EXTERNAL CHARACTERS OF CORD. 545 



column, they have the following situation : In the first two nerves 

 they lie on the neural arches of the atlas and axis. In the sacral sacral, and 

 nerves they are contained in the spinal canal ; and in the coccygeal coccygeal. 

 nerve the ganglion is usually within the sac of the dura mater. 



VESSELS OP THE SPINAL CORD. The arteries on the surface of Arteries of 

 the cord are anterior and posterior spinal. 



The anterior spinal artery occupies the middle line of the cord Anterior 

 beneath the fibrous band before alluded to in that position. It 

 begins by the union of two small branches of the vertebral artery 

 within the skull, and it is continued to the lower end of the cord 

 by a series of anastomotic branches, which are derived from the 

 vertebral and ascending cervical arteries in the neck, and from the 

 intercostal arteries in the dorsal region. Inferiorly it supplies the termina- 

 roots of the nerves forming the cauda equina, and ends on the 

 central fibrous prolongation of the cord. The branches of this offsets, 

 artery ramify in the pia mater, some entering the median fissure. 



The posterior spinal arteries, one on each side, are continued from Posterior 

 the upper to the lower part of the cord, behind the roots of the 

 nerves. These vessels are furnished from the vertebral artery 

 within the skull, and their continuity is maintained by a series of 

 anastomotic offsets, which enter the canal along the spinal nerves. 

 Dividing into small branches, the vessels of opposite sides form a lie onsides 

 free anastomosis around the posterior roots, and some twigs enter 

 the posterior septa of the cord. 



The veins of the spinal cord are very tortuous and form a plexus Veins: 

 on the surface. At intervals larger trunks arise, which accompany termina- 

 the spinal nerves to the intervertebral foramina, and end in the tion 

 veins outside the spinal canal. Near the top of the cord the veins at top of 

 are united into two or more small branches, which, communicating 

 with the intraspinal veins, join in the skull the inferior cerebellar 

 veins, or the inferior petrosal sinuses. 



The SPINAL CORD (medulla spinalis) is the elongated cylindrical Situation of 

 part of the cerebro-spinal centre, which is enclosed within the spinal 

 canal. Invested by the membranes before examined, the cord 

 occupies about two-thirds of the length of the canal, and is much 

 smaller than the bony case surrounding it. 



The extent of the spinal cord is from the lower margin of the Extent 

 foramen magnum of the occipital bone to the lower border of the below, 

 first lumbar vertebra, but its termination inferiorly may be a little 

 higher or lower than that spot. In the embryo before the third and in the 

 month the cord reaches the whole length of the spinal canal ; em ry ' 

 but it gradually recedes as the surrounding bones enlarge faster 

 than it, until it takes the position it has in the adult. Its length Length, 

 is usually from sixteen to eighteen inches. 



Above, the cord joins the medulla oblongata ; and below it ends inferior 

 in a small tapering part (conus medullaris), from which the filum h^the^duifc 

 terminate (fig. 197, d) is continued downwards. The lower end of 

 the conus medullaris is sometimes marked by one or two irregular 

 swellings. 



The size of the spinal cord is much increased where the nerves Two swell- 

 ings on it. 

 D.A. N N 



