390 



SPINAL CORD. 



slender filaments* of areolar tissue, and by a longitudinal lamella which is 

 attached to the posterior aspect of the cord. The areolar tissue is most 

 abundant in the cervical region, and diminishes in quantity from above 

 downwards ; and the longitudinal lamella is complete only in the dorsal 

 region. The arachnoid passes off' from the cord on either side with the 

 spinal nerves, to which it forms a sheath ; and is then reflected on the 

 dura rnater, to constitute its serous surface. A connexion exists in several 

 situations between the arachnoid of the cord and that of the dura mater. 

 The space between the arachnoid and the spinal cord is identical with 

 that already described as existing between the same parts in the brain, 

 the sub-arachnoidean space. It is occupied by a serous fluid, sufficient in 

 quantity to expand the arachnoid, and fill completely the cavity of the 

 theca vertebralis. The sub-arachnoidean fluid keeps up a constant and 

 gentle pressure on the entire surface of the brain and spinal cord, and 

 yields with the greatest facility to the various movements of the cord, 

 giving to those delicate structures the advantage of the principles so use- 

 fully occupied by Dr. Arnott in the hydrostatic bed. 



The Pia mater is the immediate investment of the cord ; and, like the 

 other membranes, is continuous with that of the brain. It is not, however, 

 like the pia mater cerebri, a vascular membrane ; but is dense and fibrous 

 in structure, and contains but few vessels. It invests the cord closely, 

 and sends a duplicature into the fissura longitudinalis anterior, and an- 

 other, extremely delicate, into the fissura longitudinalis posterior. It forms 

 a sheath for each of the filaments of the nerves, and for the nerves them- 

 selves ; and, inferiorly, at the conical termination of the cord, is prolonged 

 downwards as a slender ligament (filum terminate), which descends 

 through the centre of the cauda equina, and is attached to the dura mater 

 lining the canal of the coccyx. This attachment is a rudiment of the 

 original extension of the spinal cord into the canal of the sacrum and 

 coccyx. The pia mater has, distributed to it, a number of nervous plexuses 

 derived from tlie sympathetic. 



The Membrana dentata (ligamentum dentatum) is a thin process of pia 

 mater sent off from each side of the cord throughout its entire length, and 

 separating the anterior from the posterior roots of the spinal nerves. The 

 number of serrations on each side is about twenty, the first being situated 

 on a level with the occipital foramen, and having the vertebral artery and 

 hypoglossal nerve passing in front and the spinal accessory nerve behind 

 it, and the last opposite the first or second lumbar vertebra. Below this 

 point, the membrana dentata is lost in the filum terminale of the pia mater. 

 The use of this membrane is to maintain the position of the spinal cord in 

 the midst of the fluid by which it is surrounded. 



The Spinal cord of the adult extends from the pons Varolii to opposite 

 the first or second lumbar vertebra, where it terminates in a rounded 

 point; in the child, at birth, it reaches to the middle of the third lumbar 

 vertebra, and in the embry is prolonged as far as the coccyx. It pre- 

 sents a difference of diameter in different parts of its extent, and exhibits 

 tb-^ee enlargements. The uppermost of these is the medulla oblongata ; 

 the next corresponds with the origin of the nerves destined to the upper 

 extremities ; and the lower enlargement is situated near its termination, 



t 



* According to Mr. Rainey, these filaments are nervous fasciculi, having their origin 

 in the arachnoid, and passing to the arteries of the cord. See page 371. 



