128 GENERAL ZOOLOGY 



external carotid arteries. The carotid gland is a spongy struc- 

 ture containing a network of small blood vessels through which 

 the blood of the carotid artery passes. 



There is still a difference of opinion as to the function of the 

 valves in the bulbus. The present account is based on the mor- 

 phological relations and also on experimental data. If India ink 

 is injected into the postcava, the sinus venosus or the right 

 atrium, much of the ink may appear in the aortic and carotid 

 arches, but on the average, less in amount than in the pulmo- 

 cutaneous trunks. Thus it would seem that there is a con- 

 siderable separation in the bulbus of the two kinds of atrial blood 

 but that the separation is not absolute. Such experiments sup- 

 port the idea that most of the blood from the right atrium enters 

 the pulmocutaneous arches and that most of the blood from the 

 left atrium enters the aortic and carotid vessels. It would also 

 seem that the last blood to leave the heart, the least venous blood, 

 enters the carotid system since the small openings of the carotid 

 arteries are beyond the larger entrances to the aortic trunks. 



This interpretation of the action of the frog's heart can have 

 a functional significance only on the assumption that the blood 

 returned to the heart from the lungs has a higher oxygen content 

 than the blood in the right atrium. This point must be con- 

 sidered because the frog obtains a considerable portion of oxygen 

 through buccal and cutaneous respiration. It would seem to be 

 fairly obvious that the internal structure of the lung provides a 

 mechanism for the absorption by the blood of oxygen from the 

 alveolar air and for the release of carbon dioxide from the blood 

 into the cavity of the lung. The pulmonary artery runs along 

 the outer surface of the lung to the tip, giving off at right angles 

 lateral branches forming a rich capillary network in the alveolar 

 walls (Fig. 14). The pulmonary vein arising from this capillary 

 network courses along the inner surface of the lung from the tip 

 to the base. On the other hand, the great cutaneous veins from 

 the skin of the dorsal and lateral regions of the body contain 

 oxygenated blood which passes into the venous stream, but the 

 volume of this contribution to the venous stream is relatively 

 small, compared to the volume of the venous stream as a whole. 

 Therefore, it would appear that the venous blood brought to the 

 right atrium contains less oxygen than the blood entering the left 

 atrium directly from the lungs. 



