868 MEMBRANES OF THE ENCEPHALON. 



These are invested and protected by the membranes of the brain, and the 

 \vhole together constitute the encephalon (v xscpaX^, within the head). 



MEMBRANES OF THE ENCEPHALON. 



Dissection. To examine the encephalon with its membranes, the upper 

 part of the skull must be removed, by sawing through the external table 

 and breaking the internal table w r ith the chisel and hammer. After the 

 calvarium has been loosened all round, it will require a considerable de- 

 gree of force to tear the bone away from the dura mater. This adhesion 

 is particularly firm at the sutures, where the dura mater is continuous with 

 a membranous layer interposed between the edges of the bones ; in other 

 situations, the connexion results from numerous vessels \vhich permeate 

 the inner table of the skull. The adhesion subsisting between the dura 

 mater and bone is greater in the young subject and in old persons than in 

 the adult. On being torn away, the internal table will present numerous 

 deeply grooved and ramified channels, corresponding with the branches 

 of the arteria meningea media. Along the middle line will be seen a groove 

 corresponding with the superior longitudinal sinus, and on either side may 

 be frequently observed some small fossae, corresponding with the Pacchi- 

 onian bodies. 



The membranes of the encephalon are the dura mater , arachnoid mem- 

 brane , and pia mater. 



The DURA MATER* is the firm, whitish or greyish layer which is brought 

 into view when the calvarium is removed. It is a strong fibrous membrane, 

 somewhat laminated in texture, and composed of white fibrous tissue. 

 Lining the interior of the cranium, it serves as the internal periosteum of 

 that cavity ; it is prolonged also into the spinal column, under the name 

 of theca vertebralis, but is not adherent to the bones in that canal as in the 

 cranium. From the internal surface of the dura mater, processes are di- 

 rected inwards for the support and protection of parts of the brain ; while 

 from its exterior, other processes are prolonged outwards to form sheaths 

 for the nerves as they quit the skull and spinal column. Its external sur- 

 face is rough and fibrous, and corresponds with the internal table of the 

 skull. The internal surface is smooth, and lined by the thin varnish-like 

 lamella of the arachnoid membrane. The latter is a serous membrane. 

 Hence the dura mater becomes a fibro-serous membrane, being composed 

 of its own proper fibrous structure, and the serous layer derived from the 

 arachnoid. There are two other instances of fibro-serous membrane in the 

 body, formed in the same W 7 ay, namely, the pericardium and tunica albu- 

 ginea of the testicle. 



On the external surface of the dura mater the branches of the middle 

 meningeal artery may be seen ramifying; and in the middle line is a de- 

 pressed groove, formed by the subsidence of the upper wall of the superior 

 longitudinal sinus. If the sinus be opened along its course, it will be 

 found to be a triangular channel, crossed at its lower angle by numerous 

 white bands, called chordae Willisii ;f granular bodies are also occasion- 

 ally seen in its interior, these are glandule Pacchioni. 



* So named from a supposition that it was the source of all the fibrous membranes of 

 the body. 



\ Willis lived in the seventeenth century ; he was a great defender of the opinions 

 of Harvey. 



