598 NERVOUS SYSTEM 



paths by which these efforts find their way to the muscles are disturbed, 

 and the coordinating process, which is more or less automatic in health, 

 requires peculiar care and attention ; but with the aid of the sense of 

 sight and artificial supports, progression may be safely though irregu- 

 larly accomplished. The movements are jerky, and each step seems to 

 require a distinct act of volition. It is possible to imagine that in 

 disorganization of the paths of coordination in the spinal cord, the 

 coordinating centre may act in some degree through the motor paths in 

 the direct and crossed pyramidal tracts. It is certain that the want of 

 normal coordinating power is supplemented by ordinary voluntary acts 

 and by the sense of sight. 



Vertigo is not a necessary accompaniment of cerebellar ataxia. 

 Disease of the semicircular canals of the internal ear (Meniere's 

 disease) is attended with vertigo, and this is the main cause of the 

 disturbances of equilibrium. 



THE BULB 



The medulla oblongata, or bulb, connects the spinal cord with the 

 encephalic ganglia. It is composed of white and gray matter and pre- 

 sents in its substance a number of important nerve-centres. It is not 

 necessary to give anything like a complete anatomical description of 

 the bulb. Its most important conducting parts are those which are con- 

 tinuous with the columns of the cord and pass to the cerebrum and 

 cerebellum. The nuclei of origin of certain of the cranial nerves in the 

 floor of the fourth ventricle have already been mentioned. 



Physiological Anatomy. The bulb is pyramidal in form, with its 

 broad extremity above, and rests in the basilar groove of the occipital 

 bone, extending from the lower border of the pons Varolii to the atlas. 

 It is about an inch and a quarter (31.8 millimeters) in length, three- 

 quarters of an inch (19.1 millimeters) broad at its widest portion and 

 half an inch (12.7 millimeters) thick. It is flattened antero-posteriorly. 

 Like the cord, it has an anterior and a posterior median fissure. 



Apparently continuous with the anterior columns of the cord, are 

 the two anterior pyramids, one on either side. Viewed superficially, the 

 innermost fibres of these pyramids are seen to decussate in the median 

 line ; but if the fibres are traced from the cord, it is found that they 

 come from the crossed pyramidal tracts of the lateral columns and that 

 none are derived from the anterior columns. The fibres of the external 

 portion of the anterior pyramids come from the direct pyramidal tracts 

 of the cord. At the site of the decussation, the pyramids are composed 

 entirely of white matter; but as the fibres spread out to pass to the 



