366 PHYSIOLOGY 



THE HIND-BRAIN 



It will be convenient to trace first the modifications undergone by the 

 axial part of the nervous system in the brain, and then to deal with the 

 new masses of grey matter which have no homologies in the spinal cord, as 



n. c 



, *v ~r/<<Wu^ / 



C.a. 



FIG. 181. Section through the lower border of the medulla oblongata, at the 



pyramidal decussation. (BECHTEREW.) 



fla, anterior fissure ; d, decussation of the pyramids ; V, anterior columns ; 

 Ca, anterior cornu ; cc, central canal ; S, lateral columns ; //, formatio reti- 

 cularis ; ce, neck, and g, head of the posterior cornu ; rpCl, posterior root of 

 first cervical nerve ; nc, beginning of nucleus cuneatus ; ng, nucleus gracilis ; 

 H 1 , funiculus gracilis ; H 2 , funiculus cuneatus ; sip, posterior fissure. 



well as the long tracts of white matter serving to connect different levels 

 or different sides of the brain. 



In examining successive sections from the spinal cord up through the 

 medulla, the first change which makes its appearance is due to the decussa- 

 tion of the pyramids (Fig. 181). Throughout the spinal cord, fibres have 

 been crossing from one side to the other through the anterior white com- 

 missure, many of them belonging to the pyramidal system. But at the lower 

 border of the medulla we see a large mass of fibres crossing between the 

 anterior columns and the postero -lateral columns, at first cutting off the head 

 of the anterior horn and later on breaking this up altogether, so that the 

 only definite collection of grey matter left in this situation is a small part 

 of the lateral column of grey matter known as the lateral nucleus. In this 

 way are formed the big anterior columns of the medulla, which are known 

 as the pyramids, and contain all the fibres that in the cord are represented by 

 the direct and crossed pyramidal tracts. 



The next change is due to the ending of the posterior columns (Fig. 182). 

 These are the central ascending branches of dorsal nerve roots, having there- 

 fore an origin outside the cord. On their way up the cord they send in 

 collaterals to end in the grey matter of the posterior horn. The main mass 

 terminates in the medulla, just above the pyramidal decussation, in two 

 collections of grey matter the nucleus gracilis and the nucleus cuneatus 



