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CRANIAL AND PERIPHERAL NERVES 



The peripheral nervous system is divided into the cranial 

 nerves, which emerge from the brain inside the skull, and the 

 spinal nerves which leave the spinal cord between the vertebrae. 



The cranial nerves are twelve pairs. The first ten resemble those 

 of the dogfish (p. 342) in origin and function, but differ in details : 

 thus the branches of the olfactory nerves leave the olfactory bulb 

 to pass at once through the cribriform plate (p. 427). The optic 

 nerve comes from the thalamencephalon, from which a relay of 

 fibres goes forwards to the cerebral hemispheres. The facial has 

 no ophthalmic branch, and, as in the frog, its buccal branch, all 

 the lateral line nerves, and the pretrematic branches of the 

 glossopharyngeal and vagus, are lost. From the vagus ganglion 

 a pharyngeal goes to the pharynx, an anterior laryngeal forward, 

 and a thin depressor branch to the heart ; the main vagus branch 

 runs to various viscera ; it gives off also a recurrent laryngeal 

 which loops round the ductus arteriosus on the left side, the sub- 

 clavian on the right, and runs forward to the larynx. The eleventh 

 or accessory nerve arises from the side of the medulla and spinal 

 cord by a number of roots, the first of which is just behind the 

 vagus and the last at the level of the fifth spinal nerve. It supplies 

 certain muscles of the neck. The twelfth or hypoglossal nerve 

 also arises by several roots which are situated on the ventral 

 side of the medulla, outside the pyramid ; it has a branch running 

 forward to the tongue and another running backwards to the 

 larynx. Some of the nerves of the neck are shown in Fig. 364. 



There are thirty or so pairs of spinal nerves, of origin similar to 

 those of the dogfish (p. 337). The fourth and fifth nerves of the 

 neck give off a branch, the phrenic, which is conspicuous in the dis- 

 section of the neck ; it runs back through the thorax and supplies 

 the diaphragm. The last three nerves of the neck, together with 

 the first of the thorax, unite to form a network called the brachial 

 plexus ; from this several nerves run down the fore-limb. The 

 last four nerves of the lumbar region unite with each other and 

 the two sacral nerves to form a lumbosacral plexus ; from this 

 the great sciatic nerve descends the leg, and a number of other 

 branches supply the genital organs and other parts of the abdomen 

 and hip. 



Each spinal nerve has near its origin a small ramus commum- 

 cans to the autonomic or sympathetic system, which is chiefly 



