268 



ANATOMY OF VERTEBRATES 



165 



observed to decussate, as in fig. 166, changing their relative 



position within the sheath. 



The termination of efferent nerves on 

 sentient surfaces of the skin appears to 

 be plexiform : but they have been seen 

 to enter the bases of the tactile papillae 

 in the form of loops. The looped termi- 

 nation has been distinctly seen by Ilenle upon the membrana 

 nictitans of the frog, and by Valentin on parts of the formative 

 matrix of teeth, fig. 167. 



166 



Wavy course of nerve fibre?, within 

 the common sheath, ccv. 



167 



Diagram to show the decussation of the primitive fibres within 

 the trunk of a nerve, ccv. OOVX 



Amongst the nerve-fibres of the sympathetic system are some 

 of a grey colour, sometimes called 'soft fibres,' which are flattened, 



homogeneous, more minute than the 

 primitive fibres of the cerebro-spinal 

 system, and characterised by small 

 multinucleate bodies upon their sur- 

 face, fig. 168. 



§ 52. MyelencephalonqfFish.es. — In 

 the cold-blooded Vertebrates the pro- 

 portion of the mass-form, or centres, to 

 the thread-form, or conductors, of the 

 nervous system is less than in the 

 warm-blooded classes. 



In the Lancelet (Branchiostoma\ 

 fig. 169, the neural axis, m d, shows 

 no distinction between brain and 

 myelon ; it is a slender tract of nu- 

 cleated cells, inclosed in a delicate 

 pia mater, constituting a continuous 

 chord, of opaline sub-transparency, 

 ductile and elastic. It is depressed 

 or band-like along its middle third, 



Terminal nerves on the sac of the second ,... , . . . -.. . 



molar tootii of the Lower jaw in the Which IS Slightly gl'OOVed along tllC 

 sheep, showing the arrangement in. J* 1 V £ j.i_ i l r 



loops, con. medial line ot the dorsal surface, 



