THE VERTEBRAL ARTERY 623 



a descending branch, which unite with similar branches from the artery above 

 and below, so that two lateral anastomotic chains are formed on the posterior 

 surface of the vertebrae near the attachment of the pedicles. From these anasto- 

 motic chains branches are given off to supply the periosteum and the bodies of the 

 vertebrae, and to communicate with similar branches from the opposite side; from 

 these communicating branches small branches are given off which join similar 

 branches above and below, so that a central anastomotic chain is formed on the 

 posterior surface of the bodies of the vertebrae. 



Muscular branches are given off to the deep muscles of the neck, where the ver- 

 tebral artery curves around the articular process of the atlas. They anastomose 

 with the occipital and with the ascending and deep cervical arteries. 



The posterior meningeal (ramus meningeus) is a small branch given off from 

 the vertebral opposite the foramen magnum. It ramifies between the bone and 

 dura in the cerebellar fossae, and supplies the falx cerebelli. It is frequently repre- 

 sented by two small branches. 



The anterior or ventral spinal (a. spiualis ventralis, a. spinalis anterior) is a 

 small branch which arises near the termination of the vertebral, and, descending 

 in front of the medulla oblongata, unites with its fellow on the opposite side at about 

 the level of the foramen magnum. One of these vessels is usually larger than the 

 other, but occasionally they are about equal in size. The single trunk thus formed 

 descends on the front of the spinal cord, and is reinforced by a succession of small 

 branches which enter the vertebral canal through the intervertebral foramina; 

 these branches are derived from the vertebral artery and the ascending cervical 

 branch of the inferior thyroid artery in the neck; from the intercostal in the 

 thoracic region; and from the lumbar, iliolumbar, and lateral sacral arteries in 

 the lower part of the vertebral column. They unite, by means of ascending and 

 descending branches, to form a single terminal artery, which extends as far as 

 the lower part of the spinal cord. This vessel is placed in the pia along the 

 anterior median fissure; it supplies that membrane and the substance of the cord, 

 and sends off branches at its lower part to be distributed to the cauda equina, 

 and ends on the central fibrous prolongation of the cord. 



The posterior or dorsal spinal (a. spinalis dorsalis, a. spinalis posterior) arises from 

 the vertebral at the side of the medulla oblongata ; passing backward to the posterior 

 aspect of the spinal cord, it descends on each side, lying behind the posterior roots 

 of the spinal nerves, and is reinforced by a succession of small branches which 

 enter the vertebral canal through the intervertebral foramina, and by which it is 

 continued to the lower part of the cord and to the cauda equina. Branches from ' 

 the posterior spinal arteries form a free anastomosis around the posterior roots 

 of the spinal nerves, and communicate, by means of very tortuous transverse 

 branches, with the vessel of the opposite side. Close to its origin each gives off 

 an ascending branch, which terminates at the side of the fourth ventricle. 



Applied Anatomy. Bleeding into the spinal membranes or into the substance of the spinal 

 cord itself is not common, but may occur from injuries received at birth when labor is unduly 

 prolonged or instruments are used. It is also met with in chronic insanity, and in tetanus and 

 strychnine poisoning. 



The posterior inferior cerebellar artery (a. postcerebellaris, a. cerebelli inferior 

 posterior) (Fig. 449), the largest branch of the vertebral, winds backward around 

 the upper part of the medulla oblongata, passing between the origin of the vagus 

 and spinal accessory nerves, over the restiform body to the under surface of the 

 cerebellum, where it divides into two branches an internal, which is continued 

 backward to the notch between the two hemispheres of the cerebellum; and 

 an external, which supplies the under surface of the cerebellum as far as its 

 outer border, where it anastomoses with the anterior inferior cerebellar and the 



