432 SPINAL CORD. 



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

 pressure upon 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 

 usefully applied 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 its structure, and contains few vessels. It invests 

 the cord closely, and sends a duplicature into the fissura longitudinalis 

 anterior, and another, extremely delicate, into the fissura longitudinalis 

 posterior. It forms a sheath for each of the filaments of the nerves, 

 and for the nerves themselves ; and, inferiorly, at the conical termina- 

 tion 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 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 mem- 

 brane 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 oppo- 

 site 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 embryo is prolonged as far as the 

 coccyx. It presents a difference of diameter in different parts of its 

 extent, and exhibits three 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, and corresponds with the attach- 

 ment of the nerves which are intended for the supply of the lower 

 limb. 



In form, the spinal cord is a flattened cylinder, and presents on its 

 anterior surface a fissure, which extends into the cord to the depth of 

 one third of its diameter. This is the fissura longitudinalis anterior. 

 If the sides of the fissure be gently separated, they will be seen to be 

 connected at the bottom by a layer of medullary substance, the 

 anterior commissure. 



On the posterior surface another fissure exists, which is so narrow 

 as to be hardly perceptible without careful examination. This is the 



