1906.] RESPIRATORY SYSTEMS IK THE OPHIDIA, 513 



more laterally. In Boa constrictor the same area is drawn from, 

 but there is more specialisation in the veins. 



There are, in fact, two longitudinal trunks, one of which is 

 lateral in position and the other close to the medial dorsal line. 

 These arise by separate origins from the jugular. The branch of 

 the azygos which draws blood from the immediate neighbourhood 

 of the vertebral column does not form a long vessel running 

 freely in the body-cavity. It divides directly after its opening 

 near the heart into three equally sized branches, which run 

 straight to the body- wall and plunge into the parietes between 

 two successive vertebrte. This specialisation of the azygos into 

 a proximal and a more distal trunk is an approach to the con- 

 ditions observable in the Crocodilia, and is an advance upon the 

 structure which has been as yet recorded among most of the 

 Ophidia. In Python sehm, however, there is a similar division of 

 the azjrgos into two branches concerned Avith different regions of 

 the dorsal parietes*. 



A specimen of Python sebce which I have dissected since \vriting 

 the account of the azygos of that sna,ke referred to below affords 

 confirmation of that account (which is of importance in view of 

 getting at the normal arrangement of the veins in these animals) 

 and enables me to add a few details. In the indiAddual to which 

 I now refer, a female, the azygos shows the same division into a 

 more dorsal and a more lateral branch. The trunk is bifid behind 

 the point where the third intercostal is given off from the un- 

 divided trunk. The more lateral branch only supplies three 

 intercostal spaces. After this point the main trunk gives off 

 eleven branches to as many rib-spaces, the last two of which are 

 very slender. There is then a gap, but the very next rib is 

 accompanied by a vein which is the first of a continuous series 

 of fourteen intercostals arising from the right side of the 

 median line which communicate with the hepatic portal system. 

 So large a development of intercostal veins on the right side is 

 not common in Snakes. On the left side, in this specimen as in 

 other snakes, there is a strong development of the longitudinal 

 parietal vessel. 



Pemaiois of Umbilical Vein. — In the case of Boa cliviniloqua the 

 male and female examples which I dissected showed traces of 

 the umbilical vein (text-fig. 91, p. 514). I do not think that 

 there were any noticeable differences in the several examples. 

 But I made more complete notes in one case than in the other. 

 In the larger female specimen the vena cava, immediately after 

 emerging from the liver, was joined by a slender vein expanding 

 somewhat at its debouchment into the vena cava. The extreme 

 anterior end of the liver occupied the angle formed by the con- 

 fluence of the two veins. Traced backwards, this affluent of the 

 vena cava continued to be full of blood for some little distance ; 

 but soon it seemed to be impervious, and to be a mere ligamentous 



* See Beddard, " Contributions to the Anatomy of the Ophidia," P. Z. S. 1906, 

 p. 30. 



35* 



