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 following 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 the 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 sphincter, 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-glossopharyngeal nerves. 

 Its only skeleto-motor centre is that for the muscles of the tongue 



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