Srencer—TZhe Llood Vessels of Ceratodus. 9 
represented diagrammatically in figs. 2 and 5. It runs for a short distance just 
outside the cartilage at the posterior end of the skull, but soon penetrates this, 
passing dorsad of the superior jugular vein and hyoidean branch of the 7th cranial 
nerve ; it pursues an almost straight course, trending slightly forwards and towards 
the median line. Passing ventrad of the posterior carotid and of the branch between 
the 7th and 9th cranial nerves and of the main branch of the 5th nerve, it reaches the 
brain cavity, across the floor of which, behind the level of the infundibulum, it sends 
a branch which unites with one from the corresponding artery of the other side. 
There is thus formed a small czrculus cephalicus (fig. 5 c.c.), though this is apparently 
stated to be absent in Dipnoi.* 
The main branch enters the brain case and divides into (a) an anterior cerebral 
branch which supplies the front part of the brain, and (b) a posterior cerebral branch 
which supplies the hind part of the brain (fig. 5, ant. cer. art., post. cer. art.) 
(d) Posterior carotid artery (figs. 2 and 5). 
This is a larger branch than the anterior carotid. It arises on each side from 
the epibranchial artery of the first branchial arch, and immediately runs forwards into 
the cartilaginous cranium. It passes to the outer side of the auditory organ and 
then bends inwards towards the middle line until it crosses the anterior carotid, 
immediately ventrad of which it lies. The fifth nerve lies dorsad of both and 
gives off a branch which passes round the posterior side of both arteries where 
they cross one another and then turns immediately forwards ventrad of them and 
runs to the anterior end of the head. 
Shortly after entering the cartilage the artery gives off a branch which passes 
downwards and then runs forwards under the parasphenoid to the roof of the mouth. 
This is the palatine artery (fig. 5, pal. art.). 
Just after crossing the anterior carotid it divides into two branches, of which 
one runs outwards to the orbit, forming the ovéztal artery (orb. art.), and another 
runs on till it comes to the optic nerve, on which it branches. 
Of the above arteries the anterior carotid must be regarded as equivalent to the 
combined pseudobranchial and anterior carotid arteries of such forms as Mustelus and 
Raja,+ where a pseudobranch is present, and to the anterior carotid of Callorhynchus,t 
where, as in Ceratodus, no pseudobranch is present. In all the forms, though the 
* Cf. Forms of Animal Life, 2nd Edit. Jackson, p. 425. 
+ Cf. Parker, loc. cit. p. 693, fig. 1. 
t Cf. Parker, loc. cit. p. 693, fig. 17. 
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