The Spine. 489 



Direction of the Spinal Nerve Roots. The roots 

 of the first cervical ascend slightly to reach the interval 

 between the atlas and the occipital bone. The roots of the 

 second and third nerves pass horizontally outwards; that 

 of the fourth passes obliquely downwards and outwards, 

 while the remaining roots proceed, with increased degrees 

 of obliquity, to their respective foramina. 



Landmarks. The spine measures, from the atlas 

 to the tip of the coccyx, on an average, about twenty- 

 eight inches, of which, the cervical portion is five inches ; 

 the dorsal, eleven inches ; the lumbar, seven inches, and the 

 sacrum and coccyx, five inches. Viewed in profile, the 

 spinal column presents four curves, a cervical, convex 

 anteriorly; a thoracic, concave; a lumbar, convex, and 

 a pelvic, concave. The spinal cord, which may be re- 

 garded as consisting of thirty-one segments, placed, one 

 on top of the other, like a pile of checkers, measures only 

 seventeen dr eighteen inches in length, so that, the spinal 

 canal is nine or ten inches longer than the spinal cord. 

 The spinal cord extends from the foramen magnum to 

 about the lower part of the body of the first lumbar verte- 

 bra, the remaining space of the canal being occupied by 

 nerves (the cauda equina), and by the filum terminale, 

 i.e., the prolongation downwards of the pia mater, which 

 contains a very slight amount of nervous tissue. The 

 cervical enlargement of the cord, caused by the large size 

 of the nerves given off to supply the arms, extends from 

 the third cervical vertebra to the second dorsal, and is at 

 its maximum at the level of the sixth cervical vertebra; 

 whereas, the lumbar enlargement commences at the level 

 of the ninth dorsal and reaches its maximum at the level 

 of the twelfth dorsal, from which point it diminishes grad- 

 ually. Each segment of the spinal cord is connected with 

 a pair of nerves, and not only acts as a conductor for the 



