THE VERTEBRAL ARTERY. 421 



spinal arteries ; the other branch ramifies on the back part of the bodies of the ver- 

 tebrse in the same manner as similar branches derived from the intercostal and lumbar 

 arteries (see p. 422). 



(b) Muscular branches of variable size are distributed to the deep-seated cervical 

 muscles, and anastomose with the ascending cervical, deep cervical and occipital 

 arteries. 



(c) The posterior meningeal artery is a small branch, sometimes double, which 

 arises just before the artery pierces the dura mater, and ascends through the foramen 

 magnum to ramify in the lower occipital fossa, and in the falx cerebellir 



B. Within the cranium. (a) The posterior spinal artery (vi) arises from the 

 vertebral immediately after it has perforated the dura mater, and inclines downwards 

 to reach the side of the spinal cord ; reinforced by a number of small arteries (5 to 

 12 on one side) as it descends, it may be traced as a small tortuous vessel or anasto- 

 motic chain in front of the posterior roots of the nerves as far as the lower end of the 

 cord, where it terminates by communicating with the lower part of the anterior 

 spinal artery. Offsets from this postero-lateral vessel ramify and anastomose 

 together in the pia mater investing the cord, especially on its hinder surface, where 

 they form more or less regularly a second longitudinal anastomosis behind the pos- 

 terior nerve-roots. 



(5) The (posterior} inferior cerebellar artery (v), the largest of the branches, 

 arises a little above, or often in common with, the foregoing. It turns backwards, 

 passing first between the roots of the hypoglossal nerve, and then between the vagus 

 and spinal accessory nerves, and runs over the side of the restiform body and near 

 the lower lateral margin of the fourth ventricle, to enter the fore part of the vallecula, 

 where it divides into two branches. One of these is continued backwards in the 

 sulcus between the cerebellar hemisphere and the inferior vermiform process, its 

 offsets being distributed mainly to the latter structure, and anastomosing with those 

 of the opposite artery ; while the other inclines outwards and ramifies on the under 

 surface of the hemisphere as far as its outer border, over which the ultimate divisions 

 of both branches anastomose with those of the superior cerebellar artery. Before 

 dividing, this artery gives branches to the medulla oblongata and the choroid plexus 

 of the fourth ventricle. 



(c) The anterior spinal artery (vi) arises near the end of the vertebral, and 

 descends obliquely on the front of the medulla oblongata. One is usually much 

 larger than the other, and forms the commencement of the median anterior artery 

 of the cord, receiving at a variable level a small communicating branch from the 

 opposite vessel : in exceptional cases the median artery results from the union of 

 equal, or nearly equal, branches of the two vertebrals. The vessel thus constituted 

 is continued along the whole length of the cord by means of a series of arteries 

 which pass up with the anterior nerve-roots, and divide each into an ascending and 

 a descending branch ; by the union of adjacent branches a longitudinal vessel is 

 produced, varying in size in different regions, and terminating below as a slender 

 branch on the filum terminale. The number of these reinforcing arteries is commonly 

 from five to ten, and the. last one, which ascends on one of the lowest dorsal or 

 upper lumbar nerves, is constantly larger than the rest. 1 From the anterior spinal 

 artery numerous offsets pass backwards into the anterior median fissure, and 

 outwards on the surface of the cord where they anastomose freely with one another 

 and with the branches of the posterior spinal arteries. 



(d) Bulbar branches. Numerous small branches springing from the vertebral, 

 as well as from the beginning of the inferior cerebellar and anterior spinal arteries, 



1 A. Adamkiewicz, "Die Blutgefasse des menschlichen Riickenmarkes, " Wiener Sitzungsber. , 

 1881-2 ; H. Kadyi, "Die Blutgefasse des menschlichen Riickenraavkes," Lemberg, 1889. 



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