76 LABORATORY OUTLINE OF NEUROLOGY 



nections between the sensory and the motor nuclei referred to 

 in the preceding sections, there are more diffuse connections for 

 more complex reflexes through the formatio reticularis. This 

 is a complex of gray with many bundles of myelinated fibers 

 running through it in the ventro-lateral regions of the medulla 

 oblongata. Locate it in the sections and indicate it in the draw- 

 ings. This tissue is reached by fibers from all sensory nuclei 

 of the medulla oblongata and the axons of its neurons are 

 distributed to the various motor nuclei. It is the direct continu- 

 ation of the reticular formation of the cervical cord (see Herrick 

 ('15), Fig. 58, "processus reticularis") and its fibers (the formatio 

 reticularis alba) are functionally similar to the fasciculi proprii 

 of the cord. 



90. Arcuate fibers. — The correlation fibers just described in 

 part connect various nuclei of the same side of the brain, and in 

 part they cross to the opposite side. The decussating fibers are 

 called arcuate fibers. Some of them cross obliquely through the 

 deeper levels of the oblongata (internal arcuate fibers), while 

 others form a thin but dense layer of obliquely transverse fibers 

 on the extreme outer surface (external arcuate fibers, see Herrick 

 ('15), Fig. 72). Both sorts of arcuate fibers are evident in sec- 

 tions at most levels of the medulla oblongata. 



91. The spino-bulbar tracts. — Various ascending tracts from 

 the spinal cord to the brain have already been mentioned. 

 Some of these fibers pass through the medulla oblongata to end 

 in the thalamus and the cerebellum. Others, like the fasciculus 

 gracilis and fasciculus cuneatus, end in the medulla oblongata 

 and, after a synpase here, are continued upward to the thalamus 

 under a different name. The fibers of any of these tracts may 

 give off collaterals into the gray centers of the cord and the 

 correlation centers of the brain stem. Similar fibers reach the 

 midbrain (tractus spino-tectalis, see Herrick ('15), Figs. 59 and 

 75). These connections serve for reflexes of more complex 

 sorts than can be effected in the spinal cord alone, in contrast 

 with the lemniscus systems terminating in the thalamus and 

 thence connecting with the cerebral cortex, which may serve 

 conscious reactions. The spino-bulbar connections are diffuse 

 and are not readily seen in sections, though physiologically they 

 are important. 



