1904.] CIRCULATORY SYSTEM IX THE OPHIDIA. 333 



the dorsal median line. This is the case with the first fourteen ; 

 the next two are median in position. 



The posterior series of intei'costals at the middle and posteiior 

 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 intei-costal, 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 intei'costals 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 inseited 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 x&vy 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 occasional^ absent. The 

 absence, however, is rare. 



Anteriorly there ai-e many corresponding intercostal arteiies, 

 which are, of covirse, given off from the vertebral aitery. Com- 

 mencing from the origin of the vertebral from the aorta, the 

 first ten ti'unks are perfectly i-egular, each one coi-responding 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- 

 lunning 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 emerg- 

 ence here and there of an arteriole supplying the 03Sophagus. 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. 



(Eso'phageal and Gastric Arteries. — The ossophagus and stomach 

 are furnished with a very lai-ge 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 regulai- series of fair-sized gastric trunks. None of these, how- 

 ever, nearly appi'oaches in size the two ensuing mesenteric aiteries. 

 The failure of large arteiies is, however, compensated by the 



