74 MUSCULAR CONTRACTION. 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 contract 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 



