THE CRANIAL NERVES 927 



I. THE CRANIAL NERVES 



Customarily, the cranial nerves are described as comprising twelve pairs and 

 each is referred to by number. However, present knowledge of their origin, 

 central connections and peripheral distribution suggests that those enumerated as 

 the fifth, seventh, and eighth pairs under the old nomenclature are better each 

 separated into its two component nerves, each of which merits a separate descrip- 

 tion and a separate name. None of the cranial nerves corresponds closely to 

 a typical spinal nerve "wdth its motor and sensor}^ root. The so-called motor por- 

 tion of the fifth is no more its motor root than is the seventh nerve. The sensory 

 portion of the seventh is not wholly sensory and rather resembles the ninth pair in 

 distribution, and it has long been commonly referred to as a separate nerve. The 

 two parts of the eighth nerve, both sensory, are known to be wholly different in 

 functional character and are so named. Further, the names of the nerves, descrip- 

 tive of their function, are pedagogically much more efficient than the use of num- 

 bers in referring to them. 



Separating the three pairs mentioned, each into its two nerves, gives fifteen 

 pairs instead of twelve. Their names and functional nature are given in the fol- 

 lowing table. The Roman numerals given in parentheses correspond to the serial 

 numbers given when twelve pairs only are considered. It is also customary to 

 enumerate the cranial nerves from in front backward and caudalward, and this 

 custom is followed here, but again it would be pedagogically better to take them 

 in the reverse order. Then each in its turn could be directly considered as in 

 continuous series ^\'ith the spinal nerves below and the similarities to and progres- 

 sive modifications from the spinal type could be better realized. It wall be remem- 

 bered that somatic motor or efferent fibres are those which terminate directly upon 

 the fibres of skeletal muscle while visceral motor fibres transfer their impulses to 

 sympathetic neurones, and the axones of the latter terminate upon gland cells and 

 upon the fibres of cardiac and smooth muscle. 



Name Nature General Distribution 



Olfactory (I) Sensory Olfactory region, nasal epithelium. 



Optic (II) Sensory Retina. 



r»«„i«^^+^- /'TTT\ AT^+^„ / Somatic Eye-moving muscles. 



Oculomotor (III) Motor | y .g^^^.^^ ^^^^^ ^^^^^^ -^-^ 



Trochlear (IV) Motor-somatic Eye-moving muscles. 



Abducens (VI) Motor-somatic Eye-moving muscles. 



Trigeminus (V) Sensory Face, mouth, and scalp. 



Masticator (minor part or Motor-somatic Muscles of mastication. 



motor root of trigeminus). 



■p„.- 1 f\TrT\ AT^f^^ / Somatic Facial muscles. 



^^""'^^ (^ ") ^^"^^"^ \ Visceral (?) SaUvary glands, vessels(?). 



Glossopalatine. (Intermedi- ( Sensory Tongue, palate. 



ate part of facial). \ Motor- visceral Sahvary glands. 



Cochlear (auditory) (VIII). . Sensory Internal ear. 



Vestibular (equilibrator) Sensory Semicircular canals, utriculus, sac- 



(VIII). cuius. 



! Sensory Tongue, palate, pharynx. 



■jL/r . j Somatic Pharynx. 



Motor I Visceral Glands and vessels. 



Sensory AUmentary canal, lung, heart. 



Motor / Somatic Larynx, pharynx. 



\ Visceral Alimentary canal, heart, larjmx, tra- 

 chea, lung. 



Hypoglossal (XII) Motor-somatic Tongue-moving muscles. 



<3T^,•r,ol „„^o^« r-vT\ AT i /Somatic Neck and shoulder muscles. 



Spmal acessory (XI) Motor | y-^^^^^j Pharynx, larynx, heart. 



The cranial nerves, like the spinal nerves, are developed from cells of the primi- 

 tive neural tube and, beginning with the fifth pair downward, all the sensory 

 nerves are developed from the cells corresponding to those of the ganglion crest 

 which give origin to the spinal ganglia with the sensory components or dorsal roots 

 of the spinal nerves. Otherwise between the cranial nerves and the spinal nerves 

 there are many important differences. Each spinal nerve has a dorsal or sensory 

 root, which springs from the cells of a spinal ganglion; a ventral or motor root, 

 whose fibres are processes of the nerve-cells which are situated in the walls of the 

 central system, and at their attachment to the surface of the cord the two roots are 

 some distance apart. Only one of the (usually considered) twelve pairs of cranial 



