314 



THE AMERICAN NATURALIST 



[Vol. XL 



A short distance beyond the point of union of the second and 

 third arches, the systemic arch gives off a well marked vessel (H.), 

 to the end of the hyoid apparatus, and some distance beyond this 

 it gives off an occipito-vertebral artery to the vertebral column 

 and occipital region (0. V.). After passing around to the dorsal 

 side of the digestive tract, the two systemic arches unite, just above 

 the heart, to form the dorsal aorta (D. A.). 



The Joiirfh hrajirhial arch passes just posterior to the gill open- 

 ing, at the outer niiirgin of which it is connected with the third 

 arch by the ductus Botalli. Beyond the ductus Botalli it gives 

 off an artery (Sc.) to the region of the scapula, and then two 

 small arteries (D.) to the muscles of the dorsal part of the body 

 in the region of the lungs. The main branch of this arch con- 

 tinues posteriorly as the pulmonary artery (P. A.). 



Some variation in the relative sizes of the vessels of the bran- 

 chial region occurs as well as some slight variation in their dis- 

 tribution, but the normal condition is about as described above. 



The distribution of the blood vessels that arise as branches of 

 the dorsal aorta will now be described. The aorta and its branches 

 are so easily filled with the injection fluid that it is a comparatively 

 easy matter to work out their distribution, especially in the abdom- 

 inal region. In fact, the only part of the arterial system that offers 

 any difficulty is the outer part of the visceral arches, in the region 

 of the gill openings. 



The branches of the aorta will be described in order from before 

 backward. The most anterior branches are given off in the 

 region of the heart, as a pair of rather small arteries (Fig. 12, G. 

 A.) which arise nearly opposite each other and extend in a pos- 

 terior direction to supply the anterior part of the reproductive 

 organs, especially the oviducts. 



A short distance posterior to the last described arteries, are 

 given off the two subclavians {S. CL), the right vessel arising a 

 little anterior to the left. As might be expected from the slight 

 development of the anterior appendages, the subclavian arteries 

 are comparatively small vessels. On reaching the shoulder gir- 

 dle, each subclavian divides into four main branches. The most 

 anterior of these (.4. He) supplies blood to the region in front of 

 the scapula. The next branch {Br.) is the brachial, and extends 



