THE BLOOD SYSTEM 



The circulatory system is a closed scries of vessels filled with 

 blood and ramifying through all parts of the body. Its main 

 parts are (1) the heart, which by its muscular activity propels 

 the blood through the vessels, (2) the arteries, which are the 

 vessels conducting the blood from the heart to all parts of the 

 body, (3) the veins, which return the blood from the various 

 parts back to the heart, and (4) the capillaries, a system of very 

 small vessels forming the transition from arteries to veins. It 

 is during the course of the blood through capillaries that it 

 performs its main service in the body. 



'J'he order in which blood vessels are studied will depend upon 

 the way in which the study specimens have been prepared. If 

 one specimen is to serve for this whole study the veins, whether 

 injected or not, should be studied before the arteries. The 

 following directions are based upon the use of two specimens. 

 For the study of the arteries the vessels are filled with a yellow 

 starch mass injected through the ventricle which abnormally 

 distends this compartment of the heart. 



Place the frog on its back in a wax-bottomed tray and extend 

 the slit, made in injecting the animal, cephalad to the tip of the 

 lower jaw and caudad to the pelvic girdle, cutting through both 

 the skin and body wall. Note the ventral abdominal vein, which 

 may be seen as a streak along the mid ventral line. Carefully 

 separate this vessel from the body wall by gently tearing its 

 attachments. Next, make transverse slits through the skin and 

 body wall to the right and left of the midventral incision about a 

 quarter of an inch cephalad of the caudal end of the body cavity. 

 Turn back the flaps, thus formed. At this point students dis- 

 secting a female frog should remove the ovary and oviduct of the 

 animal's left side. Care must be used in separating the blood 

 vessels from the body wall, since they will break if placed under 

 too much strain. Use judgment with regard to the mechanics 

 of dissecting. If more convenient for the worker the frog may 

 be removed from the lifjuid and held in the hand. 



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