116 
HISTOLOGY OF ANIMALS. 
be met with in the walls of cells, but principally in the 
form of an investing membrane or sheath, in which case 
it is perfectly structureless and as transparent as glass. 
So far as I am aware of, no such extended membrane 
is found in vegetables unless cellular structure is visible. 
It is in this coalescence of cell-walls that animal struc- 
tures principally differ from those of vegetables ; in the 
latter the cell-wall is always present, however old or 
hard the tissue may be ; but in the former, with the 
exception of those tissues termed cellular, it soon 
disappears, and in some cases, no trace is left either of 
nucleus or nucleolus. 
It has been found very useful for the purpose of 
study, to arrange the elementary tissues of animals in a 
tabular form, and various have been the modes in which 
this has been carried out by different anatomists. One 
of the best of these tables, and that, which, with some 
slight alteration, has from the first been adopted in 
these Lectures, is given by Todd and Bowman in their 
“ Physiological Anatomy.” It would, however, be im- 
possible to go through the entire series of tissues in one 
course of lectures ; I have, therefore, thought it best to 
follow the order laid down in the above mentioned table, 
commencing with the simple tissues, and when there is 
occasion to allude to any one of the compound tissues, 
a short account of it will be given, in order that its 
relations to the subject under consideration may be the 
more readily understood. 
