7^4 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 aortd 

 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 



FI G. 359- FROG HEART WITH VENTRAL SURFACE DISSECTED AWAY TO SHOW ITS STRUC- 

 TURE (After Parker and Has^ell.} ra. Right auricle, la. Left auricle. ,a. Septum between 

 aur: .ies. sao. Smo-auricular opening, ca. Conus arteriosus. sv. Semilunar valves av 

 Aunculo-ventncular valves. Iv. Longitudinal valve, sv' . Semilunar valve. *' s' c' Pulmo- 

 cutaneous aortic and carotid trunks respectively, cc. Columns carneee. 



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

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

 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 



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



iculo-ventncular, and the ventriculo-conic junctions the continuity is to 



