74 NERVOUS SYSTEM. 



15. The spinal marrow, (Fig. 25, me.) arises from the inferior 

 part of the brain and cerebellum. It has the form of a thick, 

 whitish cord, and descends from the interior of the cranium to 

 the lowest part of the canal which pierces the vertebral column. 



16. We give the name of medulla oblonyata, to the superior 

 portion of the spinal marrow which te enclosed in the cranium. 



17. The eticephalon (which includes the brain and spinal mar- 

 row, also called, the cerekro-spiual axis,) is surrounded by different 

 membranes, which serve to prevent it from wounding itself against 

 the sides of the bony case which encloses it. One of these mem- 

 branes, called the artichnoid, is extremely fine; another called the 

 dura mufet, is, on the contrary, very strong, and in the interior of 

 the cranium forms plaits or folds which descend between the 

 hemispheres of the cerebrum, and between this organ and the 

 cerebellum, to sustain these parts, and prevent them from press- 

 ing one upon the other. 



18. A great number of soft whitish cords go from the brain 

 and spinal marrow to all parts of the body ; they are designated 

 by the name of nerves, (Fiy. 25.) 



19. These nerves arise, some from the base of the brain, 

 others from the sides of the spinal marrow. In man there are 

 forty three pairs, of which, the first thirteen arise from the brain 

 and medulla oblongata, and pass out of the cranium through holes 

 in its base: and the remaining thirty pairs arise from the spinal 

 marrow, and go out of the vertebral canal by holes, situate on 

 each side of the spine. 



20. The nerves are divided into branches and ramuscies, which 

 are spread out in the different organs, and in them become so 

 extremely fine as to escape our vision. They possess extreme 

 sensibility, and the slightest wound of one of them causes acute 

 pain. 



21. The nerves give to different parts of the body to which 

 they are distributed, the sensibility which these parts enjoy. 

 They convey the impressions received by the organs to the brain, 

 which is the seat of the perception of sensations. 



15. What is the spinal marrow? 



16. What is the medulla oblongata ? 



1 7 What are the coverings of tne encephalon ? Wh it is the dura mater ? 

 IS. What name is given to those cords which go from the brain and 

 pinil marrow to all parts of the b idy ? 



IP. What is the origin of the nerves ? How many pairs of nerves are 

 found in mm/ Wh.tis the origin o r the first thirteen pairs of nerves in man? 



20. How arc the nerves divided ? To what part are they distributed > 

 Are they s< nsibip ? .. 



21. What office i performed by the nerves ? 



