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TEXT-BOOK OF PHYSIOLOGY. 



an internal periosteum; 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 becomes continuous with the 

 periosteal tissue, while the inner layer invests the cord down to its ultimate 

 termination. (Fig. 228.) 



The arachnoid is a delicate serous membrane. 

 The external surface is smooth and well denned 

 and separated from the dura by a narrow space, 

 the subdural space. The inner surface sends in- 

 ward fine connective-tissue processes which inter- 

 lace in every direction, constituting the subarach- 

 noid tissue. This tissue is abundant in the cra- 

 nium, much less so in the spinal canal. The 

 spaces between the connective tissue, taken collec- 

 tively, constitute the general subarachnoid space. 

 Around the spinal cord this space is well denned, 

 and at the base of the encephalon expands to 

 form large cavities known as the cisterna magna, 

 cisterna pontis, etc. 



The pia mater is a delicate membrane com- 

 posed of areolar tissue. It closely invests the en- 

 cephalon 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 certain cavities within 

 the encephalon, contain a clear transparent fluid, 

 termed the encephalo-spinat fluid. This fluid has 

 an alkaline reaction and a specific gravity of 1.007 

 or 1.008. It is composed of water, proteins (pro- 

 teoses and serumglobulin) , and pyrocatechin 

 C 6 H 4 (OH) 2 , capable of reducing copper salts, 

 though not exhibiting any other of the properties 

 of sugar. In many respects this fluid resembles 

 lymph. The subarachnoid space and the general 

 encephalic cavities, termed ventricles, communicate 

 with one another by an opening in the pia mater 

 (the foramen of Magendie) as it passes over the 

 lower part of the fourth ventricle. 



It was stated in Chapter VIII that the entire 

 nerve or neuron system can be resolved into single 

 morphologic units, the neurons: the histologic fea- 

 tures and the physiologic properties of the neuron 

 were there also described; the anatomic relation of the neurons constitut- 

 ing the peripheral organs of the nerve system to the neurons constituting 

 the central organs of the nerve system, were also stated and illustrated in 

 part diagrammatically, page 49. From the statements made regarding the 



FIG. 227. THE CENTRAL 

 ORGANS OF THE NERVE 

 SYSTEM. F. T. o. Frontal, 

 temporal, and occipital 

 lobes of the cerebrum, c. 

 Cerebellum. P. Pons. mo. 

 Medulla oblongata. ms., 

 ms. The upper and lower 

 limits of the spinal cord. 

 The remaining letters in- 

 dicate the region and num- 

 ber of the spinal nerves. 

 (Quain, ajter Bourgery.) 



