THE BASILAR ARTERY. 423 



together across the bodies of the vertebrae by transverse branches. Other small 

 twigs pass backwards from the spinal branch, and anastomosing with their fellows 

 form an irregular network on the anterior surface of the arches of the vertebra and 

 the ligamenta subflava. From these interlacements numerous minute ramifications 

 proceed to the ligaments, the periosteum and the bones. 



The basilar artery (iii), the single trunk formed by the junction of the right 

 and left vertebral arteries, extends from the lower to the upper border of the pons 

 Yarolii, along the median groove of which it lies under cover of the arachnoid. The 

 length of this artery is therefore about equal to the depth of the pons, at the upper 

 border of which it ends by dividing into the two posterior cerebral arteries. 



Branches. Besides numerous small branches to the substance of the pons, the 

 basilar artery gives off the following : 



(a) The transverse arteries, several on each side, pass directly outwards. One, 

 the auditory artery, accompanies the nerve of the same name into the internal 

 auditory ineatus and supplies the labyrinth of the ear. 



(b) The anterior (inferior) cerebellar arteries (vi) pass backwards, one on each 

 side, to the anterior part of the under surface of the cerebellum, anastomosing with 

 the inferior cerebellar branches of the vertebral arteries. 



(c) The superior cerel)ellar arteries (v) arise so close to the bifurcation of the 

 basilar, that this artery is often described as dividing into four branches. Each one 

 passes outwards immediately below the third nerve, and, entering the groove between 

 the pons Yarolii and the eras cerebri, turns backwards round the latter, close to the 

 fourth 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 artery ; while others run 

 inwards to supply the vermiform process and the valve of Vieussens, and in part the 

 velum interpositum. 



The posterior cerebral artery (iv) passes outwards, on each side, parallel 

 to the superior cerebellar artery, and separated therefrom at its origin by the third 

 nerve, which comes forwards between the two vessels. It turns backwards round 

 the crus cerebri, giving outwards its anterior and posterior temporal branches to the 

 temporal lobe of the cerebral hemisphere, and then crosses the uncinate gyrus, 

 passing below the splenium of the corpus callosum, to enter the calcarine fissure 

 (fig. 347). It furnishes the postero-mesial and poster o-lateral groups of central 

 arteries, as well as one or two posterior choroid arteries, and its terminal branches 

 supply the occipital lobe of the cerebrum (see p. 415). 



Varieties. The basilar artery is originally formed by the coalescence of two separate 

 vertebral arteries, and traces of a septum are sometimes found in its interior. The occurrence 

 of a perforation of the vessel, which is occasionally seen, may be likewise attributed to the 

 incomplete fusion of the two arteries. 



The posterior cerebral artery is sometimes given off on one side from the internal carotid, 

 by an enlargement of the posterior communicating branch (p. 412). 



2. Thyroid axis (ii). 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, two or three lines from 

 its origin, it divides into branches which diverge in different directions : these are 

 the inferior or ascending thyroid, the suprascapular, and a third branch, which is 

 either the transverse cervical, or one of the branches into which that artery, when 

 present, divides, viz., the superficial cervical. 



Varieties. The thyroid axis has been seen arising beyond the scalenus anticus 

 muscle. It may be associated at its origin with another branch ; most frequently 

 with the internal mammary ; rarely with the vertebral, superior intercostal, or deep 



