THE CEBEBEO-SPINAL AXIS. CCo 



brain, is alkaline, and contains but a small quantity of albumen ; it varies 

 in quantity according to tbe 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 hydro- 

 static agent is greatly enhanced.) 



3. The Pia Mater. 



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

 membrane whose framework, essentially connective, sustains on its external 

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



Applied immediately to the surface of the encephalon 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 inter- 

 mediate sulcus two layers that lie against each other. 



The external face of the pia mater, bathed in part of its extenLby the 

 subarachnoid fluid, adheres to the visceral layer of the arachnoid bymeans 

 of _ a more or less dense and close filamentous connective tissue. From it 

 arise the cellular coverings that constitute the neurilemma 'of the nerves. 

 It detaches a multitude of filamentous or 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 simulate the adhesions established between the two layers of 

 that membrane. 



The internal face is united to the nervous substance by multitudes of 

 arterial and venous radicles or connective filaments, which leave the pia 

 mater to plunge into this substance. 



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

 detached branches that reach the medulla and encephalon. They are ac- 

 companied by nervous filaments, and surrounded hy perivascular canals, which 

 are now believed to be lymphatics. Certainly, in their interior a colour- 

 less fluid circulates, and which contains globules very like those of lymph. 



Spinal Pia Matek. — 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 lamineB 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 consti- 

 tute, on each side of the organ, a festooned band named the dentated ligam,ent 

 (ligamentum dentata, or denticulatum). These ligaments exist throughout the 

 entire length of the medullary axis, 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 prolongation (filum terminale) : 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. 



