OCR Text |
Show 1904.] CIRCULATORY SYSTEM IX.THE orillDIA. 333 the dorsal median line. This is the case with the first fourteen ; the next two are median in position. The posterior series of intercostals at the middle and posterior end of the series, where they are most typically developed, differ in a number of characters from the anterior series that have just been described, and agree in many points with the intercostal branches of the anterior vertebral artery. They are (1) paired arteries entering the body-wall to the right and left of the median line ; (2) they are regularly intercostal, corresponding to the ribs ; (3) they give off branches to the viscera. There is not, however, by any means a sudden jump from one type of intercostal to the other. The 17th, 18th, and some of the following intercostals (of the whole series, counting from the junction of the aorta) are like the preceding branches, save for the fact that there is one for each vertebra. There are eleven of these arteries, after which the intercostals become paired. The first four of the eleven alternate regularly in their insertion into the body-wall from left to right; the next two are inserted on the right side; the next four again alternate regularly, beginning with the left side ; the last of the eleven perforates the body on the right side. The origins of the arteries from the aorta vary in accordance with the point of entrance into the body-wall. Then follow the paired intercostals, of which there are rather over 60 pairs-one twig of the pair being occasionally absent. The absence, however, is rare. Anteriorly there are many corresponding intercostal arteries, which are, of course, given off from the vertebral artery. Commencing from the origin of the vertebral from the aorta, the first ten trunks are perfectly regular, each one corresponding to a vertebra. The first two branch immediately after their origin from the aorta and enter the body-wall as two tubes; the rest do not, and perforate the body-wall accurately in the middle line. Each of these vessels gives off shortly after its origin a backwardly-running and slender branch to the oesophagus. In front of the ten regularly-arranged intercostals are two which perforate the body-wall at some distance from each other, so that room is left for six other arteries, which are, however, not developed. Their former presence (?) is indicated by the emergence here and there of an arteriole supplying the oesophagus. The last intercostal is formed by the vertebral artery itself, which plunges into the parietes in the middle line at a distance of 7| inches from the tip of the snout. Oesophageal and Gastric Arteries.-The oesophagus and stomach are furnished with a very large number of minute arterial branches, which I do not stay to characterise in detail. Later on these branches become divisible into very minute and rather larger trunks, but it is not until the end of the liver that there is a regular series of fair-sized gastric trunks. Xone of these, however nearly approaches in size the two ensuing mesenteric arteries. The failure of large arteries is, however, compensated by the |