Spencer—Zhe Blood Vessels of Ceratodus. 23 
(5) One running slightly posteriorly and downwards to the margin of the upper 
jaw, behind the eye, to join a large sinus-like vessel im connection with the inferior 
jugular vein. 
Though vessels must be present bringing back blood from the brain and entering 
presumably the superior jugular, I have been unable to detect such, though by means 
of injection the above described branches were traced with comparatively little 
difficulty and also, as described previously, the cerebral arteries. 
(2) Lnferior jugular vein (figs. 6, 15, and 16). 
This is connected with the Ductus Cuvieri at the point described above. From 
this it runs slightly outwards, lying close to the surface of the hinder wall of the 
fifth cleft and then runs downwards in this superficial position parallel to the fifth 
branchial cartilage. Passing more deeply into the body it turns forwards along the 
ventral side so that its course lies at right angles to the gill arches (fig. 6). Just as 
it does so it divides into two branches, a larger and a smaller, of which the latter 
runs slightly dorsalwards and then turns forwards again dorsad of the afferent 
branchial arteries. The larger one continues its course ventrad of the same vessels. 
The smaller deeper-lying vessel receives, before passing dorsad of the fourth 
afferent artery, a small branch which passes forwards from the anterior wall of the 
pericardium (fig. 6, fev. v.). Dorsad of the arteries it runs along close to the mid 
ventral line in the region of the mesobranchial cartilages. 
The large vessel runs forwards crossing at right angles the afferent arteries 
(fig. 6) and receiving from each arch a small nutrient branchial vem (zz. 67. v.). 
Just anteriorly to the level of the first arch it bends outwards and then passes 
upwards along the outer surface of the flattened cerato-hyal cartilage immediately 
anterior to the hyoidean artery, the relationship of which has been previously 
described. 
It then turns round the angle of the jaw, swelling out as it does so into a sinus 
which may be called the Ayozdean sinus. A branch (mandibular vern) joins it from 
the lower jaw after which it passes forwards to a point beneath the orbit. Here it 
divides into (1) a sinus-like vessel running into the orbit and forming the ordztal 
stnus which is in connection with the sinus belonging to the superior jugular, (2) a 
palatine branch which passes inwards and behind the large palatine teeth and (3) a 
nasal branch passing directly forwards to the olfactory organ. 
