THE VASCULAR SYSTEM 205 



passes, ventral to the spinal nerves, through special foramina in the 

 transverse processes of the ribs (see p. 19). Since the artery receives a 

 number of reinforcements from the dorsal aorta (q.v.), which in the 

 lumbar region are quite as strong or stronger than the original artery, 

 it is probably more correct to regard the vessel as a series of segmental 

 arteries which have fused with one another, rather than as a single 

 vessel originating at the base of the skull. The latter view is, how- 

 ever, more convenient for descriptive purposes. A number of lateral 

 vessels are given off to the muscles of the back. 



(i) Costal arteries arise alongside or within the transverse processes 

 of the vertebrae. In the latter case they soon emerge, after giving off 

 a few small twigs to the Haversian canals of the bone, to run parallel 

 with the ribs. 



(ii) Intercostal arteries arise about midway between the previous 

 ones, that is approximately opposite articulations between the verte- 

 brae. They do not run directly lateralwards but in an oblique pos- 

 tero-lateral direction, the obliquity becoming more marked in the 

 lumbar region. They are also distributed to the muscles of the back. 



(iii) Cutaneous branches arise chiefly from the costal arteries and 

 pass directly dorsalwards to supply the skin of the back, also giving 

 a branch to the large cutaneous poison glands which occur along the 

 mid-dorsal line. The capillary network forms a sort of vascular cup 

 around the bases of these glands. 



(iv) Occasional anastomoses with the intraspinal arterial system. 



It must be observed that the arrangement of costal and intercostal 

 vessels above described is not absolutely rigid. Many intermediate 

 conditions are found, but on the whole these would seem to conform 

 to a basic plan in which have been interpolated secondary additions. 



In contradistinction to the Frog it should be noted that the verte- 

 bral artery is not responsible for the blood-supply to the ^'^;//r(^/ body- 

 wall, since the epigastric artery (q.v.) discharges this function. The 

 arteriae ahdominales of the Frog are therefore not present in the Sala- 

 mander as factors of the vertebral artery. 



The vertebral artery also takes some share in the blood-supply of 

 the anterior region of the oviduct, but, since these vessels usually 

 arise from anastomoses with the dorsal aorta, they will be dealt with 

 under the branches of the latter vessel together with the other ovi- 

 ducal arteries. 



The Dorsal Aorta. 



The dorsal aorta (d.a.) arises, as previously described, by the fusion 

 of the paired systemic arches in the mid-dorsal line immediately 



