THE ENCEPHALO-SPINAL MEMBRANES. 



441 



tected by three membranes named, in their order from without in- 

 ward, the dura mater, the arachnoid, and 

 the pia mater. 



The dura mater is a tough membrane 

 composed of fjj^us tissue. It consists of 

 two layers, the outer of which lines the 

 cranial cavity and forms an internal peri- 

 osteum; the inner layer is closely attached 

 to the outer except at certain regions where 

 it separates and forms supporting structures, 

 such as the falx cerebri, falx cerebelli, 

 tentorium cerebelli, etc.; at the margin of 

 the foramen magnum the outer layer be- 

 comes continuous with the periosteal tissue, 

 while the inner layer invests the cord down 

 to its ultimate termination. (Fig. 202.) 



The arachnoid is a delicate sgrj,^ 

 membrane. The external surface is smooth 

 and well defined and separated from the 

 dura by a narrow space, the subdural space. 

 The inner surface sends inward fine con- 

 nective-tissue processes which interlace in 

 every direction, constituting the subarach- 

 noid tissue. This tissue is abundant in the 

 cranium, much less so in the spinal canal. 

 The spaces between the connective tissue, 

 taken collectively, constitute the general sub- 

 arachnoid space. Around the spinal cord 

 this space is well defined, and at the base of 

 the encephalon expands to form large cavi- 

 ties known as the cisterna magna, cisterna 

 pontis, etc. 



The pia mater is a delicate membrane 

 composed of areolar tissue. It closely 

 invests the encephalon and spinal cord, 

 dipping into the various fissures. It is 

 exceedingly vascular and sends small blood- 

 vessels for some distance into the brain and 

 spinal cord. 



The Encephalo-spinal Fluid. The 

 general subarachnoid space, as well as cer- 

 tain cavities within the encephalon, contain 

 a clear transparent fluid, termed the en- 



cephalo-spinal. This fluid has an alkaline reaction and a specific 

 gravity of 1.007 or 1-008. It is composed of water, proteids (pro- 



FIG. 201. THE CENTRAL 

 ORGANS OF THE NERVE 

 SYSTEM, r. T. o. Fron- 

 tal, temporal, and oc- 

 cipital lobes of the 

 cerebrum, c. Cerebel- 

 lum. P. Pons. mo. 

 Medulla oblongata. 

 ins., ms. The upper 

 and lower limits of the 

 spinal cord. The re- 

 maining letters indicate 

 the region and number 

 of the spinal nerves. 

 (Qitain, after Bourgery.) 



