280 THE BIOLOGY OF THE FROG chap. 



is stretched across a notch or hole through which light may 

 be passed from below. The toes may be held in position 

 by small pieces of thread tied to the tips and fastened to 

 their other ends to the board. 



In a web thus prepared the blood may be seen flowing 

 rapidly in the small veins and arteries, and more slowly in 

 the capillaries. The red corpuscles will be found to become 

 elongated and narrowed as they thread their way slowly 

 through the small capillaries. The leucocytes often creep 

 slowly along the walls of the vessels, and may be seen to 

 stop frequently, and sometimes to migrate through the capil- 

 lary walls. In the arteries a pulsation due to the beating of 

 the heart may be observed ; the caliber of the arteries often 

 changes, owing to the contraction of the muscle fibers of 

 their walls. 



The capillary circulation may also be easily studied in the 

 tail of the tadpole. 



The Lymphatic System. — The lymphatic system of the 

 frog is remarkable on account of the abundance and large 

 size of the lymph spaces in various parts of the body. There 

 are no well-defined lymphatic vessels such as occur in the 

 mammals ; the lymph flows in irregular spaces between and 

 within the different organs ; the larger spaces are lined by 

 flattened endothelial cells, but are entirely devoid of a 

 muscular coat, and usually, also, of a lining of a connective 

 tissue. 



The subcutaneous lymph spaces are especially well devel- 

 oped ; they are separated from each only other by the nar- 

 row septa of connective tissue by which the skin is here 

 and there attached to the underlying muscles. One of the 

 largest of the lymph spaces within the body is the subverte- 

 bral lymph sinus, or cistern a magna, which extends above 

 must of the dorsal side of the body cavity. 



