520 ELEMENTARY PHYSIOLOGY LESa 



of the floor of the fourth ventricle produces for a while 

 an increase of the quantity of sugar in the blood, beyond 

 that which can be utilised by the organism. The sugar 

 passes off by the kidne3's, and thus this slight injury to 

 the medulla produces a temporary disorder closely re- 

 sembling the disease called diabetes. Hence we speak of 

 a diabetic centre in the bulb. Beyond this the bulb acts 

 as a great conductor of impulses ; for all impulses passing 

 up and down between the higher parts of the brain and 

 the spinal cord must make their way through the bulb 

 from or to the spinal nerves. And a similar statement 

 holds good for impulses along the cranial nerves, with the 

 exception of the olfactory, optic and third and fourth 

 nerves. 



The impulses which pass through the bulb cross, for 

 the most part, from one side to the other on their way 

 along it. In the case of the crussed pyramidnl tract the 

 crossing of the fibres which compose the tract takes place 

 by means of what is called the decussation of the 

 pyramids in the anterior columns of the bulb (Fig. 172). 

 This point is indicated in Fig. 160 by a group of small 

 converging marks on the surface of the bulb just above 

 the cut end marked M. The fibres of the small direct 

 pyramidal tract cross below the bulb in the spinal cord 

 by means of its anterior white commissure (see Fig. 143). 

 Similarly the fibres concerned in the transmission of 

 afferent impulses largely cross in the bulb by paths 

 which are varied but of which one is well marked as 

 the sensory decussation. This general decussation 

 of efferent and afferent fibres leads to the result that 

 disease or injury of one side of the brain affects the 

 opposite side of the body. Thus when, as not un- 

 frequently happens, a blood-vessel gives way in the right 

 cerebral hemisphere, leading to a destruction of nervous 

 matter there, tlie result is that the left arm, and left leg, 

 and left side of the body generally are paralysed, tliat is, 

 the will has no longer any power to move the muscles of 



