PRACTICAL EXERCISES 177 



tion the pressure in the great veins near the heart becomes negative, 

 and lymph is sucked into them (p. 210). 



(4) In some animals rhythmically-contracting muscular sacs or 

 hearts exist on the course of the lymphatic circulation. The frog 

 has two pairs, an anterior and a posterior, of these lymph hearts, 

 which pulsate, although not with any great regularity, at an average 

 rate of sixty to seventy beats a minute, and are governed by motor 

 and inhibitory centres situated in the spinal cord. The beat is not 

 directly initiated from the cord, but the tonic influence of the cord 

 is necessary in order that the lymph heart may continue to beat 

 (Tschermak). Such hearts are also found in reptiles. It is possible 

 that in animals without localized lymph hearts the smooth muscle, 

 which is so conspicuous an element in the walls of the lymphatic 

 vessels, may aid the flow by rhythmical contractions. 



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 closaly to the 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 the 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. 15, 109). 



(3) After the normal circulation has been studied thoroughly put 

 a very small drop of tincture of cantharides on the portion of the web 

 which is in the field of the microscope, using a fine pipette. Observe 

 the process of inflammation, including stasis and diapedesis (p. 53). 



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- 



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