368 THE MICROSCOPE. 



wound round in a spiral form. As a consequence, when acetic acid is 

 applied, we perceive projections of swollen cellular fibre ; and in the 

 depressions the spiral fibre, which, from not having been acted on, has 

 formed the constriction, and with the acid has given rise to the ap- 

 pearance. This forms a striking instance of the constancy of the 

 spiral, which perhaps will be seen to take on a definite course, as ob- 

 served by M. Wichura in many other departments of nature. He says, 

 "that the circularly or heliacally acting forces of nature follow an 

 unchanging, definite, lateral direction in their course. The planets 

 describe heliacal lines, winding to the right in space by virtue of their 

 circulation from west to east, since this is combined with the advance 

 iu company with the sun towards a point in the northern hemisphere. 

 In the department of physics we meet with allied phenomena in the 

 circular polarisation of light, and in the course of electro-magnetic 

 spirals. Organic life exhibits similar laws in the circulation of the 

 blood, in all cases starting from the left side of the animal's body ; and 

 in the heliacal windings of the shells of molluscs, which follow a 

 direction determinate for every species. But plants, above all, give 

 evidence of a wonderful obedience to such laws, in the direction of the 

 spiral vessels, the heliacally winding trunks of trees, winding stems 

 and leaves ; and probably, also, in the circulation of their saps." In 

 the lowest forms of animal life, and in those of insects, the spiral form 

 will be seen to be most extensively distributed throughout ; as may be 

 observed by referring to our woodcuts. 



Sometimes fat is found in the cellular tissue ; it is not secreted 

 from it, but is contained in its proper cells, and termed adipose, 

 tissue, the elementary cells of which are from the 1-3 00th to the 

 1 -600th of an inch in diameter, Plate XII., No. 4. The cell-wall is 

 very delicate and transparent : sometimes there are one or two nuclei 

 enclosed. ^Ether dissolves out the fat-cells from the tissues. Acetic 

 acid acts upon the cell-wall, and causes the contents to pass from within 

 outwards. 



Fibrous tissue, elastic and non-elastic, is usually divided into white 

 and yelloiv fibrous tissue. The yellow is elastic and of great strength, 

 consisting of bundles of fibres which are highly elastic (Plate XIII., 

 No. 2). The white, No. 1, though non-elastic, is of great strength, 

 and of a shining silvery appearance. These two kinds of fibrous 

 tissue differ from each other in many respects, but chiefly in their 

 ultimate structure, their physical properties, and their colour : both 

 are largely employed in those parts subservient to the organs of loco- 

 motion. 



