CHAPTER II 

 BRAIN PATTERN— Continued 



Having grasped a general idea of the layout of the brain, it is now 

 necessary to consider its intimate structure, wliich is revealed by 

 stud\'ing its tissue by microscopic methods. These methods are 

 the same as those used in human anatomy, and involve an elaborate 

 process which requires the use of a great variety of stains, which 

 differentiate special cells and nerve fibres or neurons. 



In this way certain cells can be distinguished, and we are entitled 

 to assume that a motor nerve cell recognised as such in human 

 histology, is a motor cell in a fish, and that a nerve cell of another 

 definite type in man has the same function in fish. The lobes are 

 thus found to consist of accumulations of nerve ceUs of different 

 types, and these are so arranged that some receive impulses or 

 messages from the sense organs of the body, both from the external 

 parts and from the viscera or internal parts, wliile others send out 

 impulses, wliich set the motor machinery in action. These messages 

 are carried to and from the brain by the nerve roots and neurons 

 as we have already mentioned. But, further, these groups of cells 

 must of necessity have relations with the other groups, so that 

 strands of nerve fibres, called tracts, carry messages from one 

 centre to another ; finally it is necessary that there should be a 

 centre to correlate all these various messages, so that there are 

 other tracts to carry messages to a higher centre which controls 

 the whole nervous system. We have described the brain of fish 

 as being an enlargement of a simple nervous tube which is known 

 as the spinal cord ; this also has tracts of fibres leading up to and 

 from the brain. Aromid the central canal there is an accumulation 

 of cells for the reception of sensory impulses and for the sending out 

 of motor impulses. In all vertebrate animals the nerves given 

 off from the sjnnal cord have two roots, and two columns of nerve 

 fibres pass up the cord to the brain. 



It Mas established by Bell just a hundred years ago that the 

 anterior (in fish ventral or inferior) column of the spinal marrow 

 and the anterior roots of the spinal nerves were for motion, and that 

 the posterior (in fish dorsal) column and posterior roots were for 



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