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Show 1904.] CIRCULATOR"! SYSTEM IX THE OPHIDIA. 339 Furthermore, this right aortic arch is a much more delicate vessel than the left aortic arch, which it joins at a point about 10 nun. behind the heart. Just after leaving the common trunk the right aortic arch is crossed superficially by the vertebral vein. The latter is formed of two branches, which emerge from the thickness of the parietes in the middle line and join before crossing the aorta as a single trunk. Neither the right nor the left aortic arch gives off any branches to the parietes before their union. In the male specimen, however, there were two such blanches. The origin of the anterior vertebral artery has been already described. It rapidly ascends to the median dorsal line and is lost to sight, becoming imbedded in the neck 2 inches in front of the heart and 5^ inches behind the tip of the snout. It gives off no backwardly running superficial posterior vertebral artery as in Python spilotes, but a number of intercostals plunging at once into the thickness of the parietes in the dorsal middle line arise from it. Of these the first is 5 m m. from the junction with the right aortic arch, the next 10 m m . from the first, and the third 18 m m . farther towards the head than the second-i. e., three in all. It also gives off a number of small branches to the oesophagus, which I do not further particularise. In the second individual there were also three intercostals given off from the anterior vertebral artery. It is important to note that this anterior set of intercostals plunge into the parietes exactly in the middle line. The single aorta is not increased in calibre at the junction and passes back in a more or less straight line ; it gives off a very large number of branches, of which the following is, I trust, an accurate account. There is, in the first place, a series of unpaired intercostal arteries which pierce the parietes along the middle line of the back. Most of these arteries are slightly convoluted, so as to allow of stretching. The first two are at regular intervals of 30 m m . from each other or from the junction of the aorta?; the third arises further away, 40 m m . or so from the second. Closely associated with this is the first of a series of veins emerging from the parietes in a similar fashion, which will be dealt with shortly. The distances separating the origins of the next six branches are 35, 25, 25, 30, 40, and 30 m m . respectively; the 10th is only 10 m m . from the 9th. The next two complete a series of 12 intercostals, all of which enter the parietes on the left side of the median dorsal line. After this point there is a more or less regular alternation in the point of entrance into the parietes; some of the arteries pierce the body on the right, some on the left. A few, moreover, are strictly paired, and each artery of the pair pierces the parietes either on the right or on the left. There were three such pairs in the specimen examined, all of them towards the end of the series. The total number of arteries which I counted was 46. I do not particularise, since they differed somewhat in the second 90* |