BLOOD-VASCULAR SYSTEM OF THE LORICATI 63 
5. Subclavian Arteries. 
The subclavian artertes (Pls. I and II, figs. 1, 5, 14 and 16; 
Sub.A.), usually,’ have their origin in a single trunk from the 
common chamber (fig. 5, C.C.). This common subclavian 
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 
artery, 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 frst neural artery (Pls. I and II, figs. 1 
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 the first sp/nal or myelon artery (P1. Il, 
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 thymus 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 system. 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 
1For exception see page 45. 
