PI A MATER. NERVES. 1 33 



All the cavities formed by the arachnoid are filled with a 

 serous fluid called the sub-arachnoid or cerebro-spinal fluid. The 

 ventricles of the brain also contain, as has been stated, a 

 quantity of serous fluid. The use of this fluid seems to be 

 to protect the organs against the effect of blows and shocks. 

 The brain and spinal cord being, as it were, suspended in 

 the arachnoid, are held in place in the gentlest possible 

 manner by the sub-arachnoid fluid, and by that in the ven- 

 tricles which moistens the surfaces and prevents all friction. 



Pia mater. This is the name given to the membrane 

 which immediately surrounds the nervous centre. It is a 

 vascular net-work of extreme delicacy and fineness of texture, 

 and may be considered as the nutritive membrane of the 

 cerebro-spinal organs. In its tissue, the arteries running to 

 the brain are ramified with infinite minuteness, and inosculate 

 with the radicles of the veins which spring from it. It 

 follows closely all the cerebral convolutions; penetrates the 

 furrows, and into the ventricles, and covers even the thin 

 layers of the cerebellum. It becomes denser and more 

 fibrous around the spinal cord, of which it forms the neuri- 

 lemma or envelope, as well as at the origin of the nerves. 



To recapitulate, the greater and lesser brain, and the spinal 

 cord, contained in the skull and spinal canal inclosed in 

 three superposed membranes, are united by a common 

 centre, the isthmus of the encephalon. Nerves spring from 

 the brain and spinal cord. 



Nerves. This is the name given to white or grayish threads 

 which are attached by one extremity to the cerebro-spinal 

 nervous centre, and at the other are distributed to the organs. 

 The nerves are composed of very fine filaments, united at 

 the point at which they spring from the nervous centre in 

 bundles called the roots of the nerves; these roots unite and 

 form the trunks which ramify and disappear, as it were, in 

 the tissues of the body. A sheath of cellular tissue called 

 neurilemma or perineurium, envelops the nerves, and pene- 

 trates between the fibres formed by the union of the nerve- 

 tubes spoken of in treating of the tissues. The ramifications 

 of the nerves unite and seem to be confounded at certain 

 points where they form a very complicated net-work, which is 



