198 ANIMAL BIOLOGY. [Parti. 



form the dorsal aorta (d. ao.\ which runs backwards beneath 

 the spine, and becomes the caudal artery. But before thus 

 uniting, each epibranchial gives off a subclavian artery (s. a.) 

 to the pectoral fin, while the right epibranchial gives off also 

 a cceliac (cce. a.) and a mesenteric artery (me. a.) to supply the 

 stomach and intestines. 



From the stomach and intestines the blood is delivered by a 

 portal vein to the liver, and thence passes by the hepatic veins 

 (he. v.) to the sinus venosus. The blood from the posterior 

 parts of the body collects in posterior cardinal veins (p. card. v.\ 

 which pass through the substance of the kidney. 1 The blood 

 from the upper parts of the head collects in anterior cardinal 

 veins (a. card. v.). The anterior and posterior cardinals of 

 each side unite to form ductus Cuvieri (d. c.), which pass down- 

 wards to the sinus venosus. The left ductus receives near its 

 dorsal aspect a spermatic or ovarian vein from the organs of 

 generation. The right ductus receives near its opening into 

 the sinus venosus an inferior jugular vein from the lower 

 parts of the head. Note that the epibranchials, by curving 

 round and uniting anteriorly, form a closed circle (circulus 

 cephalicus), which lies beneath the skull, and may be seen by 

 removing the lower jaw and the mucous membrane of the roof 

 of the mouth. In the figure the afferent and efferent branchial 

 arteries and the ductus Cuvieri are placed side by side. They 

 are connected by the capillaries of the gills, and curve upwards 

 from the ventral aspect, where the heart lies, to the dorsal 

 aspect, along which run the epibranchials and cardinals. The 

 vessels containing arterial blood are shaded. 



The Frog. The heart of the frog consists of four divisions : 



1. Sinus venosus: dorsal, thin- walled, receiving venous blood 

 by the precavals and postcaval. 



2. Atrium : anterior, thin-walled, divided into two chambers : 



(a) right auricle, receiving venous blood from the sinus venosus; 



(b) left auricle, receiving arterial blood from the pulmonary 

 vein. 



1 That of the left side is aborted for some distance in the kidney. 



