SECTION XII 

 THE FUNCTIONS OF THE BRAIN STEM 



THE brain stem may be taken to include all those parts lying between the 

 cerebral hemispheres and the spinal cord, from the optic thalamus in front 

 to the medulla oblongata behind. The brain may be divided into the follow- 

 ing parts from before back : 



(1) Thalamencephalon, including the corpus striatum, the cerebral 

 hemispheres and rhinencephalon, or olfactory lobes. 



(2) Diencephalon, i.e. the fore-brain, especially the optic thalamus. 



(3) Mesencephalon, or mid-brain, including the quadrigemina, the iter of 

 Sylvius, and the crura cerebri. 



(4) Metencephalon, composed of the pons Varolii, the upper part of the 

 fourth ventricle, and cerebellum. 



(5) Myelencephalon, or bulb, consisting of the medulla oblongata. 



We may get some idea of the part played by these different regions of 

 the brain in determining the reactions of the individual as a whole by 

 examining the behaviour of the animals in whom all the rest of the brain 

 in front of the part in question has been removed. If, however, we take 

 into account the numberless connections existing between the different levels 

 in the central nervous system, the interdependence between the different 

 portions, and the subordination, especially in the higher animals, of the 

 functions of the lower to those of the higher levels, we must acknowledge 

 that such experiments can only give us an imperfect idea of the possibilities 

 of each level when in connection with all other portions of the nervous system. 



THE FUNCTIONS OF THE MEDULLA OBLONGATA 



OR MYELENCEPHALON 



The possibilities of any given nervous centre are determined by the 

 afferent impressions which enter it, and by the connections made by the 

 nerves carrying these impulses with the motor tracts within the centre. The 

 bulb receives afferent impressions of ' taste ' from the tongue through the 

 nervus intermedius, from the alimentary canal as low as the ileocolic sphinc- 

 ter, from the lungs, the heart, and the larger blood-vessels, i.e. from the most 

 important of the viscera of the body, by the fibres of the vago-glossopharyn- 

 geal nerves. Its only skeleto-motor centre is that for the muscles of the 

 tongue (the hypoglossal). It sends also efferent fibres to the viscera, which 

 arise from cells in the nucleus ambiguus. These fibres carry motor impulses 



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