614 VEKTEBRATED ANIMALS. 



He lays great stress on the freshness of the specimen, which 

 should be taken from the body as soon as possible after death ; 

 and when a successful preparation has been made, it should be 

 preserved in Goadby's solution. The shape of the fibres can 

 only be properly seen in cross sections ; and these are best made 

 by drying a piece of muscle, so that very thin slices can be cut 

 with a sharp instrument, which on being moistened again, will 

 resume in great part their original characters. Striated muscu- 

 lar fibres are readily obtainable from the limbs of Crustacea and 

 of Insects ; and their presence is also readily distinguishable in 

 the bodies of Worms, even of very low organization ; so that it 

 may be regarded as characteristic of the Articulated series gene- 

 rally. On the other hand, the Molluscous classes are for the 

 most part distinguished by the non-striation of their fibre ; there 

 are, however, two remarkable exceptions, strongly striated fibre 

 having been found in the Terebratula and other Brachiopods, and 

 also in many Potyzoa. Its presence seems always related to 

 energy and rapidity of movement; whilst the non-striated pre- 

 sents itself, where the movements are slower and feebler in their 

 character. 



429. Nerve-substance. Whenever a distinct Nervous system can 

 be made out, it is found to consist of two very different forms of 

 tissue ; namely, the vesicular, which are the essential components 

 of the ganglionic centres, and the tubular, of which the connect- 

 ing trunks consist. The " nerve-vesicles" or " ganglion- glo- 

 bules" are cells, whose typical form may be regarded as globular ; 

 but they often present an extension into one or more long pro- 

 cesses, which give them a "caudate" or a "stellate" aspect. 

 These processes have been traced into continuity, in some in- 

 stances, with the axis-cylinders of nerve-tubes ; whilst in other 

 cases they seem to inosculate with those of other vesicles. The 

 vesicles are filled with a finely-granular substance, which extends 

 into the prolongations ; and they also usually contain pigment- 

 granules, which give them a reddish or yellowish-brown color ; 

 but these are commonly absent among the lower animals. It is 

 the presence of this pigment, however, which gives to collections 

 of ganglion-globules in the warm-blooded Yertebrata that pecu- 

 liar hue, which causes it to be known as the cineriiious or gray 

 matter. Each of the nerve-tubes, on the other hand, of which 

 the trunks are composed, consists, in its most completely deve- 

 loped form, of a delicate membranous sheath, within which is a 

 hollow cylinder of a material known as the " white substance 

 of Schwann," whose outer and inner boundaries are marked out 

 by two distinct lines, giving to each margin of the nerve-tube 

 what is described as a "double contour." The centre or axis 

 of the tube is occupied by a transparent substance, which is 

 known as the "axis-cylinder;" and there is reason to believe 

 that this last is the essential component of the nervous fibre, 

 and that the hollow cylinder that surrounds it, serves, like the 



