BLOOD-VASCULAR SYSTEM OF THE LORICATI 63 



5. Subclavian Arteries. 



The subclavian arteries (Pis. I and II, figs, i, 5, 14 and 16; 

 Sub. A.), usually,' have their origin in a single trunk from the 

 common chamber (fig. 5, C.C.). This common stibclavian 

 trunk (PL I, fig. 5) arises above and between the dorsal 

 aorta and the cceliaco-mesenteric artery. For a short dis- 

 tance it runs parallel to the aorta and the cceliaco-mesenteric 

 artery, and then branches at nearly right angles ; the right 

 subclavian passing obliquely above the cceliaco-mesenteric 

 arter}', the right dorsal branchial retractor muscle, and the 

 right head kidney to the right pectoral fin ; while the left sub- 

 clavian passes between the aorta and the cceliaco-mesenteric 

 artery, above the left dorsal branchial retractor muscle and left 

 head kidney to the left pectoral fin. 



After crossing the head kidney the course of each subclavian 

 is ventrad, passing with the combined trunk of the first and 

 second spinal nerves across the outer surface of the head kidney 

 and cardinal vein to the inner surface of the pectoral fin. In 

 the region of the dorso-lateral edge of the head kidney the sub- 

 clavian gives off the first neural artery (Pis. I and II, figs, i 

 and 16 ; Neu. A.^,), which is somewhat analogous to the verte- 

 bral artery in mammals. This vessel runs obliquely dorsad in 

 front of the combined trunk of the first and second spinal 

 nerves, and then passes over the second and first epibranchial 

 arteries, but behind the thymus gland. When the atlas is 

 reached it gives off t\\Q first spinal or myelon artery (PI. II, 

 fig. 16; Sp.'A.'), which enters a foramen in the exoccipital 

 and usually anastomoses with the myelonal artery. The main 

 stem, however, continues dorsad, terminating in a cephalic, 

 and a dorsal branch. The cephalic branch supplies the 

 trapezius muscle and sends a branch ventrad, which probably 

 supplies the th^^mus gland. This small vessel I have been 

 able to trace to the thymus, but never have seen it penetrate the 

 gland. Strange to say the arterial supply for the thymus is 

 more difficult to trace than the venous S3^stem. The dorsal 

 branch of the first neural artery is destined to supply the super- 

 ficial, the levator, and the depressor muscles of the first dorsal 



^ For exception see page 45. 



