CIRCULATORY SYSTEM OF ELASMOBRANCHS, 439 
2. The Post-Orbital Sinus. 
3. The Anterior Cardinal Sinus. 
3. i. The Hyoidean Sinus. 
3. u. The Posterior Cerebral Vein. 
3,11. The Myelonal Veins. 
3.1v. The Nutrient Branchial Veins. 
A, The Inferior Jugular Sinus. 
v g 
1. The Orbital Sinus (Parker) [ Pl. I., Or.] is a large irregular 
sinus occupying the whole of the cavity of the orbit not occupied 
by the eyeball and its muscles. It is easily recognised as a blood 
sinus, for it almost always contains a considerable amount of 
clotted blood in the freshly-killed animal and even in preserved 
specimens. 
It receives, in addition to the orbito-nasal vein, the anterior 
cerebral vein, which enters it on its lateral wall a little way 
behind the point of origin of the inferior oblique muscle. The 
two orbital sinuses communicate with one another by means of 
an inter-orbital vein. 
1.i. The Nasal Sinus [Pls. 1. & II., N.S.j is a well-marked 
crescent-shaped sinus situated somewhat ventrally on the 
inner and hinder margins of the olfactory sac. ‘The two sinuses, 
although approximating very closely in the middle line, are 
completely separated by the cartilaginous inter-nasal septum. 
They colleet the blood from the snout and olfactory sacs by a 
number of more or less indefinite trunks, some of which come 
from the anterior end of the roof of the mouth. They are figured 
in an embryo of 26 mm. by Grosser (2 a, fig. 4). 
Lui. The Orbito- Nasal Vein [ Pls. I. & Te, O.N.] is a small but 
distinct vein passing through a canal in the cartilage separating 
the olfactory sac from the orbit and entering the latter at its 
lower, inner, anterior corner by a well-marked orbito-nasal 
foramen. It conveys the blood from the nasal to the orbital 
sinus. 
This vein appears to correspond to the anterior facial vein of 
Parker, who states that in Mustelus antarcticus he was unable to 
get a satisfactory injection of it. In Scylliwm it is fairly easy 
to inject with gelatine mass from the orbital sinus by means of 
a pancreatic canula which will jamb into the orbito-nasal 
foramen. 
l. iii. The Anterior Cerebral Vein (Parker) |Text-fig. 2, A.C.] 
is formed by the union of three factors at the posterior dorso- 
lateral border of the diencephalon, whence it passes directly 
outwards through a foramen in the cranial wall and empties 
itself into the orbital sinus. The anterior factor is itself com- 
posed of two smaller tributaries, one of which starts on the 
ventral surface of the olfactory lobe and passes forwards and 
then upwards to the dorsal surface. Here it runs backwards to 
