BLOOI>VASCULAR SYSTEM, ETC., OF AMIURUS CATUS. 427 



the vessel passes through the pseudobranchia. It theii enters the 

 wide flat anterior portion of the brain cavity as the nasal artery 

 (Fig. 1, n), 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, ant. 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 

 cir cuius 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 left cerebral hemispheres 

 at their base, where they distribute themselves. 



The median and smallest lies behind the optic nerve and runs 

 backwards 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 tori semiculares distributed 

 upon the surface covered by the tecta optica, and a posterior passing 

 30 



