644 



MEDULLA OBLONGATA AND THE CRANIAL NERVES 



sense-organs which then diverge centrally to seek the respective centers 

 for these functions. Again, a certain sense-organ may distribute its 

 ingoing fibers to two different cranial nerves, after which they reunite to 

 attain a common center. 



This structural divergency implies that the cranial nerves may be 

 efferent or afferent in their function, as well as mixed. The 

 efferent fibers arise, of course, in cells situated within the domain of 

 the cerebrum, isthmus and medulla, while the cells of the afferent 

 fibers are situated in special ganglia at some distance from these parts. 

 In the latter case, the same arrangement is found to exist as in 

 the spinal ganglia, i.e., the sensory cell sends out an axon which soon 

 divides into two branches, one of them tending toward the brain, and 

 the other toward the peripheral sense-organ. The trophic centers 

 of the motor fibers, therefore, are situated within the brain, and those of 

 the sensory fibers in the peripheral ganglia. 



With the exception of the first and second pairs, the cranial nerves 

 arise from the medulla oblongata and neighboring parts, their nuclei 

 being situated chiefly in the gray matter below the floor of the fourth 

 ventricle and its prolongation below the aqueduct. 



1. The olfactory nerve, or nerve of smell, forms the connection be- 

 tween the olfactory region of the nose and the olfactory center. These 



Olfactory tract 



Granule cell 

 ' Mitral cell 

 Glomerulus 

 Olfactory nerve 

 Ethmoid bone 

 '""^---^ Olfactory epithelium 



Fig. 318a. — Diagram of the Connections of the Olfactory Bulb. (Herrick.) 



fibers arise in the olfactory cells of the aforesaid area, whence they 

 attain the primary center within the olfactory bulb by passing through 

 the cribriform plate of the ethmoid bone. The arborizations formed by 

 these fibers in this particular locality, are known as glomeruli and repre- 

 sent synapses between the primary and secondary olfactory neurons. 

 The latter, which begin here, are known as the mitral cells. Their 

 axons continue inward and form the so-called olfactory tract, ending 

 finally in the secondary olfactory nucleus,^ at the base of the olfac- 

 tory bulb. The olfactory center is then attained by three paths which 

 are known as the medial, intermediate and lateral olfactory striae. - 



1 Zwaardemaker, Ergebn. d. Physiol., i, 1902; also: Edinger, Vergl. Anat. des 

 Gehirns, Leipzig, 1908. 



