384 MEDULLA SPLNALIS. 



of the nerves, and for the nerves themselves ; and, inferiorly, at the 

 conical termination of the cord, is prolonged downwards as a slender 

 ligament, 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 Menibrana dentata is a process of the 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. Between 

 each of the nerves it forms a serration, which is attached to the 

 dura mater, and unites the two layers of the arachnoid membrane 

 at that point. The processes are about twenty in number at each 

 side. Their use 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 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 oblangata ; the next corresponds with the origin 

 of the nerves destined to the upper extremities ; and the lower en- 

 largement is situated near to its termination, and corresponds with 

 the attachment 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 groove, which extends into the cord to the depth 

 of one third of its diameter. This is the sukus longitudinalis ante- 

 rior. If the sides of the groove 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 sulcus longitudinalis posterior. It extends much more deeply 

 into the cord than the anterior sulcus, and terminates in the gray 

 substance of the interior. These two fissures divide the medulla 

 spinalis into two lateral cords, which are connected to each other 

 merely by the white commissure which forms the bottom of the 

 anterior longitudinal sulcus. 



On either side of the sulcus longitudinalis posterior is a slight line, 

 which bounds on each side the posterior median columns. These 

 columns are most apparent at the upper part of the cord, near to the 

 fourth ventricle, where they are separated by the point of the cala- 

 mus scriptorius, and where they form a bulbous enlargement at 'each 

 side, called the processus clavatus. 



Two other lines are observed on the medulla, the anterior and 

 posterior lateral sulci, corresponding with the attachment of the 

 anterior and posterior roots of the spinal nerves. The anterior 

 lateral sulcus is a mere trace, marked Only by the attachment of the 



