286 



CLASS AMPHIBIA. 



empty and interpose much less resistance to the entry of blood than do 

 the aortic and carotid arteries (as will be explained in a moment). In 

 the second phase of the ventricular systole, the conus itself contracts and 

 the free edge of the longitudinal valve becomes applied to its ventral 

 wall, whereby its cavity becomes converted into two separate chambers. 

 Of. these the cavum pulmo-cutaneurn is closed behind so that no more 

 blood can enter it from the ventricle ; but it remains open in front to the 

 dorsal division of the ventral aorta and so to the pulmonary arteries. 

 The result of the contraction of the conus upon the cavum pulmo 

 cutaneum will be to empty it and to drive the blood through the pul- 

 monary arteries, thus rendering easy the entrance of blood into these 

 stractures at the next ventricular systole The cavum aorticum on the 

 other hand remains open to the ventricle during the second phase of the 

 ventricular systole. It contains probably a certain amount of venous 



blood already driven 

 into it and receives 

 now all the rest of the 

 veiitricul a r b lo o d, 

 namely, that from the 

 left side. This must 

 be largely arterial, for 

 most of it must have 

 entered the ventricle 

 from the left (p*ulmo- 

 nary) auricle. This 

 blood, together with 

 the venous blood al- 

 ready in the cavum 

 aorticum, must be en- 

 tirely delivered throTigh 

 the cavum aorticum 

 into the ventral divi- 

 sion of the ventral 

 aorta, and so into the 

 carotid and systemic 

 arteries. The question 

 now arises whether 

 there is any arrange- 

 ment whereby this blood is sifted and the most arterial of it sent into 

 the carotid arteries. It is said that observation has shown that the carotids 

 receive more purely arterial blood than the systemic aortae. The following 

 structural features have been alleged as contributing to this, from a 

 physiological point of view desirable, result. The three arches of the left 

 side lie in n line with the conus and ventral aorta, the arches of the right 

 side passing off at an angle (Fig. 159). The effect of this will be that the 

 blood in order to enter the right aortic arch and the two carotids, which 

 as we have seen come off from this arch, will have to pass round a corner. 

 It? will be easier therefore for it to flow straight on into the left systemic 

 arch. In this way it is possible that the first blood from the cavum 

 aorticum, i.e. mixed venous and arterial, will pass into the left arch, leaving 

 only the last blood which enters the conus, i.e. as we have shown the most 

 purely arterial, to enter the right arch and the carotids. It has further 

 been alleged that the carotid glands (described below) interpose ad 



FIG. 160. Two transverse sections through the conus of Rana 

 platyrrhina. A at the junction of the conus and ventral 



L aorta ; B a little further back (after Boas) ; a the point 

 where valve No. 1 is fused with the left wall of conus ; Ib 

 dorsal part, la ventral part of valve No. 1 ; 2 dorsal, 

 3 ventral of the three valves of the anterior row ; trs, hori- 

 zontal septum of the ventral aorta. The sections are viewed 

 from the front side. The longitudinal valve of the conus 

 is not shown, but if posterior sections were represented, it 

 would appear as a continuation of valve Ib and la, the 

 pocket of this valve with the horizontal septum trs dis- 

 appearing. 



