BLOOD-VASCULAR SYSTEM, ETC., OF AMIURUS CATUS. 427 
the vessel passes through the pseudobranchia. It then enters the 
wide flat anterior portion of the brain cavity as the nasal artery 
(Fig. 1, 2), and joins the olfactory tract at the bulb, from which point 
dividing it distributes itself to the nasal sac, and also gives a strong 
lateral branch to the large maxillary barblet. It is difficult to under- 
stand why the internal and external carotids should cross their 
‘branches in order to supply these two parts. 
The three arteries to the brain may be designated as anterior, 
medial and posterior. (Fig. 1, wnt. med. post.) 
The anterior runs at first beneath and then along the posterior 
surface of the optic nerve direct to the optic chiasma, where a trans- 
verse stem unites it with its fellow of the opposite side. The union 
of this pair and also the posterior pair in the median line closes a 
circulus cephalicus, but within the brain-case. From this connecting 
stem a small anterior and a posterior artery are given off to the 
perilymphatic tissue of the brain-case. From the point of junction 
the arteries run backwards parallel to one another upon the dorsal 
surface of the optic tract, turn upwards behind the cerebral commis- 
sure, and enter respectively the right and lett cerebral hemispheres 
at their base, where they distribute themselves. 
The median and smallest lies behind the optic nerve and runs 
back wards about the angle of the floor and side of the skull, lateral 
to the hemispheres, and divides into a stem to the thalamencephalon 
and another to the lobus inferior. 
The posterior and largest lies above the former, behind and slightly 
above the optic nerve and runs backwards along the side of the 
skull in the same plane. It passes inwards along the anterior 
margin of the fourth nerve and gives off a branch which is continued 
along this nerve behind the optic lobes to the anterior under surface 
of the cerebellum, which it enters at its base. The artery turning 
slightly forwards passes under the brain and joins its fellow in the 
median line immediately behind the saccus vasculosis, to which a 
vessel is at once supplied. From this point a single median stem 
runs backwards and ends on the medulla oblongata. Three branches 
from this median artery pierce the floor of the ventricle and form 
centres of distribution to the median and posterior parts of the brain. 
The first gives off three pairs of branches: an anterior to the inner 
surface of the tecta optica, a median to the tort semiculares distributed. 
upon the surface covered by the tecta optica, and a posterior passing 
30 
