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CHAPTER VI. 

 THE NERVOUS SYSTEM OF THE FROG. 



The nervous system consists of the following parts :-^^ 



1. A central portion, the "brain and spinal cord, which lies 



in the cartilaginous and bony tube formed by the 

 cranium and vertebral column, and which is the centre 

 where sensations are felt, and whence motor impulses 

 causing the muscles to contract take their origin, 



2. A peripheral portion, the nerves themselves, which 



connect the central ]3ortion with the skin, sense 

 organs, muscles, viscera, etc., and serve to convey 

 sensory impulses from these parts to the brain and 

 cord, or motor impulses from the central organs to 

 the muscles. These two functions are fulfilled by 

 different nerves, which may accordingly be distin- 

 guished as (a) afferent or sensory nerves, conveying 

 impulses to the brain or cord and (6) efferent or 

 motor nerves, conveying impulses frojn the brain 

 or cord. 



lig. 13. The nervous system of the edible frog {Rana esculenfa)^ from 

 the ventral surface. (From Ecker.) 



F, facial nerve : G, ganglion of pneumogastric nerve : He, cerebral 

 hemisphere : Lc, optic tract : Lop, optic lobe : M, boundary between 

 medulla oblongata and spinal cord : M 1-10, the spinal nerves: MS, con- 

 nection between fourth spinal nerve and sympathetic chain : N, nasal 

 sac : Ni, sciatic nerve : No, crural nerve : o, eyeball : S, trunk of 

 sympathetic : S 1-10, the sympathetic ganglia : Sp, continuation of 

 symi)athetic into head. 



I, olfactory nerve : II, optic nerve : III, motor oculi : IV, fourth 

 nerve : V, trigeminal and facial nerves : Va, ophthalmic branch of 

 trigeminal '• Vc, maxillary branch of trigeminal : Vd, mandibular branch 

 of trigeminal : Ve, hyomandibular branch of facial: Vg, Gasserian gang- 

 lion : _Vs, uiiper end of sympathetic trunk, in connection with Gasserian 

 ganglion : VI, abducens nerve, : VII, facial nerve : VIII, auditory nerve: 

 A, glossopharyngeal and pneumogastric nerves : XI, ramus anterior of 

 glossopharyngeal : X2, ramus posterior of glossopharyngeal : X 3-4, 

 branches of pneumogastric. 



