MEMBRANES OF THE SPINAL MARROW. 335 



face, being spread over it, and thus giving it the glistening ap- 

 pearance. The processes enclosing the fasciculi of the spinal 

 nerves, are particularly conspicuous about the Cauda Equina. 



Of the Pia Mater of the Medulla Spinalis^ 



This third envelope of the spinal marrow forms also a com- 

 plete investment of the latter, and adheres very closely to it. 

 Its external face is smooth, and is in contact with the arach- 

 noidea, from which it may be readily separated by inflating 

 the latter. But from the middle of its internal face both ante- 

 riorly and posteriorly, a process or partition penetrates into 

 the middle fissures of the medulla spinalis, and reaches to their 

 bottoms. From these partitions there proceeds a great num- 

 ber of small vascular canals, that pass in various directions 

 through the medulla, and anastomose freely with each other. 

 This arrangement is rendered sufficiently obvious by injecting 

 and then destroying the medulla in an alkaline solution; or if 

 the medulla be hardened by neutral salts or acids, it splits into 

 many longitudinal radiated laminae, divisible into chords, where- 

 by the arrangement is made equally manifest. A fact of some 

 consequence is thus established, to wit, the similitude between 

 the structure of a nerve and of the medulla spinalis. 



At the inferior end of the medulla the pia mater becomes a 

 single chord, which is continued among the cluster of nerves 

 constituting the Cauda Equina to the lower end of the tube 

 formed by the dura mater, and there it joins with the latter. 

 As a membrane, the pia mater is much more complete than 

 the corresponding one of the brain, has more strength, but is not 

 so vascular. Its thickness increases in its descent. It is of a 

 yellowish-white colour. It seems to hold the medulla some- 

 what in a state of compression, for when a puncture is made 

 through it, the medullary substance protrudes like a hernia. 

 It goes from the medulla to the fasciculi of nerves and forms 

 their neurileme or sheath. 



The pia mater seems to impart great elasticity to the spinal 

 marrow, for when the latter is detached from the spine by the 

 severance of its nerves, it contracts suddenly and forcibly to 

 the amount of from one to two inches. 



