THE CEREBROSPINAL AXIS. 753 



according to the relative size of the cerebro-spinal axis and its containing cavity, 

 or with the amount of blood sent to this region. By affording, under all 

 circumstances, an equable pressure on the brain and spinal cord, and the nerves 

 emanating from these, its importance as a hydrostatic agent is greatly enhanced.) 



3. The Pia Mater. 



The pia mater — the proper envelope of the cerebro-spinal axis — is a thin 

 membrane, the framework of which, essentially connective tissue, sustains on its 

 external face a very abundant network of blood-vessels and nerves. 



Applied immediately to the surface of the brain and spinal cord, it adheres 

 firmly to that surface and follows all its inequalities — penetrating between the 

 cerebral or cerebellar convolutions, and forming in each intermediate sulcus two 

 layers that lie against each other. 



The external face of the pia mater, bathed in part of its extent by the sub- 

 arachnoid fluid, adheres to the visceral layer of the arachnoid by means of a 

 more or less dense and close connective tissue. From it arise the cellular 

 coverings that constitute the neurilemma of the nerves. It detaches a multitude 

 of filamentous and lamellar prolongations to the internal face of the dura mater, 

 which traverse the arachnoid cavity in the same manner as the nerves and 

 vessels, by being enveloped, like these, in a sheath furnished by the arachnoid 

 membrane. Always very short, these prolongations resemble the adhesions 

 between the two layers of that membrane. 



The internal face is attached to the nerve substance by multitudes of arterial 

 and venous radicles or connective filaments, which leave the pia mater to plunge 

 into that substance. 



The vessels of the pia mater form a very close network, from which are 

 detached branches that reach the medulla oblongata and brain. They are 

 accompanied by nerve-filaments, and are surrounded hj perivascular canals, which 

 are now believed to be lymphatics. Certainly, in their interior a colourless fluid 

 circulates, which contains lymph-corpuscles. 



Spinal Pia Mater. — Less vascular than the cranial pia mater, with which 

 it is continuous towards the medulla oblongata, this membrane is remarkable for 

 the arrangement of the prolongations that arise from its two faces. 



The internal prolongations form longitudinal layers at the fissures of the cord, 

 and enter these fissures. 



The external prolongations attach, as we have said, the pia mater to the 

 external meninge. A very large number are filamentous in form, and are 

 dispersed over the superior and inferior surfaces of the cord. Others constitute, 

 on each side of the organ, a festooned band named the dentated ligament 

 {ligamentum denticulatum or dentata). These ligaments exist throughout the 

 entire length of the spinal cord, between the superior and inferior nerve- 

 roots : their inner border is confounded for its whole length with the pia mater ; 

 and their outer margin, cut into festoons, attaches itself to the dura mater by 

 the summit of the angles separating these festoons. 



To complete this description of the spinal pia mater, there may be noticed 

 a posterior or coccygeal 2)rolongation {filum termijiale) — a very narrow process 

 formed by this membrane at the posterior extremity of the cord, situated in the 

 midst of the cauda equina nerves, and attached to the bottom of the conical 

 cul-de-sac at the termination of the dura mater. 



(This ligament, or memhrana dentata, serves to maintain the position of the 



