MEDULLA OBLONGATA 157 



myelinated axons immediately decussate and descend in the f . longi- 

 tudinalis medialis tlirough the entire length of the spinal cord. These 

 two cells are collectors of nervous impulses from many sources and 

 are part of the apparatus of control of mass movements of the 

 musculature of the trunk. Their specific functions have not been 

 fully explained, though they have been much studied. They are mag- 

 nified and highly specialized examples of the elements of the nucleus 

 motorius tegmenti of the medulla oblongata. 



We lack sufficient knowledge of the structural and physiological 

 properties of this tegmentum to frame satisfactory hypotheses of the 

 actual mechanism of the integrative and co-ordinating functions 

 which seem to be operating here. The intermediate tegmentum is 

 structurally so intimately interwoven with the tegmentum of the 

 motor zone that both probably act in these operations as a functional 

 unit. 



At the anterior end of the medulla oblongata the trigeminal teg- 

 mentum merges with the isthmic tegmentum, and dorsally of this 

 junction the nucleus cerebelli occupies the position of the intermedi- 

 ate zone. In higher animals the gray of this nucleus is incorporated 

 within the body of the cerebellum as the deep nuclei. The definitive 

 cerebellum, accordingly, is a derivative of both sensory and inter- 

 mediate zones. 



MOTOR ZONE 



Little can be added here to the general description in chapter v. 

 The larger cells of the motor field may spread their dendrites through 

 the whole of the motor and intermediate zones and also upward into 

 the sensory zone and across the ventral commissure to the opposite 

 side ('14a, figs. 29, 30, 31, 37, 41, 42; '446, figs. 7, 8). This type of 

 structure is extended forward into the isthmus, where it is more 

 specialized and the small and large cells are segregated, though not 

 completely so. Most axons of the large cells are myelinated in the 

 adult. In midlarval and late larval stages, thick unmyelinated axons 

 of the large cells take curious courses. Some descend uncrossed in the 

 ventral or ventrolateral funiculi. Others decussate in the ventral 

 commissure. The latter may cross transversely as internal or external 

 arcuate fibers, or they may take long ascending or descending courses 

 before crossing obliquely and then turning spinal ward. Before or 

 after crossing, these fibers may divide into long ascending and de- 

 scending branches, and they may give long branched collaterals along 

 the entire course. The terminals of an individual fiber may reach a 



