Dissection of Sheep^s Brain. 163 



the Basilar artery ; this vessel receiving its name from the 

 fact of its being in close relation with the basilar process of 

 the Occipital bone : after traversing the Pons in the median 

 line and reaching the anterior border of the Pons, the Basilar 

 artery divides into two terminal branches, named the posterior 

 Cerebral arteries, which run in an oblique direction forwards 

 just internal to two nerve twigs (the third pair of nerves), and 

 then bifurcate, one branch returning in a posterior direction, 

 the oih.Qv continuing forwards to join the internal Carotid arteries 

 which enter the Brain posteriorly to the Optic Commissure, 

 i.e. that mass of nerve matter lying transversely, limiting, as it 

 were, the longitudinal furrow between the Cerebral Hemi- 

 spheres posteriorly. 



The * Common ' Carotids^ after leaving the arteria innomi- 

 nata, divide each of them in the neighbourhood of the Thyroid 

 cartilage into two branches, named * external' and 'internal' 

 carotids ; of these, the former is distributed to the external 

 parts of the cranium, viz. the head and face ; the latter to 

 the internal parts of the cranium. After the bifurcation of 

 the * common ' carotid, the * internal ' branch passes ventrad 

 of the three transverse processes of the three upper cervical 

 vertebrae to the Carotid foramen in the lesser wing of the 

 sphenoid, forms a plexus in the cavernous sinus just external 

 to the sella Tursica, and, passing just within the clinoid pro- 

 cesses, reaches the base of the Brain a little posterior to the 

 Optic Commissure*. Immediately on their reaching the base 

 of the Brain the two internal Carotids are each met by a branch 

 of the posterior Cerebral Artery\ of its own side ; they then pro- 

 ceed forwards round the Optic Commissure, and bifurcate, 

 one branch (the ' middle cerebral ') running up the fissure of 

 Sylvius (i.e. the depression between the anterior and middle 



* "When the Brain has been removed from the Skull the cut ends of the 

 internal Carotids are sometimes so short as to require careful search. 



t Proceeding from the Basilar Artery, vide supra. 



