XVI 



THE NERVOUS SYSTEM 



285 



Both the cord 

 and the brain are 

 surrounded by 

 membranes which 

 are designated by 

 Gaupp as fol- 

 lows : Externally 

 is the dura mater, 

 consisting of two 

 layers sepa- 

 rated by a lymph 

 space (interdural 

 space) the outer 

 layer of this is 

 pigmented and 

 closely applied to 

 the inner surface 

 of the cranium 

 and neural canal; 

 the inner layer is 

 devoid of pigment 

 and lies close to 

 the brain and 

 cord. Within the 

 dura mater is a 

 thin vascular layer 

 corresponding to 

 the pia mater and 

 arachnoid of the 

 higher verte- 

 brates ; only here 

 and there does it 

 present a division 



mn'V 



FlG. 79. The central nervous system oT the frog. 

 The roof of the skull and vertebral column re- 

 moved to show the brain and spinal cord, at, 

 atlas, or first vertebra; au, auditory capsule; 

 b.s, brachial enlargement of the cord ; f.tm, filum 

 terminale; g.tr, prootic ganglion (trigeminus, or 

 Gasserian ganglion of many authors) ; l.s, lum- 

 bar enlargement of cord; mn ( I/'"), mandibular 

 branch of fifth nerve ; mx ( F"), maxillary branch 

 of trigeminus nerve; my, myelon.or spinal cord; 

 tia, right nasal sac; no! ', left nasal bone; n.c, 

 neural canal; ol.n, olfactory nerve; oph (V), 

 ophthalmic branch of fifth nerve ; //, optic nerve. 

 (After Howes.) 



