STRUCTURE OF HEART 



87 



takes this direction and not the other, and how does it make 

 its way from tiie artery of a given organ into the vein ? 



Internal Structure of the Heart.— To answer the first 

 question— why the blood leaves the heart by the arteries 



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fiul.cutr 



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!■ IG. 22. — Tliii htart of the frog from the \'eiitral .ispect, with the cavities hiid open 

 a, a,' hristle passed into left carotid trunk ; an. z'. v, auriculo-ventricLiiar valves 

 b. b.' hristle in left systemic trunk ; t:, f,' hristle in left pulmo-cutaneous trunk 

 car. a. carotid artery ; cai: gl. carotid plexus ; c. art. conus arteriosus ; car. tr. 

 carotid trunk ; /. an. left auricle ; Ig. a. lingual artery ; /. v. longitudinal valve 

 pill. en. tr. pulmo-cutaneous trunk ; put. t. aperture of pulmonary veins ; r, a?/ 

 right auricle ; i'. a?/, ap. sinu-auricular aperture ; spt. aitr. septum auricidarum 

 T', ?'.' valves ; 7'/. \entricle. (From Parker tmil Haswell's Z(>(V('.^'l',) 



and returns to it by the veins, and not vice versa — we must 

 examine the heart itself in some detail. 



The ventricle is a hollow structure with thick spongy walls 

 and a small cavity (Fig. 22, w/), and there are two perfectly 



