CRAM A L XERVES 743 



THE PERIPHERAL NERVOUS SYSTE3I 



The cerebru-spinal nerves, together witli tlie end-organs of these nerves, the 

 syiiipatlietic system and the gangha whieh are eonneeted to both the eerebro-spinal 

 and the symi)athetic nerves, make up tlie ])eripheral nervous system. The cerebro- 

 spinal nerves are mvariably paired, and, with a few exceptions (notably the vagus), 

 are synnnetrical in their origin, course, and distribution on the two sides of the body. 

 The cranial nerves arise directly from the brain and pass out from the skull through 

 foramina in the cranial wall. The spinal nerves arise by anterior and posterior 

 roots from the spinal cord, and leave the spinal canal by passing through the inter- 

 vertebral foramina. The first spinal nerve is somewhat exceptional, as its origin 

 is partly from tlie medulla, and, moreover, it leaves the spinal canal by passing 

 between the occipital bone and the atlas. 



THE CRANIAL NERVES 



The cranial nerves are classified into nine pairs by Willis, and into twelve 

 j)airs by Soemmerring. AMllis's classification depends upon the manner in which 

 the nerves pierce the dura mater; for example, the glosso-pharyngeal, vagus, and 

 spinal accessory nerves all pierce that membrane opposite the jugular foramen, 

 lience they collectively form a cranial nerve (the eighth) of "Willis. In Soemmer- 

 ring's classification each nerve-trunk is considered separately. AMllis' s classification 

 was formerly in use; it is now, however, generally discarded in favor of Soemmer- 

 ring's classification. The latter Avill be followed in this work. 



The following table will explain the relation of "Willis's to Soemmerring's 

 classification: — 



Willis Soemmerring Xanies 



First pair of nerves First pair of nerves Olfactory 



Second ,, Second ., Optic 



Third ., Third ,, Oculo-niotor 



Fourtli ,, Fourth ,, Pathetic or trudilear 



Fifth ., Fifth ,, Trigeminal or trifacial 



Sixth ,, Sixth ., Alxlucent 



Q .1 f Portio dura Seventh ,, Facial 



&e\entli ., j Portio mollis Eighth ., Auditory 



f Ninth ,, Glosso-i)haiyngeal 



Eighth ,, Tenth ,, Pneumogastric or vagus 



( Eleventh ,, Spinal accessorj' 



Ninth ,, Twelfth ,, Hj'poglossal 



It will be well to notice here that the olfactory bulb and tract and optic nerves 

 are not serially homohjgous with the other cranial nerves, but are ratlier outgrowths 

 of the cerebral sul^stance itself. The 'filaments of the olfactory nerve,' which 

 pierce the cribriform ])late of the ethmoid bone, correspond collectively to a cranial 

 nerve. In the case of the optic nerve, the retina, as a study of its development 

 shows, is a portion of the brain extruded beyond the cranial wall, and the gangli- 

 onic layer of the retina in all probability corresptmds to the ' nucleus of origin ' of 

 an ordinary cranial nerve. The nervous elements which intervene between the 

 ganglionic layer of the retina and the rods and cones therefore represent tlie true 

 optic nerves. 



Superficial and deep origins. — The ])oint at wliich a cranial nerve emerges 

 from the substance of the brain is called its su])erficial origin; while the collection 

 of nerve-cells to which its fibres can be followed is called its dee]) origin. It must 

 be clearly understood, however, that the deep origin is only the proximate origin 

 of the nerve, the real origin is in the cerebral cortex. For example, injuries to the 

 lower portions of the ascending parietal and ascending frontal convolutions almost 



