PIA MATER. 389 



takes its name from its extreme tenuity and transparency. 

 It lines the dura mater, and is reflected from it upon the 

 nerves to the spinal cord, which it loosely covers. Hence 

 this membrane is much larger than the spinal cord, and 

 allows of a considerable space between the two, called the 

 sub-araclmoid space. This space is occupied by a serous 

 fluid, which dilates the arachnoid, and is said completely to 

 fill the cavity of the theca-vertebralis. This fluid is of use 

 in keeping up a gentle pressure and giving support to the 

 cord, and in filling up all the inequalities on its surface; 

 allowing the greatest freedom in all its movements, and 

 guarding it against concussions. The portion of the arach- 

 noid covering the dura mater is called parietal, that upon 

 the cord is the visceral, and the space between the two con- 

 stitutes the arachnoid cavity, not found to contain much 

 fluid. It forms a sheath for the nerves and is reflected back 

 upon the theca, forming at each nerve a small cul de sac. 

 Between the anterior and posterior roots of the nerves 

 little reflections or tooth-like processes of this membrane, 

 about twenty-two in number, are seen to extend from the 

 pia mater, laterally along the spinal cord, and in a regular 

 series, to be connected by their little points, to the inner 

 surface of the dura mater. These processes are called the 

 ligamenta denticulata; each encloses a fibrous band or thread, 

 and the whole are designed to separate the anterior and 

 posterior roots of the nerves, and to give support to the 

 cord in the lateral direction. 



The structure of the arachnoid is serous, and presents 

 the usual smooth, glistening, delicate and transparent ap- 

 pearance of serous membranes every where. 



The Pia Mater forms the third and innermost covering 

 of the cord. It is considered the same and continuous with 

 that of the brain. But there is this important difference, 

 that while the pia mater of the brain is essentially vascu- 

 lar, the pia mater of the spinal marrow is essentially 

 fibrous, that of the brain being loaded with vessels, while 

 that of the cord has very few. It is dense and strong, and 

 compresses the cord to such a degree that, when opened, the 



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