THE MAMMALIAN NERVOUS SYSTEM 79 



Visceral reflexes may be effected by short and very simple 

 connections between the nucleus of the fasciculus solitarius and 

 the motor nuclei of the bulb. There is also a visceral lemniscus 

 conducting visceral impulses upward to the diencephalon, but 

 the course of these fibers in the human brain is unknown. 



89. The reticular formation (see Herrick ('18), Figs. 69, 73, 

 81, and 83). In addition to the direct and relatively simple 

 connections 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 drawings. This tissue is reached by fibers from all sen- 

 sory nuclei of the medulla oblongata and the axons of its neu- 

 rons are distributed to the various motor nuclei. It is the 

 direct continuation of the reticular formation of the cervical 

 cord (see Herrick ('18), 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 ('18), Fig. 72). Both sorts of arcuate fibers 

 are evident in sections 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 fascicu- 

 lus 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 all 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 



