 



venosus, sv, is brought into view lying in the posterior pericardial wall. 

 At its junction with the auricle a transverse crescentic line, white in 

 colour, is to be made out, marking the limits of the two cavities. Run- 

 ning from a point in the mid line of the posterior ventricular wall, and 

 about 2 mm. from the auricular ventricular groove, is a thin cord of 

 fibrous tissue which is attached to the posterior pericardial wall. This 

 is the frenum, and contains a small blood vessel. Entering the sinus 

 from below is the single large inferior vena cava, whose outline can 

 be made out below the pericardium. Replace the ventricle and gently 

 displace the two auricles by means of a seeker. In this way the upper 

 end of the sinus can be exposed with the two smaller superior venae 

 cavae running into them. 



3. Note that at each contraction the ventricle becomes pale in 

 colour, and with each distension, which occurs very suddenly, the 

 ventricle becomes deep red. This change is dependent on the structure 

 of the heart wall, which has the form of a sponge-work of muscle bundles 

 between which the blood passes during dilatation and causes the dark 

 colour. With each contraction the blood is driven out of the sponge- 

 work, leaving the muscular tissue only, which has a pale colour. 



4. Determine the rate at which the heart is beating by timing it 

 with a watch. Make a note of the number of beats per minute. It 

 will be found to vary very greatly in different frogs— from 20 up to 

 100 per minute, and in some cases even passing these limits in either 

 direction. 



5. Make out the course of the contraction as it travels over the 

 heart. This is of some little difficulty even when the heart is beating 

 slowly. The contraction starts at the sinus, thence passes to the 

 auricle, then to the ventricle, and finally to the bulbus arteriosus. 



6. RECORD THE BEAT OF THE HEART. One of the most 

 convenient ways is that known as the SUSPENSION METHOD. Pre- 

 pare a lever in the following way (fig. 83). A light straw, s, is selected, 

 and near to one end an 8-shaped hook is attached by means of a little 

 cement. The hook may be conveniently made from a fine entomological 

 pin. 3 cm. from the hook the straw is transfixed by a fine needle, x, 

 and is then cut of such a length that when the writing point is fixed 

 its length from needle to writing point is about 15 cm. The magni- 

 fication will then be fivefold. 



The last 12 cm. of the straw may with great advantage be replaced by a glass 

 writing point. This is made of a piece of glass tubing drawn out to a fine 

 capillary, which is then bent into the form of an elongated triangle, g l, by 

 heating it in the flame of a match. The two ends at the acute angle are then 

 fused and the knob of fused glass drawn out by another piece of glass tubing 

 into a fine rounded point projecting at right angles to the plane of the 



