Of the pia muter. ( Tunica intima vel vasca pia mater is the most internal mem brane of those which have been eniimerated as belonging to the spinal cord and brain.
Pia muter of the spinal cord—This mem brane stands in precisely the same relation to the spinal cord as the neurilemma does to the nerves ; and as long as the spinal cord could be, as it formerly was, reg,arded merely as a bundle of nervous fibres, the analogy of this membrane to the nervous sheath would be perfect. Et is composed almost entirely of white fibrous tissue ; it closely invests the cord and supports the minute bloodvessels which carry the nutrient fluid to it. Not only does it thus form a complete sheath to the cord, but it likewise sends in processes which dip into the anterior and posterior median fissures of that organ. That which passes into the anterior median fissure is a true fold or duplicature of the pia mater ; but the posterior fissure, which is much narrower than the ante rior, is occupied only by a single and extremely delicate layer, which at some parts almost entirely disappears, and seems to consist merely of a few minute capillary vessels. The pia mater becomes continuous with the neurilemma of the roots of the nerves on each side of the cord, and at its inferior extremity it tapers in accor dance with the shape of the spinal cord, and is prolonoed as a delicate thread vvhich is inserted into the extremity of the dura mater. This prolongation is quite gradual, so that at the upper part it encloses a portion of the medullary substance of the cord ; in the greater part of its extent, however, it is merely a membranous thread, and, therefore, goes by the name ft/. form prolongation of the pia mater (filion terminak). The late Dr. Macartney used to regard it as highly elastic, but my friend Mr. Bowman has called my attention to the fact that it consists almost entirely of white fibrous tissue, which cannot confer elasticity. And if a portion removed from the cord be stretched, it will be found to possess very little elasticity ; but if the cord be held up by the filiform prolongation, and a slight jerking move ment be communicated to it, it may be made to dance about as if by the elastic reaction of the tilliform process. The movement which may
be thus produced is very well calculated to deceive, and Dr. Macartney must have founded his opinion upon that experiment alone, omitting to try the effect of stretching a detached portion of the process. The fact is that when the cord is suspended in this way, the pia mater becomes stretched, and its anterior and posterior por tions are approximated and the cord flattened ; when it is raised with a jerk, this tension of the pia mater is diminished, and the cord re turns to its previous form until it falls again, stretches the pia mater, and becomes once more flattened, producing a degree of reaction which favours its elevation, but which alone would be insufficient for that purpose. Thus it appears that the elastic reaction, which Dr. Alacartney attributed to the filiform process, is in reality due to the compression and conse quent flattening of the cord by the tension of the pia mater. It should be stated, further, that this process is not formed of pia mater alone, but also of a continuation of the liga mentunt denticulatum on each side to be described by-and-bye.
The pia mater is abundantly supplied by bloodvessels, many of which are extremely tortuous. These vessels are derived from the anterior and posterior spinal arteries. Along the anterior surface of the spinal cord in front of the anterior median fissure there is a narrow band of fibrous tissue which is stretched across this fissure like a bridge, and occupies its whole length. No such armngement exists on the posterior surface.
The pia mater of the spinal cord possesses considerable strength and density. The ner vous matter may by pressure be squeezed out of it, leaving a hollow cylindrical membrane, or it may be dissolved out by the action of liquor potass. In the quite recent state, while the cord is as yet firm, the pia mater may be readily dissected off, its adhesion to tbe cord being through the medium of nu merous exceedingly minute capillary vessels. On its exterior the pia mater adheres to the visceral layer of the amchnoid membrane by means of a loose fibrous tissue.