MEDULLA OBLONGATA 1.59 



them decussating in the ventral commissure. The fibers of all these 

 kinds are widely dispersed, and their analysis is still incomplete. 

 Arcuate fibers are everywhere present in the medulla oblongata, 

 and each of the five groups of secondary connections mentioned 

 above is represented in these arcuates. There are also deep arcuates 

 from the intermediate and motor zones to the ventral commissure. 

 These are doubtless part of the apparatus by which the patterns of 

 performance of synergic groups of muscles are organized, but the 

 details of actual operation of this apparatus are unknown. It seems 

 to be well established that the primary organization of these motor 

 patterns is intrinsic to the motor and intermediate zones. These 

 intrinsic mechanisms may be activated (1) in a nonspecific way from 

 the sensory zone, with resulting mass movement; (2) more specifically 

 from parts of the sensory field dominated by one or another system of 

 peripheral sense organs, with local motor response; (3) from higher 

 centers of control which co-ordinate all motor activity in the interest 

 of integrated behavior of the body as a whole. The second and third 

 of these types of behavior are recognizably present in Ambly stoma, 

 though at a very primitive level of specialization. 



SENSORY CORRELATION TRACTS a AND b 



All peripheral somatic sensory fibers which enter the medulla 

 oblongata terminate in a common pool of neuropil, which pervades 

 the sensory zone within which the various qualities of sense seem to 

 be more or less completely merged. The secondary fibers which leave 

 these fields are, however, not strictly equipotential. There is evi- 

 dently an incipient localization of function among these fibers, but 

 the apparatus by which this specialization is achieved is not clear. 

 It seems probable that part of this apparatus is to be sought in two 

 longitudinal tracts of correlation, which were first described by 

 Kingsbury ('95) in Necturus. To these he gave noncommittal names, 

 "tracts a and 6," one lying above, the other below, the fascicles of 

 lateral-line nerve roots (figs. 9, 87-90). I have written a general 

 description of these tracts of Amblystoma ('446) and, in particular, of 

 their relations posteriorly with the dorsal funiculus of the spinal cord 

 and the nucleus funiculi and anteriorly with the cerebellum. Some 

 additional features may now be added. 



Tract a. — This tract follows the taenia of the fourth ventricle for 

 most of its length, anteriorly passing below and through the dorsal 

 island of neuropil, under the floor of the lateral recess of the fourth 



