NERVOUS SYSTEM. 



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tached to the dura, the ligamentum dentata. Its posterior ex- 

 tremity forms a narrow prolongation, the filum terminate. 



BRAIN. 



This is the central portion of the nervous system, situated in 

 the cranial cavity. 



Membranes. 



The dura mater, externally is a strong fibrous membrane, 

 adherent to the bony walls, giving off prolongations into the brain 

 and forming depressions for the venous sinuses (see Veins'). It 

 also is prolonged on the nerves. The prolongations of the dura 

 are the (A) jalx cerebri, passing between the two hemispheres of 

 the brain and attached to the crista galli. It contains in its meshes 

 small tubercles, the pacchionian bodies. 



(B) Tentorium cerebelli, which forms a horizontal partition be- 

 tween the cerebrum and cerebellum, and attached to the parieto- 

 temporal crest, and parietal protuberance. 



(C) Pituitary fold, circumscribes the sella turcica and envelops 

 the pituitary gland. 



The arachnoid is the middle membrane, composed of a base- 

 ment membrane, attached to the dura externally and a covering 

 of flattened endothelium internally. It is not properly a vascular 

 membrane. It does not pass into the fissures or convolutions, 

 and between it and the pia is a space filled with cerebro-spinal 

 fluid, the sub-arachnoid space. 



Pia mater. 



The most internal of the membranes; is of thin structure, hold- 

 ing in its meshes a large number of blood-vessels which have peri- 

 vascular lymph spaces about them. The pia passes into all the 

 inequalities of the surface of the brain, and supplies it with blood. 



Weight. 



In the horse, 22 ounces, 15 drachms; ox, 16 ounces, 15 drachms. 



In human male, 49^ ounces. 



Parts of the brain. 



Cerebrum, or large brain; cerebellum, or small brain; the. isth- 

 mus and medulla oblongata. 



Medulla Oblongata. 



The medulla is the upper expanded portion of the cord, ending 

 at the pons, and rests on the basilar process. It shows the su- 

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