268 



ANATOMY OF VERTEBEATES 



observed to decussate, 



as in fig. 166, changing their relative 



po.sition within the slieath. 



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 papilla3 

 in the form of loops. The looped termi- 

 nation has been distinctly seen by Henle upon the membrana 

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

 matrix of teeth, fig. 167. 



166 



165 



"Wavy course of nerve fibres, "n-ithin 

 the common slieath. ccv. 



167 



Diagram to show the derussntion of the primitive tibres within 

 the tnmlv of a !Ri-\ e. t'Cy. ccvi. 



Amongst the nerve-fibres of the sympathetic system are some 

 of a grey colour, sometimes called ' soft fibres,' vs'hich 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. Mi/elencephalon of Fishes. — 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-lilcc along its middle third, 

 which is slightly grooved along the 

 medial line of the dorsal surface, 



Tcnninul iicrvca on the s;io n[ tlio p 

 iinil;ir tuoUi of Uie Imvcr j;iw i 

 BlK'cp, sliitwiiig llie ju'rantrt'iiiei 

 Jonp.s. C'l.^vi. 



riHid 



