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CHAPTER X. 



ARRANGEMENT OF THE NERVOUS SYSTEM IN VERTEBRATA. IN INVER- 



TEBRATA. NERVOUS CENTRES IN MAN. THEIR COVERINGS OR 



MENINGES. THEIR VENOUS SINUSES. THE SPINAL CORD. THE 



ENCEPHALON. THE CIRCULATION WITHIN THE CRANIUM. 



THE leading subdivision of the animal kingdom into Vertebrate 

 and Invertebrate animals is so obviously sanctioned by the dis- 

 position of the nervous system peculiar to each, that no natu- 

 ralist hesitates to adopt it. In the vertebrate animals, an osse- 

 ous or cartilaginous column, composed of separate pieces united 

 by amphiarthrosis, forms the principal support and bond of con- 

 nexion for the other parts of the trunk. This column encloses a 

 canal, within which is placed that portion of the nervous centres 

 called the spinal cord, or marrow, with some of its nerves. At its 

 anterior or upper extremity, the component pieces of the column are 

 so modified as to form a dilated cavity, the cranium, in which 

 another portion of the central nervous system, the brain, or ence- 

 phalon, with part of the nerves connected with it, is contained. 

 In the invertebrate animals generally there is no internal skeleton, 

 if we except the slight traces which exist in the cephalopodous 

 mollusks ; but in many of them a modification of the external 

 integument affords the requisite amount of protection and support 

 to the soft tissues and organs. The nervous system, the central 

 part of which is disposed either in detached masses, or in a series 

 along the abdominal surface of the animal, receives no special 

 protection from this external skeleton. 



The brain and spinal cord, in the vertebrate classes, form a cen- 

 tral axis with which all other parts of the nervous system are 

 connected. The former is evidently an aggregate of gangliform 

 swellings, each possessing the characters of a nervous centre, but so 

 connected with the others, that their functions are in no small 

 degree mutually dependent. The latter has, throughout its entire 

 length, all the characters of one uniform nervous centre, of cylindrical 

 shape; but experiment has shewn that, if divided into segments, 

 in animals tenacious of vitality, each portion may exert an inde- 

 pendent influence on that segment of the body whose nerves are 

 connected with it. From this fact we may properly regard the cord 



