726 TEXT-BOOK OF PHYSIOLOGY. 



the right side of the ventricle it will at first receive only venous blood, which 

 on the contraction of the conus might pass either into the bulbus aortae 

 or into the aperture of the pulmo-cutaneous trunks. But the carotid and 

 systemic trunks are connected with a much more extensive capillary system 

 than the pulmo-cutaneous and the pressure in them is proportionally great, 

 so that it is easier for the blood to enter the pulmo-cutaneous trunks than to 

 force aside the valves between the conus and the bulbus. A fraction of a 

 second is, however, enough to get up the pressure in the pulmonary and 

 cutaneous arteries, and in the meantime the pressure in the arteries of the 

 head, trunk, etc., is constantly diminishing, owing to the continual flow of 

 blood toward the capillaries. Very soon therefore the blood forces the 

 valves aside and makes its way into the bulbus aortae. Here again the 

 course taken is that of least resistance; owing to the presence of the carotid 



CC. 



FIG. 377. FROG HEART WITH VENTRAL SURFACE DISSECTED AWAY TO SHOW ITS STRUC- 

 TURE. (After Parker and Hasw ell), ra. Right auricle, la. Left auricle, sa. Septum between 

 auricles, sao. Sino-auricular opening, ca. Conus arteriosus. sv. Semilunar valves, av. 

 Auriculo-ventricular valves. Iv. Longitudinal valve, sv' . Semilunar valve, p' s' c' . Pulmo- 

 cutaneous aortic and carotid trunks respectively, cc. Columnae carneae. 



gland, the passage of blood into the carotid trunks is less free than into 

 the wide elastic systemic trunks. These will therefore receive the next 

 portion of blood which, the venous blood having been mostly driven to the 

 lungs, will be a mixture of venous and arterial. Finally as the pressure 

 rises in the systemic trunks the last portion of blood from the ventricle, 

 which coming from the left side is arterial, will pass into the carotids and so 

 supply the head" (Parker and Haswell). 



The muscle fibers composing the walls of the heart from the sinus venosus 

 to the conus arteriosus are continuous, though at the sino-auricular, the 



