REVIEW OF LITERATURE. 11 



Such, then, is the basis for our present understanding of the meninges, in regard 

 to their characteristic morphology and particularly their functional relationship to 

 the cerebro-spinal fluid. Without a consideration of the circumambient fluid morpho- 

 logical studies of these membranes would be incomplete, for in order to understand 

 the meninges knowledge concerning the cerebro-spinal fluid is necessary. 



THE COMPARATIVE ANATOMY OF THE MENINGES. 



Sterzi 153 ) has published a comprehensive report of the comparative anatomy of 

 the spinal meninges. From his studies he has advanced hypotheses, supported by 

 observations on a limited number of fetuses, regarding the development of the human 

 meninges. On account of the interest of this subject in relation to the present dis- 

 cussion a brief summary of Sterzi's work will be here included. 



In the acrania there is no special envelope of the central nervous system, but 

 rather a fibrous sheath corresponding to the meninges of higher forms. This 

 fibrous sheath is largely made up of circular fibers, except in the median ventral line, 

 where there occurs a ventral ligament of longitudinal fibers. In cyclostomes, how- 

 ever, there is found a single "primitive meninx " vascular and composed of white 

 and elastic fibrils coursing in a longitudinal direction. Some of these fibrils traverse 

 the perimeningeal spaces (filled with star-like cells, with some fatty tissue) and are 

 attached to the inner surface of the vertebrae. This same general plan of a single 

 "primitive meninx" is likewise found in fishes (elasmobranchs, teleosts, etc.); the 

 membrane here is often pigmented and follows closely the external architecture of 

 the spinal cord. The perimeningeal space is filled by mucus in elasmobranchs, but 

 in teleosts this is replaced by fat. For the most part there are found dorsal and 

 ventral ligaments and two lateral ligaments. 



The next stage in the development of a more complete form of spinal covering 

 is found in the urodele amphibia. A "primitive meninx," formed of two layers, 

 often artificially separated from each other, replaces the simpler meninx of cyclo- 

 stomes and fishes. Of the two layers in this membrane the external is thin and 

 free from pigment; the inner, strongly pigmented, adheres to the spinal cord. The 

 meninx is perforated by the denticulate ligaments. 



In amphibia (Anura) Sterzi found the first evidence of a "secondary meninx," 

 corresponding to the pia-arachnoid. Surrounding this membrane, but separated 

 from it, is the dura, thin and transparent; between the two meninges is the intra- 

 dural (subdural) space. The dura lies in the peridural space. The spinal pro- 

 longations of the endolymphatic canals he in the dorsal part of the peridural space. 

 Both the dura and the "secondary meninx" continue outward along the roots of 

 the spinal nerves and along the filum terminate. Embryologically the perimedul- 

 lary mesenchyme is differentiated into these two meninges in the Anura. 



This arrangement of the two meninges in Anura is followed out in reptiles. 

 The dura, thin as in the amphibia, is covered by endothelium and is vascular. The 

 "secondary meninx" possesses laterally the denticulated ligaments and ventrally 

 the ventral ligament. Both the peridural and mtradural spaces are very small. 



