Chap. X.] THE HEART AND CIRCULATION. 199 



3. Ventricle : posterior, muscular, undivided, receiving venous 

 and arterial blood simultaneously from the right and left auricles. 



4. Truncus arteriosus : ventral, anterior, and to the right ; 

 thick-walled and muscular, receiving successively venous and 

 arterial blood from the ventricle. 



To ensure the passage of blood in the right direction there are 

 the following valves : 



(a) Sinu-auricular valve : between the sinus venosus and right 



auricle. 



(b) Auricula-ventricular valve : between the atrium and the 



ventricle prevented from flapping back into the auricles 

 by fine tendinous cords. 



(c) Semi-lunar valves in the truncus arteriosus (two sets). 



(d) A median longitudinal valve, attached to the dorsal wall of 



the truncus arteriosus, and partially dividing it into two 



The arrangement of the great vessels which enter and leave 

 the heart will be seen from the diagram Fig. 60, which shows 

 the ventral aspect. The truncus arteriosus (tr. a.) bifurcates, and 

 each branch gives rise to three arteries. The carotid (ca.) 

 anteriorly, the systemic aorta (sy. ao.) in the middle, and the 

 pulmo-cutaneous (p. cu.) posteriorly. The carotid supplies the 

 head and brain. On it is developed a carotid gland (ca. gl.\ 

 a muscular vesicle with septa in its interior. Close to the 

 carotid gland the artery gives off a lingual branch (li.) to the 

 tongue. The systemic aorta of each side curves round to meet 

 its fellow in the middle line. It gives off the vertebral (ve.) and 

 the subclavian arteries (s. cl. a.) to the vertebral column and 

 fore-limb. At or near the point where the two systemic arches 

 meet to produce the dorsal aorta which runs down the roof of 

 the pleuro-peritoneal cavity, a coeliaco-mesenteric (c. m.) is given 

 off to supply the stomach and intestines. It soon branches into 

 a codiac (c.) to stomach and liver, and a mesenteric (m.). A small 

 inferior mesenteric artery supplies the base of the large intes- 

 tine. 



The blood is returned from the stomach and intestines by the 

 portal vein (por.), which breaks up into a capillary plexus in the 



