ANATOMY OF THE FROG 45 



and two pairs in the female — viz. those of the ureters and 

 oviducts. 



The fat-bodies should be noticed; they are finger-shaped 

 fatty masses of a bright yellow colour lying in front of the 

 testes or ovaries. 



All parts of the frog's body are supplied with a nutrient 

 fluid, the blood, by means of a system of blood-vessels which 

 take their origin from a hollow muscular organ, the heart. In 

 order that the relations of the blood-vessels to one another and 

 to the heart may be clearly understood, it should be borne in 

 mind that the blood is the great medium of exchange in the 

 body. It brings nutrient material, collected from the walls of 

 the alimentary canal to the organs for their repair, and receives 

 in exchange effete waste material which it carries away to 

 organs — the kidneys and lungs — whose function it is to separate 

 out the waste material and expel it from the body. A large 

 part of this waste matter is carbonic acid gas, which is exhaled 

 through the lungs ; and the blood, in its passage through the 

 lungs, receives in return oxygen which it carries to all parts of 

 the body. 



Bearing these facts in mind, we may study the mechanism 

 by means of which the circulation of the blood is effected. 



The heart is a hollow muscular organ, of roughly triangular 

 shape, the apex pointing backwards, lying in the mid-ventral 

 line, immediately above the ventral ends of the shoulder-girdle 

 and below the oesophagus. It is enclosed in a very thin 

 membranous sac, the pericardium, on the ventral walls of 

 which some muscular slips from the great internal oblique 

 muscle of the abdominal wall are inserted. The pericardial 

 cavity is a portion of the coelom, which has been cut off from 

 the remainder by the growth of a partition, in the substance of 

 which run the great veins leading to the heart. Its relations 

 are rather complicated, and will be better understood at a later 

 stage ; but it may here be noticed that the pericardial cavity is 

 a ventral portion of the coelom, cut off by the above-mentioned 

 partition from a dorsal portion continuous behind with the 

 abdominal ccelomic space, in which the lungs lie. The heart 

 seems to lie in the pericardial cavity, but in reality it is outside 

 of it, just as the gut and other abdominal viscera are seemingly 

 inside, but really outside, of the abdominal portion of the 

 loelom. The pericardium has an extremely thin lining 



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