VI 



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 



Fic. 22. — The heart of the frog from the ventral aspect, with the cavities laid open. 

 a, a/ bristle passed into left carotid trunk ; arj. v. v, auriculo-ventricular valves ; 

 b. b.' bristle in left systemic trunk ; c, c,' bristle in left pulmo-cutaneous trunk ; 

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

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

 piil. cu. tr. pulmo-cutaneous trunk ; pul. v. aperture of pulmonary veins ; r. au. 

 right auricle ; s. au. ap, sinu-auricular aperture ; spt. aur. septum auricularum ; 

 7/, V.' valves ; 7it. ventricle. (From Parker and Haswell's Zoology.) 



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, vt), and there are two perfectly 



