168 A MANUAL OF PHYSIOLOGY 



PRACTICAL EXERCISES ON CHAPTER II. 



1. Microscopic Examination of the Circulating Blood. (i) Take 

 a tadpole and lay it on a glass slide. Cover the tail with a large 

 cover-slip, and examine it with the low power (Leitz, oc. III., obj. 3). 

 Generally the tail will stick so closely to ihe slide, and the animal will 

 move so little, that a sufficiently good view of the circulation can be 

 obtained. If there is any trouble, destroy the brain with a needle. 



Observe the current of the blood in arteries, capillaries and veins. 

 An artery may be easily distinguished from a vein by looking for a 

 place at which the vessel bifurcates. In veins the blood flows in the 

 two branches of the fork towards the point of bifurcation, in arteries 

 away from it. 



Sketch a part of a field. 



To Pith a Frog. Wrap the animal in a towel, bend the head 

 forwards with the index-finger of one hand, feel with the other for the 

 depression at the junction of the head and backbone, and push a 

 narrow-bladed knife right down in the middle line. The spinal cord 

 will thus be divided with little bleeding. Now push into the cavity 

 of the skull a piece of pointed lucifer match. The brain will thus 

 be destroyed. The spinal cord can be destroyed by passing a blunt 

 needle down inside the vertebral canal. 



(2) Take a frog and pith its brain only, inserting a match to prevent 

 bleeding. Pin the frog on a plate of cork into one end of which a 

 glass slide has been fastened with sealing-wax. Lay the web of one 

 of the hind-legs on the glass and gently separate two of the toes, if 

 necessary by threads attached to them and secured to the cork plate. 

 Put the plate on the microscope-stage and fasten by the clips (see 

 pp. 26, 107). 



2. Anatomy of the Frog's Heart. Expose the heart of a pithed 

 frog by pinching up the skin over the abdomen in the middle line, 

 dividing it with scissors up to the lower jaw, and then cutting through 

 the abdominal muscles and the bony pectoral girdle. The external 

 abdominal vein, which will be observed on reflecting the skin, can 

 be easily avoided. The heart will now be seen enclosed in a thin 

 membrane, the pericardium, which should be grasped with fine- 

 pointed forceps and freely divided. Connecting the posterior surface 

 of the heart and the pericardium is a slender band of connective 

 tissue, the fraenum. A silk ligature may be passed around this with 

 a threaded curved needle and tied, and then the fraenum may be 

 divided posterior to the ligature. The anatomical arrangement of the 

 various parts of the heart should now be studied. Note the single 

 ventricle with the bulbus arteriosus, the two auricles, and the sinus 

 venosus, turning the heart over to see the latter by means of the 

 ligature. Observe the whitish crescent at the junction of the sinus 

 venosus and the right auricle (Fig. 58). 



3. The Beat of the Heart. Note that the auricles beat first, and 

 then the ventricle. The ventricle becomes smaller and paler during its 

 systole, and blushes red during diastole. Count the number of beats 

 of the heart in a minute. Now excise the heart, lifting it by means 



