DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 827 



bundles or fasicles of fibers surrounded and held together by connective tissue. 

 Most of the peripheral nerve fibers are myelinated. The connective tissue 

 which surrounds a nerve is called the perineurium and that which penetrates 

 inward between the fibers is the endoneurium (fig. 358C). 



4. The Origin, Development and Functions of the 

 Cranial Nerves 



Consult diagrams, figures 356A and B, also 3551. 



0. Terminal 



The nervus terminalis is a little understood nerve closely associated with 

 the olfactory nerve. It was discovered by F. Pinkus in 1894, in the dipnoan 

 fish, Protopterus, after the other cranial nerves were described. In consequence 

 it does not have a numerical designation. (Consult Larsell, '18, for references 

 and discussion.) 



1. Olfactory 



Arises from bipolar cells located in olfactory epithelium. These cells give 

 origin to fibers which grow into the olfactory bulb to synapse with olfactory- 

 bulb neurons (fig. 356B). 



Summary of functional components: Special visceral afferent fibers. 



II. Optic 



The optic nerve arises from neurons located in the retina of the eye. They 

 grow mediad along the lumen of the optic stalk to form the optic nerve. 

 In mammals part of the fibers from the median half of each retina decussate, 

 i.e., cross over, and follow the fibers from the lateral half of the retina of 

 the other eye into the brain (fig. 356B). In birds, however, decussation of 

 the optic nerve fibers is complete, as it is in reptiles and fishes, and probably 

 also in amphibians. 



Summary of functional components: Special somatic afferent fibers, cell 

 bodies in the retina. In fishes, there are efferent fibers in the optic nerve con- 

 trolling, possibly, movements of retinal elements (Arey, '16, and Arey and 

 Smith, '37). 



III. Oculomotor 



The third cranial nerve is composed mainly of somatic motor fibers which 

 originate from neuroblasts in the anterior basal area of the mesencephalon. 

 These fibers grow latero-ventrad from the mesencephalic wall to innervate 

 the premuscle masses of the inferior oblique, inferior, medial and superior 

 rectus muscles of the eyeball (fig. 356A). 



Summary of functional components: ( 1 ) Somatic motor fibers controlling 

 eye muscles indicated, (2) general somatic afferent (sensory) fibers, i.e. pro- 



