193 



with which the lymphatics are even more richly provided than the 

 veins, hinder a backward and favour an onward flow. The contractions 

 of the intestines, and especially of the villi, aid the movement of 

 the chyle. By the contraction of the diaphragm, substances may 

 be sucked from the peritoneal cavity into the lymphatics of its 

 central tendon, through the stomata in the serous layer with which 

 its lower surface is clad. It is even possible by passive movements of 

 the diaphragm in a dead rabbit to inject its lymphatics with a coloured 

 liquid placed on its peritoneal surface. Passive movements of the 

 limbs and massage of the muscles are also known to hasten the sluggish 

 current of the lymph, and are sometimes employed with this object in 

 the treatment of disease. 



(3) The movements of respiration aid the flow. At every inspiration 

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

 lymph is sucked into them (p. 226). 



(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 ol 

 sixty to seventy beats a minute, and are governed by motor and inhibi- 

 tory centres situated in the spinal cord. The beat is not directly ini- 

 tiated from the cord, but the tonic influence of the cord is necessary in 

 order that the lymph hearts 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 III. 



i. 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 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, 118). 



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

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



13 



