224 



ANIMAL PHYSIOLOGY. 



[LESSON 66. 



rately agrees with the structure of the muscular fibre of the tenta- 

 cles of the recent Cuttle-fishes (Fig. 345). 



987. The fibres are of two sizes, those near the external surface 

 are remarkably fine (a, Fig. 345), whilst those of deeper layer (a, 



FIG. 346. 



FIG. 345. 



Fossil muscular fibre, Belemnite. 



Fossil muscular fibre, Belemnite. 



FIG. 347. 



Fig. 346) are much broader ; both present the same characteristics, 

 which consist chiefly in the possession of a great number of nuclei 

 (b, Fig. 345). Wherever two layers of fibres are found (in the deep- 

 seated muscle), one layer of fibres crosses the other at right angles 

 (c, Fig. 346). The fibres of the outer layer show the pigment cells 



in great abundance, and in a 

 wonderful state of preserva- 

 tion (a, Figs. 346 and 347). 



988. In addition to the 

 muscular fibre, a portion of the 

 integument (a, Fig. 347) is also 

 shown ; it occurs to some ex- 

 tent in connection with the 

 muscular fibre, in which situa- 

 tion it cannot well be seen, 

 but the detached portion is very interesting, as it shows epidermic 

 scales. In this preparation the fibres are seen singly, and the nuclei 

 well shown. 



989. Amongst the active animals of the Articulate sub-kingdom, 

 the striped fibre prevails, except as regards the muscles of the nutri- 

 mental organs. This form of the fibre in insects, and lulidae, has 

 often been figured, but a general misconception appears to prevail 

 with regard to its true structure. 



990. The transverse lines on an insect fibre are so well marked, 



Fossil muscular fibre, a, Integument, Belemnite. 



