478 
NATURE 
| March 22, 1883 
The consideration of the different kinds of tumours is 
followed by a chapter on their ztiology, in which Cohn- 
heim’s embryonic hypothesis is discussed at some length, 
and the objections to its general acceptance pointed out- 
In answer to the question, How does the tumour assume 
properties distinct from those of its surroundings? there 
is as a reply, “‘ We believe that the phenomenon is ulti- 
mately due to some change affecting individual elements 
of a tissue whereby they are rendered dissimilar to their 
neighbours.”’ The change is manifested especially in this 
—that the normal checks to the indefinite growth of the 
proliferous cells are inoperative or inadequate, either 
because the formative and productive energy is increased, 
or because the restraining influence of the surrounding 
structures is diminished, or from both causes together. 
The last section of the present volume is devoted to 
Parasites. On comparing the German account of animal 
parasites with the English, we note very considerable 
additions and improvements. The chapter contains a 
sufficiently complete account of the structure and life- 
history of the ordinary parasites for all practical pur- 
poses. The chapter on Bacteria is extremely valuable. 
The editor has been careful to incorporate in the text all 
the important recent discoveries, and references are given 
to all the memoirs that the student or investigator is 
likely to require to consult. We thus have in a connected 
form the results of nume1ous inquiries into the nature of | 
the organisms which for some time have been claiming 
not a little of the attention of biologists and physicians. 
In describing the bacteria, reference is made to the 
influence of temperature and of the surrounding medium 
on their growth and development, also to the influence 
they exercise on the nutrient liquid, and to their presence 
without and within the living body. 
In reference to the existence of bacteria within the 
body we read :— 
“ Bacteria are perpetually entering the body with the 
food we eat and the air we breathe. They must, there- 
fore, be at times found in the tissues, especially in places 
where access is direct. The fact that they are not easy to 
demonstrate is readily explained. It must be only a small 
number that are able to multiply in the tissues they have 
penetrated ; the majority must quickly perish.” Bacteria | 
are described as pathogenous and non-pathogenous, the 
latter being harmless unless the normal secretions under- 
go some alteration, or the bacteria develop to an 
unusual extent. Under such conditions, inflammation 
may be set up, or the whole system may be influenced 
by the absorption of the soluble products of decomposi- 
tion, some of which are extremely poisonous, and capable 
according to Hiller, of altering or even destroying the | 
tissues exposed to them. “The pathogenous bacteria 
have the power of settling, not merely in the ingesta 
and secretions, or in dead tissue, but also in living tissue. 
This happens chiefly in the mucous membranes and in 
the lungs. The uninjured skin is protected against 
invasion by the horny epidermis.” 
“Many of the bacteria can settle in perfectly healthy 
mucous membranes. 
imagine that they do not find a proper soil for their 
development, unless the mucous membrane is injured or | 
Of course injury or alteration of this kind may | 
altered. 
In the case of others we must | 
seem to make the outer skin or any other accessible | 
tissue the starting-point of a bacterial invasion (wound- 
infection). All that is necessary is that a bacterium 
should reach a spot that affords the conditions of its 
development. If this occurs, it multiplies and forms 
colonies or swarms. These may, according to the 
species of the fungus and the nature of its soil, remain 
in aggregation, forming heaps or masses, or may spread 
through the tissues. Such a settlement is never without 
effect on the affected tissues. The bacteria may force 
their way into the substance of the constituent elements, 
and especially into the tissue-cells, which are sometimes 
found to be crammed with bacteria.” 
All that is necessary is that a bacterium should reach 
a spot that affords the conditions for its development, Ze. 
“the temperature of the body must be such as favours its 
development; it must be able to abstract fit nutriment 
from the tissues in which it settles; it must nowhere 
encounter substances which check or injure it.” When 
in the tissues, the increase of the bacteria may be 
arrested by the aggregation of living cells resulting from 
the inflammation they set up, assisted by the regenera- 
tive action of the fixed tissue-cells. If this does not 
happen, they spread into the surrounding tissues, usually 
reaching the lymphatics and blood-vessels, some to perish, 
others rapidly to multiply. 
The bacteria are supposed to lead to disease by with- 
drawing nourishment, setting up chemical changes— 
partly by their direct action on the nutrient material, and 
partly by the action of the unorganised ferments they 
form ; and finally, as a result of these changes, by pro- 
ducing poisonous matters. In doing this they enter into 
conflict with the tissue-cells, influencing their nutritive 
activity, changing them or even leading to their destruction. 
Whether it isa change in the fermentive action of the 
cells, or a disturbance of the functions of the central 
| nervous system which leads to fever, has not been deter- 
mined. Neither is it known whether the unsusceptible 
condition of the tissues which usually follows when the 
bacteria have been eliminated, results from “a modifica- 
tion in the chemical constitution of the tissues, or to a 
change in the vital activity of the cells.” 
In referring to the relation of bacteria to infective dis- 
eases it is stated “that among the infective diseases there 
are certainly some which are due to the invasion of a 
microphyte, and that it is highly probable the others have 
a like origin.” This chapter further gives a short account 
of the various diseases which have been described as 
resulting from the influence of bacteria, and concludes 
by discussing the burning question of the present moment 
—the mutability of bacterial species. It is well known that 
Naegeli, Buchner, and others believe “that both the 
morphological and the physiological characters of the 
bacteria are mutable” ; that “a given bacillus does not 
invariably produce bacilli of the same structure, and does 
not always pass through the same developmental stages.” 
“‘ A bacterium which, under given conditions, gives rise 
to a definite kind of fermentation, may lose this property 
when cultivated under different conditions.” Koch and 
others believe that bacteria do not alter in their proper- 
ties, and that ‘‘even when the nutrient medium is 
altered from time to time no recognisable differences 
are produced.” 
The authors point out that “at present we are unable 
