1878. } A Lesson in Comparative Histology. 343 
encircle the canal. Among the fibres occur elongated nuclei. 
Each muscular fibre in fact is a greatly eine and peculiarly 
modified cell. But into this matter I 
cannot enter here; but I wish to point 
out that the lining membrane of the 
canals and ducts of animal bodies 
is generally if not always an epithel- 
ium, and that we frequently find the 
epithelium surrounded by a muscular 
coat. Thus may etree facts be 
observed on a single or 
In the body of the aed there te 5 Aus ot lees alk 
are long tubes, often pigmented, of Vesieida: scuba 
and opening into the digestive canal in the hind end of the 
stomach—they are the Malpighian vessels, so named after their 
illustrious discoverer. They make very beautiful preparations, if 
merely picked out, colored with carmine and mounted in glycer- 
ine, and are interesting to us because they have an epithelium 
which is very different from that above described. An optical 
section of part of one of them is represented on Plate II, Fig. 2. 
There is a very delicate external membrane which is hardly 
noticeable, though it forms a continuous external coating. In- 
side the epithelium is very distinct, but the cells which compose 
it instead of being high in proportion to their breadth are com- 
pressed ; the nucleus is rounder, and the cell itself different from 
those of the seminal vesicle. The mass of matter which sur- 
rounds the nucleus is termed the protoplasm. Now, in the 
epithelium under examination the protoplasm of the cells is 
charged with coarse spherical granules. We naturally regard 
these peculiarities as somehow connected with the special function 
of these tubes, but in the majority of cases we are still unable to 
trace the relations of histological appearance to the physiological 
functions of organs. We have now made the acquaintance of a 
second kind of epithelium, and have learned to recognize cells by 
the presence of the nuclei, which, as far as we know, always have 
the property of being more darkly stained by various dyes than 
any other part of the cell. Moreover, each nucleus corresponds 
to a single cell, and there are never two nuclei in one cell. 
There are, however, some exceptions; thus the nervous cells 
(ganglia) of the sympathetic ganglia of vertebrates and of the 
