INVOLUNTARY MUSCULAR FIBRE. 557 



It further appears, that in the pig's intestine the muscular elements are, on 

 the one hand, capable of an extraordinary degree of extension, and, on the other 

 hand, are endowed with a marvellous faculty of contraction, by which they may be 

 reduced from the condition of very long fibres to that of almost globular masses 

 In the extended state they have a soft, delicate, and usually homogeneous aspect, 

 which becomes altered during contraction by the supervention of highly refract- 

 ing transverse ribs, which grow thicker and more approximated as the process 

 advances. Meanwhile, the " rod-shaped" nucleus appears to be pinched up by 

 the contracting fibre till it assumes a slightly oval form, with the longer diame- 

 ter transversely placed. 



I will only further remark, that these properties of the constituent elements of 

 involuntary muscular fibre explain, in a very beautiful manner, the extraordi- 

 nary range of contractility which characterizes the hollow viscera. 



EXPLANATION OF PLATE XV. 



Fig. 1 represents part of a fibre-cell from the pig's intestine, drawn out into a very fine thread. 



Figs. 2 and 3, fibre-cells from the same situation, considerably extended. 



Fig. 4, fibre-cells exhibiting faint longitudinal striation. 



Figs. 5, 6, and 7, fibre-cells imperfectly contracted. 



Figs. 8 and 9, small fibre-cells considerably contracted. 



Figs. 10, 11, 12, 13, 14 and 15, fibre-cells extremely contracted. 



Fig. 16, a fibre-cell curled up, showing the position of the nucleus embedded in its substance. 



Fig. 17, part of a moderately contracted fasciculus of unstriped muscle from the pig's intestine, 

 as seen under a rather low magnifying power. 



Fig. 18, a small artery from the frog's web, under a rather low magnifying power. 



Fig 19, part of the same vessel highly magnified, showing the spiral arrangement of the mus- 

 cular fibre-cells. 



Figs. 20 and 21, muscular fibre-cells from another artery. In fig. 20, the spirals are much 

 closer than in fig. 19 ; and in fig. 21, the sphral is quite close. 



Figs. 22 and 23 represent some fibre-cells in arteries of extreme minuteness, and show the 

 section of the nucleus surrounded by that of the fibre-cell. 



VOL. XXI. PART IV. 7 L 



