NE11VOUS SYSTEM. 151 



In the brain of the Frog only eight separate nerves are found, the 

 facial, glosso-pharyngeal, accessory of Willis, and hypoglossal, ex- 

 hibiting no distinct roots ; the facial is still supplied as a branch 

 from the acoustic ; the vagus, which arises behind the auditory 

 nerve from the most posterior limits of the medulla oblongata, 

 passes at a right angle through an opening lying to the outer side 

 of the articulating tubercle of the occipital bone ; some very deli- 

 cate nervous radicles, arising from the inferior Hacts of the medulla 

 oblongata near to its anterior fissure, unite with it and appear to cor- 

 respond with the glosso-pharyngeal. Some of the roots of the ac- 

 cessory nerve appear to be absent, and the hypoglossal is given off 

 by the first pair of cervical nerves. In the Frogs, as in all the Rep- 

 tilia, even the Ichthyodea, which live habitually in water, the olfac- 

 tory nerves are of very considerable size, and proceed generally 

 from some medullary ganglia situated in front of the hemispheres. 

 The optic nerves are flat and form a chiasma, and at this seat of 

 union there is found a partial decussation formed by the overlapping 

 of several distinct laminiform faciculi of nervous matter. 



In the Tortoises, and probably all the Squamigerous Reptiles, all 

 the twelve pairs of cerebral nerves are to be found, and their roots 

 admit of being traced to the base of the encephalon. 



As regards the Spinal nerves, their origin by two roots is always 

 very distinct, and those of the Amphibia, especially of the Frogs, 

 are particularly well adapted for the purposes of physiological experi- 

 ment. Their number varies very much, ten pairs being found in the 

 Frog, and several more in the Tortoise. In the Squamigerous Rep- 

 tilia the last cervical and first dorsal nerves usually form the brachial 

 plexus ; from the lumbar nerves is given off the crural, and from it 

 and the sacral nerves the ischiadic plexus. 



The Sympathetic nerve, the existence of which was formerly over- 

 looked in the Serpents, has now been proved of general occurrence^ 

 throughout the present class. In the Frogs the ganglia admit of 

 being very readily demonstrated lying upon the sides of the v,erte- 

 bral column ; they are here situated near to those small white ves^ 

 icles which protrude by becoming swollen chiefly during the spring 

 of the year, and contain numerous microscopic calcareous crystals. 

 The plexuses of the sympathetic nerve unite with the vagus, and by 

 a ganglion with each of the spinal nerves. The sympathetic then 

 enters the skull through the condyloid foramen, and unites with the 

 ganglion of the fifth pair or trigeminus, and also with the other cer- 

 ebral nerves. The cranial portion of the sympathetic in the Ser- 



