74 MUSCULAR CONTRAOTION.—NERVOUS TISSUE. 
fibre passes into the state of contraction, or when, by the 
relaxation of the whole muscle, its ends are separated again 
to their full extent. 
59. Now the alternate contraction and relaxation, which 
is thus made to produce a continued contraction in ordinary 
muscles, elsewhere occasions a different effect. Thus in the 
heart, all the fibres of the ventricles seem to contracts to- 
gether and all to relax together,—those of the auricles contract- 
ing whilst the others are relaxing, and vice versd ;—and in this 
way the alternate contractions and dilatations of that most 
important organ are continually kept up. Again,in the muscular | 
coat of the intestinal canal, we observe the contraction of each 
part to be almost immediately followed by its relaxation ; but 
the peculiarity of its movement is, that the contraction is pro- 
pagated on (as it were) to the succeeding part, which in its turn 
contracts and then relaxes, producing the same action in the 
part that follows it,—and so on along the whole canal. This 
peristaltic motion (§ 215), as it is called, is obviously adapted - 
to propel the contents of the intestinal tube from one ex-— 
tremity of it to the other; just as the peculiar action of the 
heart is adapted to receive and propel the blood alternately, — 
or as the mode of contraction of the ordinary muscles enables — 
them to keep up a continued strain for a great length of time. | 
It is much less rapid and energetic than the action of the 
heart ; for it is the characteristic of the non-striated fibre, that 
its contraction follows much less closely on the application of 
the stimulus, and is much less rapidly succeeded by relaxa- 
tion, than that of the striated fibre. | 
60. The Nervous tissue consists of two distinct structures, 
of one of which the trunks of the nerves are entirely made 
up, whilst the other enters largely into the composition of 
the ganglia or centres of action (§ 61). The former, termed 
the white or fibrous tissue, consists of straight fibres, lying 
side by side, and bound together by areolar tissue into 
bundles (fig. 22); these, again, are united with others into a 
larger group ; and by the union of a considerable number of! 
such groups, the nervous trunks are formed, which are dis- | 
tributed through the body, especially to the skin and muscles, 
Nervous Fibre, like muscular, presents itself in the higher 
animals under two forms, of which one may be considered as 
more completely developed than the other ; these are known, 
