CIRCULATORY SYSTEM OF ELASMOBRANCHS. 44] 
foramen through which the sixth and main branches of the fifth 
and seventh cranial nerves leave the cranium. It is situated 
directly under a well-marked ridge in the floor of the cranial 
cavity immediately behind the pituitary body. 
2. The Post-Orbital Sinus [ Pls. I. & II., P.O.] 1s a moderate- 
sized vessel placing the orbital sinus in communication with the 
anterior cardinal sinus. It passes backwards from the posterior 
external part of the orbit around the auditory capsule dorsally to 
the spiracle. Its path around the capsule is indicated by a well- 
‘marked groove, the post-orbital groove, which is to be found 
between the ridge formed by the horizontal semi-circular canal 
and the smooth surface for the articulation of the hyomandibular 
cartilage. 
3. The Anterior Cardinal Sinus* (Jugular Vein, Parker) 
[Pls. I. & IJ., A.C.] is a very large and irregular sinus running 
along the internal dorsal ends of the gill-clefts. It starts close 
behind the spiracle at the anterior end of the first gill-cleft and 
runs toa point a short way behind the fifth gill-cleft. At its front 
end it receives the post-orbital sinus. The two anterior cardinal 
sinuses are situated at about the same depth below the dorsal 
surface of the animal as the vertebral column, with which they 
are approximately parallel. The pharyngeo-branchial cartilages 
project into the floor of the sinuses, forming well-marked ridges 
which possess membranous flaps. The sinuses are readily exposed 
from the dorsal side of the fish when making a dissection of the 
cranial nerves, and, indeed, the branchial branches of the tenth 
cranial nerves pass freely through their cavities, while the visceral 
branches of the same are partially embedded in their mesial 
walls. Just behind the fifth gill-cleft the anterior cardinal sinus 
narrows down considerably, and passing ventrally opens into the 
posterior cardinal sinus by an opening provided with a valve 
(vide footnote, p. 444). 
The anterior cardinal sinus receives small tributaries from the 
dorsal ends of the gill-bars and from the surrounding musculature, 
but their position is extremely difficult to ascertain owing to the 
indefiniteness and irregularity of the walls of the sinus. At its 
front end the anterior cardinal sinus receives the posterior 
cerebral vein and the hyoidean sinus. 
3.1. The Hyoidean Sinus (Parker) [Pls. I. & II, H.] is a 
moderate-sized vessel situated in front of the first gill-cleft and 
running parallel with it from the dorsal to the ventral side of the 
fish. It lies in a shallow groove on the external side of the hyo- 
mandibular cartilage, and at its dorsal end enters the anterior 
cardinal sinus near the point where the latter receives the post- 
ovbital sinus. The ventral end joins the inferior jugular sinus — 
at the level of the posterior end of the thyroid gland, 
The hyoidean sinus doubtless receives the blood from the 
tissues surrounding it, but it is extremely difficult to make out 
_* This name is used because, as Parker points ont, the vein is not in any way 
homologous with the jugular vein of Iigher animals. 
