346 THE CAT. [chap. x. 



ventricular portion, which latter comes to He transversely. Mean- 

 time a septum grows up, dividing the ventricular part into a right 

 and a left chamber. The septum extends on into the bulbus, which 

 becomes divided into two channels, those of the aorta and of the 

 pulmonary artery respectively. As soon as the ventricular septum 

 is complete, another septum begins to divide the auricular portion 

 into a right and left chamber, but this septum remains incomplete 

 even at birth, when an aperture, the /o?Y/;>?e« ovale, still remains, and 

 allows the blood to pass directly from the right to the left auricle 

 without passing through the lungs. It is the existence of this 

 perforation in the new-born kitten * which renders the drowning of 

 it so long a process. 



The more important arteries arise as follows. The aortic bulb 

 gives off at first two arteries which run forwards till they reach the first 

 visceral arch, inside which they pass upwards to beneath the spine, 

 where they unite together to form the incipient dorsal aorta. As 

 the length between the most anterior part of these primitive aortic 

 arches and the heart, lengthens (from the growth of the embryo), 

 other arches successively arise from the forwardly extending bulb, 

 and ascend to the backwardly extending root of the dorsal aorta, 

 forming, as it were, a succession of short cuts on each side to the 

 dorsal aorta (Fig. 156, i - ^ *). Four vessels _ thus arise on each 

 side behind the first, and respectively ascend inside successive visceral 

 arches to the aorta. jThere are thus five such ascending arches 

 developed on each side, though only three exist at the same time, 

 the earlier and more anterior disappearing as the later ones arise. 



Ultimately the fourth arch of the left side persists,^ grows, and 

 becomes the arch of the adult aorta. That of the right side becomes 

 the innominate and right subclavian and carotid arteries. The more 

 anteriorly situated arches become transformed into branches of the 

 carotids. The fifth arch disappears on the right side altogether, but 

 that of the left becomes the pulmonary artery, a communication 

 between it and the great aorta persisting till a little after birth as 

 the ductus arteriosus (Fig. 157, d a), a relic of which is seen in the 

 fibrous cord which connects together these arteries in the adult. 



The descending aorta at first gives off" on each side a large omphah- 

 meseraic artery (Fig. 156, oma), which goes to the vitelline sac, or 

 umbilical vesicle ; but these branches dwindle as the aorta comes to 

 give origin to two other considerable and more posteriorly situated 

 arteries (ua) called vmbilical (the future hypogastric ones), which 

 proceed into the allantois and placenta. From each umbilical 

 artery a compai-atively small branch (I/a), the future i/iac artery, is 

 given off, and goes to the incipient pelvic limb. 



In the process of their devei-opmext the more notable veins 

 undergo remarkable changes. The first important veins to appear 

 are two which pass upwards from the umbilical vesicle to the heart, 

 and are called the right and left omphalo-meseraic veins (Fig. 156 , 



* 1 liavc found this forniiicn still open a week after birtb. 



