386 THE SUBCLAVIAN ARTERY. 



■u-hich it divides into two terminal branches, the posterior arteries of 

 the cerebrum. 



Brandies. — Besides numerous small brandies to the substance of the pons, tlie 

 basilai' artery gives off the following- : — 



(fl) The transverse arteries, several on each side, pass du-ectly outwards. One, 

 the artery of the avditory nerve, accompanies that nerve into the internal auditory 

 meatus and labyrinth of the ear. 



(Z>) The anterior inferior eerehella r artevies pass backwards, one on each side, to 

 the anterior pai-t of the under surface of the cerebellum, anastomosing with the 

 inferior cerebellar branches of the vertebral arteries. 



(c) The superior eerehellar arteries arise so close to the bifurcation of the 

 basQar, that this artery is described by several anatomists as dividing into four 

 branches. Each one turns backwards and outwards immediately behind the third 

 nerve, and entermg the gi-oove between the pons Varolii and the cms cerebri, 

 tui-ns round the latter, close to the foui-th nerve, to reach the upper surface of 

 the cerebellum, where it divides into branches. Of these some extend outwards, 

 and one or more backwards along the superior vermiform process, to reach the 

 circumference of the cerebellum, where they anastomose with the branches of the 

 inferior cerebellar arteries : while others run urwards to supply the vemiiform 

 process and the valve of Vieussens, and in part the velum interpositum. 



The posterior cerebral artery on each side, resuUing from the 

 division of the basilar, passes outwards, parallel to the superior cere- 

 bellar artery, and separated from it at its origin by the third nerve, 

 ■which comes forwards between the two vessels. It turns backwards 

 round the crus cerebri, and then runs outwards and upwards on the 

 under surface of the posterior lobe of the cerebrum, passing near the 

 posterior extremity of the corpus callosum. It divides beneath the 

 posterior lobe into many branches, which ramify upon the under, 

 median, and outer surfaces, and anastomose with the other cerebral 

 arteries. 



Branches. — Immediately after its origin the posterior cerebral artery gives off 

 numerous small parallel branches, which perforate the substance of the brain 

 between the crura, at the point which is called from this circumstance the pos- 

 terior perforated spot. As it turns backwards, a short distance from its origin, 

 this artery is joined by the imderior communicating artery, and in this way con- 

 tributes as already described (p. 363) to form the circle of Willis. Lastly, the 

 posterior cerebral gives origin to a small branch, the ^;mft'?'(V?' choroid, which, 

 arising external to the junction of the communicating artery, turns backwards 

 over the crus cerebri and the tubercula quadiigemina, supplying these with 

 branches, and ending in the velimi interpositum and choroid plexus, in the 

 interior of the brain. 



Varieties. — The basUar artery is originally formed by the coalescence of two 

 separate veii;ebral arteries, and traces of a septum are sometimes found hi its 

 interior. (Davy, " Researches," &c., vol. i. p. 301.) The occurrence of a perfora- 

 tion of the vessel, which has also been observed, may be likewise attributed to the 

 incomplete fusion of the two arteries. 



The posterior cerebral artery is occasionally given off on one side from the 

 internal carotid artery. 



2. — Thyroid axis. — The thyroid axis springs from the fore part of 

 the subclavian artery, close to the inner side of the anterior scalenus 

 muscle. It is a short thick trunk, and receives the name of '• axis," 

 because, at a line or two from its origin, it divides into branches, which 

 diverge in different directions, viz., the inferior or ascending thyroid, 

 the suprascapular, and a third branch, which is either the transverse 



