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With Hopes Her Dream of Becoming A veterinarian Comes True
Special Thanks to Deborah Furness of the University College London
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Transcriber's Note:
I try to edit my e-texts so they can easily be used with voice
speech programs, I believe blind people and children should also
be able to enjoy the many books now available electronically. I
use the -- for an em-dash, with a space either before or after
it depending on its usage. This helps to keep certain programs
from squishing the words together, such as down-stairs. Also to
help voice speech programs I've enclosed upper case text
between - and _ (-UPPER CASE TEXT_), and used underscores to
show chapter and section headers. I also added a second
contents that shows the other sections of this e-text. This
e-text was made with a "Top can" text scanner, with a bit of
correcting here and there.
This book is volume one of two. It was later reworked by A. M.
Davies in 1898 under the title "Text-book of Zoology",
then revised and rewritten by J. T. Cunningham about 1909 and
W. H. Leigh-Sharpe around 1932. Although these editions gave
Wells the main credit, most of Wells' writing and all his
drawings were removed; only his rough outline seems to have
been used. It was re-published by University Tutorial Press.
The First Edition, as well as The Second and Revised Edition
(with dissections redrawn by Miss A. C. Robbins) are used in this
e-text. The First Edition had some small minor errors, as well as
dissection abbreviations that are shown on the Dissection Sheets,
but no mention of them was listed in the text. Certain figures on
the Dissections Sheets are missing (such as Figures 1, 2, 4, with
no mention to a 3, as if Mr. Wells drew a Figure 3 but found it
was not needed and removed it from the book). Rather then leaving
it as is, I put {} marks around my notes saying things like
{No Figure 3}. For the "Second and Revised Edition" Wells was
able to change some of these errors and missing parts, but many of
the same printing tablets were used and with almost each addition
other things were removed, (in one instance one entire section
from a chapter), and many of the helpful suggestions were
shortened or removed so other things could be explained more. In
an ideal version of the book both could have been used, but with
reprinting the entire book from the first to the second editions
almost as many things were lost as were gained, so I've tried to
indicate where both text go separate paths with the following;
[Second Edition only text] and -First Edition only text,- and also
{Lines from Second Edition only.} and {Lines from First Edition
only.} were more then just a sentence is added or removed. Other
things to notice is how some words are spelt or punctuated
differently throughout the book, such as;
Blood Vessels
Blood-Vessels
Bloodvessels
I've tried to keep these as close to the original book as possible.
University Correspondence College Tutorial Series.
-Text-Book Of Biology._
by
H. G. Wells,
Bachelor of Science, London., Fellow of the Zoological Society.
Lecturer in Biology at University Tutorial College.
With An Introduction by G. B. Howes,
Fellow of the Linnean Society, Fellow of the Zoological Society.
Assistant Professor of Zoology, Royal College of Science, London.
Part 1.-- Vertebrata.
Contents
Introduction
Preface
The Rabbit--
1. External Form and General Considerations
2. The Alimentary Canal of the Rabbit
3. The Circulation
4. The Amoeba, Cells and Tissue
5. The Skeleton
6. Muscle and Nerve
7. The Nervous System
8. Renal and Reproductive Organs
9. Classificatory Points
10. Questions and Exercises
The Frog--
1. General Anatomy
2. The Skull of the Frog (and the Vertebrate Skull generally)
3. Questions on the Frog
The Dog-Fish--
1. General Anatomy
2. Questions on the Dog-Fish
Amphioxus--
1. Anatomy
2. The Development of Amphioxus
3. Questions on Amphioxus
Development--
The Development of the Frog
The Development of the Fowl
The Development of the Rabbit
The Theory of Evolution
Questions on Embryology
Miscellaneous Questions
Note on Making Comparisons
Syllabus of Practical Work
{Contents part 2}
Key for Dissection Sheets, and Abbreviations
-Introduction_
In the year 1884 I was invited to give tuition by correspondence, in
Biology. Although disposed at the time to ridicule the idea of
imparting instruction in natural science by letter, I gladly accepted
the opportunity thus afforded me of ascertaining for myself what
could and could not be accomplished in that direction. Anyone
familiar with the scope of biological enquiry, and the methods of
biological instruction, will not need to be reminded that it is only by
the most rigorous employment of precise directions for observation,
that any good results are to be looked for at the hand of the
elementary student. True to this principle, I determined to issue to
my correspondence pupils rigid instructions, and to demand in return
faithful annotated drawings of facts observed in their usage. In the
case of two among the few students who passed through my hands,
the result far exceeded my most sanguine anticipations. The notes
sent in by one of them-- a man working at a distance, alone and
unaided-- far excelled those wrung from many a student placed
under the most favourable surroundings; and their promise for the
future has been fulfilled to the utmost, the individual in question
being now a recognised investigator. It thus became clear that,
not-with-standing the complex conditions of work in the biological
field, tuition by correspondence would suffice to awaken the latent
abilities of a naturally qualified enquirer. The average members of a
University Correspondence Class will be found neither better nor
worse than those of any other, and they may therefore pass
unnoticed; if however, the correspondence system of tuition may
furnish the means of arousing a latent aptitude, when the
possibilities of other methods of approach are excluded-- and in
so doing, of elevating the individual to that position for which he was
by nature qualified, ensuring him the introduction to the one sphere
of labour for which he was born-- it will have created its own defence,
and have merited the confidence of all right-thinking people. The
plucking of one such brand from the burning is ample compensation
for the energy expended on any number of average dullards, who but
require to be left alone to find their natural level.
Mr. Wells' little book is avowedly written for examination purposes,
and in conformity with the requirements of the now familiar "type
system" of teaching. Recent attempts have been made to depreciate
this. While affording a discipline in detailed observation and
manipulation second to that of no other branch of learning, it
provides for that "deduction" and "verification" by which all science
has been built up; and this appears to me ample justification for its
retention, as the most rational system which can be to-day adopted.
Evidence that its alleged shortcomings are due rather to defective
handling than to any inherent weakness of its own, would not be
difficult to produce. Although rigid in its discipline, it admits of
commentatorial treatment which, while heightening the interest of
the student, is calculated to stimulate alike his ambition and his
imagination. That the sister sciences of Botany and Zoology fall
under one discipline, is expressed in the English usage of the term
"Biology." Experience has shown that the best work in either
department has been produced by those who have acquired on
all-round knowledge of at least the elementary stages of both; and,
that the advanced morphologist and physiologist are alike the better
for a familiarity with the principles-- not to say with the progressive
advancement-- of each other's domain, is to-day undeniable. These
and other allied considerations, render it advisable that the
elementary facts of morphology and physiology should be presented
to the beginner side by side-- a principle too frequently neglected in
books which, like this one, are specially written for the biological
neophyte. Although the student is the wiser for the actual
observation of the fact of nature, he becomes the better only when
able to apply them, as for example, by the judicious construction of
elementary generalizations, such as are introduced into the pages of
this work. So long as these generalizations, regarded as first
attempts to deduce "laws" in the form of "generalized statement of
facts based observation," are properly introduced into an elementary
text-book, intended for the isolated worker cut off from the lecture
room, their intercalation is both healthy and desirable.
Mr. Wells has kept these precepts constantly in mind in the
preparation of his work, and in the formulation of his plans for its
future extension, thereby enhancing the value of the book itself, and
at the same time, discouraging the system of pure cram, which is
alien to the discipline of biological science.
G. B. Howes
Royal College of Science,
South Kensington;
November 30, 1892.
-Preface_
No method of studying-- more especially when the objects of study
are tangible things-- can rival that prosecuted under the direction and
in the constant presence of a teacher who has also a living and vivid
knowledge of the matter which he handles with the student. In the
ideal world there is a plentiful supply of such teachers, and easy
access to their teaching, but in this real world only a favoured few
enjoy these advantages. Through causes that cannot be discussed
here, a vast number of solitary workers are scattered through the
country, to whom sustained help in this form is impossible, or
possible only in days stolen from a needed vacation; and to such
students especially does this book appeal, as well as to those more
fortunate learners who are within reach of orderly instruction, but
anxious to save their teachers' patience and their own time by some
preliminary work.
One of the most manifest disadvantages of book-work, under the
conditions of the solitary worker, is the rigidity of its expressions; if
the exact meaning is doubtful, he can not ask a question. This has
been kept in view throughout; the writer has, above all, sought to be
explicit-- has, saving over-sights, used no uncommon or technical
term without a definition or a clear indication of its meaning.
In this study of Biology, the perception and memory of form is a very
important factor indeed. Every student should draw sketches of his
dissections, and accustom himself to copying book diagrams, in
order to train his eye to perception of details he might otherwise
disregard. The drawing required is within the reach of all; but for
those who are very inexperienced, tracing figures is a useful
preliminary exercise.
By the time the student has read the "Circulation of the Rabbit"
(Sections 34 to 49), he will be ready to begin dissection. It is
possible to hunt to death even such a sound educational maxim as
the "thing before the name," and we are persuaded, by a
considerable experience, that dissection before some such
preparatory reading is altogether a mistake. At the end of the book
is a syllabus (with suggestions) for practical work, originally drawn
up by the writer for his own private use with the evening classes of
the University Tutorial College-- classes of students working mainly
in their spare time for the London examination, and at an enormous
disadvantage, as regards the number of hours available, in
comparison with the leisurely students of a University laboratory.
This syllabus may, perhaps by itself, serve a useful purpose in some
cases, but in this essential part of the study the presence of some
experienced overlooker to advise, warn, and correct, is at first almost
indispensable.
A few words may, perhaps be said with respect to the design of this
volume. It is manifestly modelled upon the syllabus of the
Intermediate Examination in Science of London University. That
syllabus, as at present constituted, appears to me to afford
considerable scope for fairly efficient biological study. The four types
dealt with in this book are extremely convenient for developing the
methods of comparative anatomy and morphological embryology.
Without any extensive reference to related organisms, these four
forms, and especially the three vertebrata, may be made to explain
and illustrate one another in a way that cannot fail to be educational
in the truest sense. After dealing with the rabbit, therefore, as an
organic mechanism, our sections upon the frog and dog-fish, and
upon development, are simply statements of differences, and a
commentary, as it were, upon the anatomy of the mammalian type.
In the concluding chapter, a few suggestions of the most elementary
ideas of it is hoped to make this first part of our biological course
complete in itself, and of some real and permanent value to the
student. And the writer is convinced that not only is a constant
insistence upon resemblances and differences, and their import,
intellectually the most valuable, but also the most interesting, and
therefore the easiest, way of studying animal anatomy. That chaotic
and breathless cramming of terms misunderstood, tabulated
statements, formulated "tips," and lists of names, in which so many
students, in spite of advice, waste their youth is, I sincerely hope, as
impossible with this book as it is useless for the purposes of a
London candidate. On the other hand, our chief endeavour has been
to render the matter of the book clear, connected, progressive, and
easily assimilable. In the second part Plants, Unicellular Organisms,
and Invertebrata will be dealt with, in a wider and less detailed view
of the entire biological province.
{Lines from First Edition only.}
-In this volume, we study four organisms, and chiefly in their
relation to each other; in the next, we shall study a number of
organisms largely in relation to their environment. In this part
our key note is the evidence of inheritance; in our second part
it will be of adaptation to circumstances.-
This book will speedily, under the scrutiny of the critical reader,
reveal abundant weakness. For these the author claims the full
credit. For whatever merit it may posses, he must however,
acknowledge his profound indebtedness to his former teacher,
Professor Howes. Not only has the writer enjoyed in the past the
privilege of Professor Howes' instruction and example, but he has,
during the preparation of this work, received the readiest help,
advise, and encouragement from him-- assistance as generous as it
was unmerited, and as unaffected as it was valuable.
{Lines from Second Edition only.}
[The publication of a second and revised edition of this Part affords
the author an opportunity of expressing his sense of the general
kindliness of his reviewers, and the help they have him in improving
this maiden effort. To no one is there vouchsafed such a facility in
the discovery of errors in a book as to its author, so soon as it has
passed beyond his power of correction. Hence the general tone of
encouragement (and in some cases the decided approval) of the
members of this termination to a period of considerable remorse and
apprehension.]
I have been able through their counsel, and the experience I have had
while using this book in teaching, to correct several printer's errors
and to alter various ambiguous or misleading expressions, as well as
to bring the book up to date again in one or two particulars.
My thanks are particularly due to my friend Miss Robbins, who has
very kindly redrawn the occasionally rather blottesque figures of the
first edition. Not only have these plates gained immensely in grace
and accuracy, but the lettering is now distinct-- an improvement that
any student who has had to hunt my reference letters in the first
edition will at once appreciate.
H. G. Wells
November, 1892. {First Edition.}
December, 1893. {Second Edition.}
-The Rabbit._
1. _External Form and General Considerations._
Section 1. It is unnecessary to enter upon a description of the
appearance of this familiar type, but it is not perhaps superfluous, as
we proceed to consider its anatomy, to call attention to one or two
points in its external, or externally apparent structure. Most of our
readers know that it belongs to that one of two primary animal
divisions which is called the vertebrata, and that the distinctive
feature which place it in this division is the possession of a spinal
column or backbone, really a series of small ring-like bones, the
vertebrae (Figure 1 v.b.) strung together, as it were, on the main
nerve axis, the spinal cord (Figure 1 s.c.). This spinal column can be
felt along the neck and back to the tail. This tail is small, tilted up,
and conspicuously white beneath, and it serves as a "recognition
mark" to guide the young when, during feeding, an alarm is given
and a bolt is made for the burrows. In those more primitive (older and
simpler-fashioned) vertebrata, the fishes, the tail is much large and
far more important, as compared with the rest of the body, than it is
in most of the air-inhabiting vertebrates. In the former it is invariably a
great muscular mass to propel the body forward; in the latter it may
disappear, as in the frog, be simply a feather-bearing stump, as in
the pigeon, a fly flicker, as in the cow or horse, a fur cape in squirrel,
or be otherwise reduced and modified to meet special requirements.
Section 2. At the fore end, or as English zoologists prefer to say,
anterior end, of the vertebral column of the rabbit, is of course the
skull, containing the anterior portion of the nerve axis, the brain
(Figure 1 br.). Between the head and what is called "the body," in
the more restricted sense of the word, is the neck. The neck gives
freedom of movement to the head, enables the animal to look this
way and that, to turn its ears about to determine the direction of a
sound, and to perform endless motions in connexion with biting and
so forth easily. We may note that in types which swim through the
water, the neck dose not appear-- in the fish and frog, for instance--
and the head simply widens out as one passes back to the body.
The high resistance offered by water necessitates this tendency to a
cigar or ship outline, just as it has determined the cigar shape of the
ordinary fish torpedo.
Section 3. In the body of the rabbit, as examined from the outside,
we can make out by feeling two distinct regions, just as we might in
the body of a man; anteriorily a bony cage, having the ribs at the
sides, a rod-like bone in the front, the sternum (Figure 1 -st.-, [stm.]),
and the backbone behind, and called the chest or thorax; and
posteriorily a part called the abdomen, which has no bony protection
over its belly, or ventral surface. These parts together with the neck
constitute the trunk. As a consequence of these things, in the
backbone of the rabbit there are four regions: the neck, or cervical
part, consisting of seven vertebrae, the thoracic part of twelve joined to
ribs, the abdominal (also called the lumbar) region of seven without
ribs, and the tail or caudal of about fifteen. Between the lumbar and
caudal come four vertebrae, the sacral, which tend to run together into
a bony mass as the animal grows old, and which form a firm
attachment for the base of the hind limb.
Section 4. The thorax and abdomen are separated by a partition, the
diaphragm (Figure 1 dia.). This structure is distinctive of that class of
the vertebrata called mammals, and which includes man, most of
the larger and commoner land animals, and whales and manatee.
We shall find later that it is essentially connected with the perfection
of the air breathing to which this group has attained. Another
characteristic shared by all mammals, and by no other creature, is
the presence of hair. In birds we have an equally characteristic cover
in the feathers, the frog is naked, and the fishes we find either naked
skins or scales.
Section 5. The short strong fore limbs are adapted to the burrowing
habit, and have five digits; the hind limbs are very much longer and
muscular, enable the animal to progress rapidly by short leaps, and
they have four toes. If the student thinks it worth while to attempt to
remember the number of digits-- it is the fault of examiners if any
value dose attach to such intrinsically valueless facts-- he should
associate the number 54 (5 in front, 4 behind) with the rabbit, and
observe that with the frog the reverse is the case.
Section 6. We may note here the meaning of certain terms we shall
be constantly employing. The head end of the rabbit is anterior, the
tail end posterior, the backbone side of the body-- the upper side in
life-- is dorsal, the breast and belly side, the lower side of the
animal, is ventral. If we imagine the rabbit sawn asunder, as it were,
by a plane passing through the head and tail, that would be the
median plane, and parts on either side of it are lateral, and left or
right according as they lie to the animal's left or right. In a limb, or in
the internal organs, the part nearest the central organ, or axis, is
proximal, the more remote or terminal parts are distal. For instance,
the mouth is anteriorly placed, the tongue on its ventral wall; the
tongue is median, the eyes are lateral, and the fingers are distal to
the elbow. The student must accustom himself to these words, and
avoid, in his descriptions, the use of such terms as "above,"
"below," "outside," which vary with the position in which we conceive
the animal placed.
Section 7. So much for the general form; we may note a few facts of
general knowledge, in connection with the rabbit's life-activity. In a
day of the rabbit's life a considerable amount of work is done-- the
animal runs hither and thither, for instance; in other words, a certain
mass of matter is moved through space, and for that we know force
must be exerted. Whence comes the force?
Section 8. We find the rabbit occupies a considerable amount of its
time in taking in vegetable matter, consisting chiefly of more or less
complex combustible and unstable organic compounds. It is a pure
vegetarian, and a remarkably moderate drinker. Some but only a
small proportion, of the vegetable matter it eats, leaves its body
comparatively unchanged, in little pellets, the faeces, in the process
of defaecation. For the rest we have to account.
Section 9. We find, also, that the rabbit breathes air into its lungs,
which is returned to the atmosphere with a lessened amount of
oxygen, and the addition of a perceptible amount of carbon dioxide.
The rabbit also throws off, or excretes, a fluid, the urine, which
consists of water with a certain partially oxydised substance
containing nitrogen, and called urea, and other less important salts.
The organs within the body, by which the urine is separated, are
called the kidneys.
Section 10. Repeating these facts in other words, the rabbit takes
into its body complex and unstable organic compounds containing
nitrogen, carbon, hydrogen, a certain amount of oxygen, a small
quantity of sulphur, and still smaller amounts of other elements. It
also breathes in oxygen.
Section 11. It returns a certain rejected part of its food comparatively
unchanged. Besides this, it returns carbon dioxide and water, which
are completely oxydised, and very simple and stable bodies, and
urea-- a less completely oxydised compound, but a very simple one
compared with the food constituents.
Section 12. Now the chemist tells us that when a stable body is
formed, or when an unstable compound decomposes into simpler
stable ones, force is evolved. The oxydation of carbon, for instance,
in the fireplace, is the formation of the stable compound called
carbon dioxide, and light and heat are evolved. The explosion of
dynamite, again is the decomposition of an unstable compound.
Hence, we begin to perceive that force-- the vital force-- which keeps
the rabbit moving, is supplied by the decomposition and partial
oxydation of compounds continued in its food, to carbon dioxide,
water, urea, and smaller quantities of other substances.
Section 13. This is the roughest statement of the case possible, but it
will give the general idea underlying our next chapters. We shall
consider how the food enters the body and is taken up into the
system, how it is conveyed to the muscles in the limbs, to the nerve
centres, and to wherever work is done, to be there decomposed and
partially oxydised, and finally how the products of its activity-- the
katastases, of which the three principal are carbon dioxide, water,
and urea-- are removed from the body.
Section 14. There are one or two comparatively modern terms that
we may note here. This decomposition of unstable chemical
compounds, releasing energy, is called kataboly. A reverse process,
which has a less conspicuous part in our first view of the animal's life
action, by which unstable compounds are built up and energy
stored, is called anaboly. The katastases are the products of
kataboly.
Section 15. In an ordinary animal, locomotion and other activity
predominate over nutritive processes, which fact we may express, in
the terms just given, by saying that kataboly prevails over anaboly.
An animal, as we have just explained, is an apparatus for the
decomposition and partial oxydation of certain compounds, and
these are obtained either directly or indirectly-- through other animals,
in the case of meat-eaters-- from the vegetable kingdom.
As the student will learn early in his botanical reading, the typical
plant has, in its green colouring matter, chlorophyll, a trap to catch
the radiating energy of the sun, and to accomplish, by the
absorption of that energy, the synthesis (building up) of those
organic compounds which the animal destroys. The typical plant is,
on whole, passive and synthetic, or anabolic; the typical animal,
active and katabolic; and the excess of kataboly over anaboly in the
animal is compensated for by the anabolic work stored up, as it
were, by the plant, which is, directly or indirectly, the animal's food.
2. _The Alimentary Canal of the Rabbit_
Section 16. Figure 1 represents the general anatomy of the rabbit,
but is especially intended to show the alimentary (= food) canal,
shortened to a certain extent, and with the proportions altered, in
order to avoid any confusing complications. It is evidently simply a
coiled tube-- coiled for the sake of packing-- with occasional
dilatations, and with one side-shunt, the caecum (cae.), into which
the food enters, and is returned to the main line, after probably
absorbent action, imperfectly understood at present. A spiral fold in
this cul-de-sac {bottom-of-sack}, which is marked externally by
constrictions, has a directive influence on the circulation of its
contents. The student should sketch Figure 1 once or twice, and
make himself familiar with the order and names of the parts before
proceeding. We have, in succession, the mouth (M.), separated from
the nasal passage (Na.) above the palate; the pharynx (ph.), where
the right and left nasal passages open by the posterior nares into
the mouth; the oesophagus (oes.); the bag-like stomach, its left
(Section 6) end being called the cardiac (cd.st.), and its right the
pyloric end (py.); the U-shaped duodenum (ddnm.) and the very long
and greatly coiled ileum (il.). The duodenum and ileum together form
the small intestine; and the ileum is dilated at its distal end into a
thick-walled sacculus rotundus (s.r.), beyond which point comes the
large intestine. The colon (co.) and rectum (r.) continue the main line
of the alimentary canal; but, at the beginning of the large intestine,
there is also inserted a great side-shunt, the caecum (cae.), ending
blindly in a fleshy vermiform appendix (v.ap.). The figure will indicate
how the parts are related better than any verbal description can.
Between the coiling alimentary tube and the body walls is a space,
into which the student cuts when he begins dissecting; this is the
peritoneal cavity (pt.). A thin, transparent membrane, the mesentery,
holds the intestines in place, and binds them to the dorsal wall of
this peritoneal space.
Section 17. The food stuffs of an animal, the unstable compounds
destined ultimately to be worked into its life, and to leave it again in
the form of katastases (Section 13), fall into two main divisions. The
first of these includes the non-nitrogenous food stuffs, containing
either carbon together with hydrogen and oxygen in the proportion of
H2O (the carbo-hydrates), or carbon and hydrogen without oxygen
(the hydrocarbons). The second division consists of the nitrogenous
materials, containing also carbon, hydrogen, a certain amount
of oxygen, sulphur, and possibly other elements. Among the
carbohydrates, the commonest are starch and cellulose, which are
insoluble bodies, and sugar, which is soluble. The hydrocarbons,
fats, oils, and so on, form a comparatively small proportion of the
rabbit's diet; the proverb of "oil and water" will remind the student
that these are insoluble. The nitrogenous bodies have their type in
the albumen of an egg; and muscle substance and the less modified
living "protoplasm" of plants, a considerable proportion of the
substance of seeds, bulbs, and so on, are albuminous bodies, or
proteids. These also are insoluble bodies, or when soluble, will not
diffuse easily through animal membranes.
Section 18. Now the essential problem which the digestive canal of
the rabbit solves is to get these insoluble, or quasi-insoluble, bodies
into its blood and system. They have to pass somehow into the
circulation through the walls of the alimentary canal. In order that a
compound should diffuse through a membrane, it must be both
soluble and diffusible, and therefore an essential preliminary to the
absorption of nutritive matter is its conversion into a diffusible soluble
form. This is effected by certain fluids, formed either by the walls of
the alimentary canal or by certain organs called glands, which open
by ducts into it; all these fluids contain small quantities of organic
compounds of the class called ferments, and these are the active
agents in the change. The soluble form of the carbohydrates is
sugar; proteids can be changed into the, of course, chemically
equivalent but soluble and diffusible the peptones; and fats and oils
undergo a more complicated, but finally similar change.
Section 19. We shall discuss the structure and action of -a gland-
[glands] a little more fully in a subsequent chapter. Here we will
simply say that they are organs forming each its characteristic fluid
or secretion, and sending it by a conduit, the duct, to the point
where its presence is required. The saliva in our mouths, tears, and
perspiration, are examples of the secretions of glands.
Section 20. In the month of the rabbit the food is acted upon by the
teeth and saliva. The saliva contains ptyalin, a ferment converting
starch into sugar, and it also serves to moisten the food as it is
ground up by the cheek teeth. It does not act on fat to any
appreciable extent. The teeth of the rabbit are shown in Figure XVIII.,
Sheet 4. The incisor teeth in front, two pairs above and one pair below
(i.), are simply employed in grasping the food; the cheek teeth-- the
premolars (pm.) and molars (m.) behind-- triturate the food by a
complicated motion over each. Their crowns are flat for this purpose,
with harder ridges running across them.
Section 21. This grinding up of the food in the mouth invariably
occurs in herbivorous animals, where there is a considerable amount
of starch and comparatively little hydrocarbon in the food. By finely
dividing the food, it ensures its intimate contact with the digestive
ferment, ptyalin. In such meat-eaters as the cat and dog, where little
starchy matter and much fat is taken, the saliva is, of course, of less
importance, and this mastication does not occur. The cheek teeth of
a dog ({Section 91}), and more so of a cat, are sharp, and used for
gnawing off fragments of food, which are swallowed at once.
Between the incisors and premolars of a dog come the
characteristic biting teeth, or canines, absent in the rabbit.
Section 22. The student will probably ask why the cheek teeth,
which are all similar in appearance, are divided into premolars and
molars. The rabbit has a set of milk molars-- a milk dentition-- which
are followed by the permanent teeth, just as in man. Those cheek
teeth of the second set, which have predecessors in the first series,
are called premolars; the ones posterior to these are the molars.
Section 23. After mastication, the food is worked by the tongue and
cheeks into a saliva-soaked "bolus" and swallowed. The passage
down the oesophagus is called deglutition. In the stomach it comes
under the influence of the gastric juice, formed in little glandular pits
in the stomach wall-- the gastric (Figure VIII. Sheet 3) and pyloric
glands. This fluid is distinctly acid, its acidity being due to about
one-tenth per cent {of a hundred} of hydrochloric acid, and it
therefore stops any further action of the ptyalin, which can act only
on neutral or slightly alkaline fluids. The gastric juice does not act on
carbo-hydrates or hydrocarbons to any very noticeable degree. Its
essential property is the conversion of proteids into peptones, and the
ferment by which this is effected is called pepsin. Milk contains
a peculiar soluble proteid, called casein, which is precipitated by a
special ferment, the rennet-ferment, and the insoluble proteid, the
curd, thus obtained is then acted on by the pepsin. In the
manufacture of cheese, the rennetferment obtained, from the
stomach of a calf is used to curdle the milk.
Section 24. After the food has undergone digestion in the stomach it
passes into the duodenum, the U-shaped loop of intestine
immediately succeeding the stomach. The duodenum is separated
from the stomach by a ring-like muscular valve, the pylorus; this
valve belongs to the class of muscles called sphincters, which, under
ordinary circumstances, are closed, but which relax to open the
circular central aperture. The valve at the anus, which retains the
faeces, is another instance of a sphincter.
Section 25. The food at this stage is called chyme; it is an acid and
soup-like fluid-- acid through the influence of the gastric juice. The
temperature of the animal's body is sufficiently high to keep most of
the fat in the food melted and floating in oily drops; much of the
starch, has been changed to sugar, and the solid proteids to soluble
peptones, but many fragments of material still float unchanged.
Section 26. It meets now with the bile, a greenish fluid secreted by
that large and conspicuous gland the liver. The bile is not simply a
digestive secretion, like the saliva or the gastric juice; it contains
matters destined to mix in, and after a certain amount of change to
be passed out of the body with, the faeces; among these
substances, of which some portion is doubtless excretory, are
compounds containing sulphur-- the bile salts. There is also a
colouring matter, bili verdin, which may possibly also be excretory. If
the student will compare Sections 10 and 11, he will notice that in
those paragraphs no account is taken of the sulphur among the
katastases, the account does not balance, and he will at once see
that here probably is the missing item on the outgoing side. The bile,
through the presence of these salts, is strongly alkaline, and so
stops the action of the gastric juice, and prepares for that of the
pancreas, which can act only in an alkaline medium. The fermentive
action of the bile is trifling; it dissolves fats, to a certain extent, and
is antiseptic, that is, it prevents putrefaction to which the chyme
might be liable; it also seems to act as a natural purgative.
Section 27. The bile, as we shall see later, is by no means the sole
product of the liver.
Section 28. The pancreatic juice, the secretion of the pancreas is
remarkable as acting on all the food stuffs that have not already
become soluble. It emulsifies fats, that is, it breaks, the drops up
into extremely small globules, forming a milky fluid, and it
furthermore has a fermentive action upon them; it splits them up into
fatty acids, and the soluble body glycerine. The fatty acids combine
with alkaline substances (Section 26) to form bodies which belong to
the chemical group of Soaps, and which are soluble also. The
pancreatic juice also attacks any proteids that have escaped the
gastric juice, and converts them into peptones, and any residual
starch into sugar. Hence by this stage, in the duodenum, all the food
constituents noticed in Section 17 are changed into soluble forms.
There are probably, three distinct ferments in the pancreatic juice
acting respectively on starch, fat, and proteid, but they have not
been isolated, and the term pancreatin is sometimes used to
suggest the three together.
Section 29. A succus entericus, a saliva-like fluid secreted by
numerous small glands in the intestine wall (Brunner's glands,
Lieberkuhnian follicles), probably aids, to an unknown but
comparatively small extent, in the digestive processes.
Section 30. The walls of the whole of the small intestine are engaged
in the absorption of the soluble results of digestion. In the
duodenum, especially, small processes, the villi project into the
cavity, and being, like the small hairs of velvet pile, and as thickly
set, give its inner coat a velvety appearance. In a villus we find
(Figure IX., Sheet 3) a series of small blood-vessels and with it
another vessel called a lacteal. The lacteals run together into larger
and larger branches until they form a main trunk, the thoracic duct,
which opens into the blood circulation at a point near the heart; but
of this we shall speak further later. They contain, after a meal, a fluid
called chyle.
Section 31. Emulsified fats pass into the chyle. Water and diffusible
salts certainly pass into the vein. The course taken by the peptones
is uncertain, but Professor Foster favours the chyle in the case of
the rabbit-- the student should read his Text-book of Physiology,
Part 2, Chapter 1, Section 11, if interested in the further discussion
of this question.
Section 32. The processes that occur in the remaining portions of
the alimentary canal are imperfectly understood. The caecum is so
large in the rabbit that it must almost certainly be of considerable
importance. In carnivorous animals it may be so much reduced as to
be practically absent. An important factor in the diet of the
herbivorous animals, and one absent from the food of the carnivora,
is that carbohydrate, the building material of all green-meat- [food],
cellulose, and there is some ground for thinking that the caecum is
probably a region of special fermentive action upon it. The pancreatic
juice, it may be noted, exercises a slight digestive activity upon this
substance.
Section 33. Water is most largely absorbed in the large intestine,
and in it the rejected (mainly insoluble) portion of the food gradually
acquires its dark colour and other faecal characteristics.
3. _The Circulation_
Section 34. The next thing to consider is the distribution of the food
material absorbed through the walls of the alimentary canal to the
living and active parts of the body. This is one of the functions of the
series of structures-- heart and blood-vessels, called the circulation,
circulatory system, or vascular system. It is not the only function.
The blood also carries the oxygen from the lungs to the various parts
where work is done and kataboly occurs, and it carries away the
katastases to the points where they are excreted-- the carbon
dioxide and some water to the lungs, water and urea to the kidneys,
sulphur compounds of some kind to the liver.
Section 35. The blood (Figure 4, Sheet 2) is not homogeneous;
under the low power of the microscope it may be seen to consist of--
(1.) a clear fluid, the plasma, in which float--
(2.) a few transparent colourless bodies of indefinite and changing
shape, and having a central brighter portion, the nucleus with a still
brighter dot therein the nucleolus-- the white corpuscles (w.c.), and
(3.) flat round discs, without a nucleus, the red corpuscles (r.c.),
greatly more numerous than the white.
Section 36. The chyle of the lacteals passes, as we have said, by
the thoracic duct directly into the circulation. It enters the left vena
cava superior (l.v.c.s.) near where this joins the jugular vein (ex.j.)
(see Figure 1, Sheet 2, th.d.) and goes on at once with the rest of
the blood to the heart. The small veins of the villi, however, which also
help suck up the soluble nutritive material, are not directly
continuous with the other body veins, the systemic veins; they
belong to a special system, and, running together into larger and
larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.)
veins, which unite to form the portal vein (p.v.) which enters the liver
(l.v.) and there breaks up again into smaller and smaller branches.
The very finest ramifications of this spreading network are called the
(liver) capillaries, and these again unite to form at last the hepatic
vein (h.v.) which enters the vena cava inferior (v.c.i.), a median
vessel, running directly to the heart. This capillary network in the
liver is probably connected with changes requisite before the
recently absorbed materials can enter the general blood current.
Section 37. The student has probably already heard the terms vein
and artery employed. In the rabbit a vein is a vessel bringing blood
towards the heart, while an artery is a vessel conducting it away.
Veins are thin-walled, and therefore flabby, a conspicuous purple
when full of blood, and when empty through bleeding and collapsed
sometimes difficult to make out in dissection. They are formed by
the union of lesser factors. The portal breaks up into lesser branches
within the liver. Arteries have thick muscular and elastic walls, thick
enough to prevent the blood showing through, and are therefore pale
pink or white and keep their round shape.
Section 38. The heart of the rabbit is divided by partitions into four
chambers: two upper thin-walled ones, the auricles (au.), and two
lower ones, both, and especially the left, with very muscular walls,
the ventricles (vn.). The right ventricle (r.vn.) and auricle (r.au.)
communicate, and the left ventricle (l.vn.) and auricle (l.au.).
Section 39. The blood coming from all parts of the body, partly
robbed of its oxygen and containing much carbon dioxide and other
katastases, enters the right auricle of the heart through three great
veins, the median vena cava inferior from the posterior parts of the
body, and the paired venae cavae superiores from the anterior. With
the beating of the heart, described below, it is forced into the right
ventricle and from there through the pulmonary artery (p.a.) seen in
the figure passing under the loop of the aorta (ao.) to the lungs.
Section 40. The lungs (lg. Figure 1, Sheet 1) are moulded to the
shape of the thoracic cavity and heart; they communicate with the
pharynx by the trachea (tr. in Figure 1, Sheet 1) or windpipe, and
are made up of a tissue of continually branching and diminishing
air-tubes, which end at last in small air-sacs, the alveoli. The
final branches of the pulmonary arteries, the lung capillaries, lie
in the walls of these air-sacs, and are separated from the air by an
extremely thin membrane through which the oxygen diffuses into,
and the carbon dioxide escapes from, the blood.
Section 41. The mechanism of respiration will be understood by
reference to Figure 3, Sheet 2. It will be noted, in dissecting that the
lungs have shrunk away from the walls of the thorax; this
collapse occurs directly an aperture is made in the thorax wall, and
is in part due to their extreme elasticity. In life the cavity of the
thorax forms an air-tight box, between which and the lungs is a
slight space, the pleural cavity (pl.c.) lined by a moist membrane,
which is also reflected, over the lungs. The thorax wall is muscular
and bony, and resists the atmospheric pressure on its outer side, so
that the lungs before this is cut through are kept distended to the
size of the thoracic cavity by the pressure of the air within them. In
inspiration (or breathing-in) the ribs are raised by the external
intercostal (Anglice, between-ribs, e.i.c.m.) and other allied
muscles, and the diaphragm (dia.) contracts and becomes flatter;
the air is consequently sucked, in as the lungs follow the movement
of the thorax wall. In expiration the intercostals and diaphragm relax
and allow the elastic recoil of the lungs to come into play. The
thoracic wall is simultaneously depressed by the muscles of the
abdominal area, the diaphragm thrust forwards, as the result of the
displacement and compression of the alimentary viscera thus
brought about. (r.r.r. in the Figure mark ribs.)
Section 42. The oxygen and carbon dioxide are not carried in
exactly the same way by the blood. The student will know from his
chemical reading that neither of these gases is very soluble, but
carbon dioxide is sufficiently so in an alkaline fluid to be conveyed
by the liquid plasma. The oxygen however, needs a special portative
mechanism in the colouring matter of the red corpuscles, the
haemoglobin, with which it combines weakly to form
oxy-haemoglobin of a bright red colour, and decomposing easily in
the capillaries (the finest vessels between the arteries and veins), to
release the oxygen again. The same compound occurs in all true
vertebrata, and in the blood-fluid of the worm; in the crayfish a similar
substance, haemocyanin, which when oxygenated is blue, and when
deoxydized colourless, discharges the same function.
Section 43. The blood returns from the lungs to the left auricle (l.au.)
by the pulmonary veins, hidden in the Figure by the heart, passes
thence to the thick-walled left ventricle (l.vn.), and on into the aorta
(ao.).
Section 44. The beating of the heart is, of course, a succession of
contractions and expansions of its muscular wall. The contraction,
or systole, commences at the base of the venae cavae and passes
to the auricles, driving the blood before it into the ventricles, which
then contract sharply and drive it on into the aorta or pulmonary
artery; a pause and then a dilatation, the diastole follows. The flow of
the blood is determined in one direction by the various valves of the
heart. No valves occur in the opening of the superior cavae but an
imperfect one, the Eustachian valve, protects the inferior cava; the
direction of the heart's contraction prevents any excessive back-flow
into the veins, and the onward, tendency is encouraged by the suck
of the diastole of the ventricles. Between the left ventricle and auricle
is a valve made up of two flaps of skin, the mitral valve, the edges of
the flaps being connected with the walls of the ventricle through the
intermediation of small muscular threads, the chordae tendinae,
which stretch across its cavity to little muscular pillars, the papillary
muscles; these attachments prevent the mitral valve from flapping
back into the auricle, and as the blood flows into and accumulates in
the ventricle it gets behind the flaps of the valve and presses its
edges together. When the systole of the ventricle occurs, the
increased, tension of the blood only closes the aperture the tighter,
and the current passes on into the aorta, where we find three
watch-pocket valves, with the pocket turned away from the heart,
which are also closed and tightened by any attempt at regurgitation
(back-flow). A similar process occurs on the right side of the heart,
but here, instead of a mitral valve of two flaps between auricle and
ventricle, we have a tricuspid valve with three. The thickness of the
muscular walls, in view of the lesser distance through which it has to
force the blood, -are- [is] less for the right ventricle than the left.
Section 45. The following are the chief branches of the aorta. The
student should be able to follow them with certainty in dissection;
they are all displayed in the Figure; but it must not be imagined for a
moment that familiarity with this diagram will obviate the necessity
for the practical work; (in.) is the innominate artery; it forks into
(s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid.
Each carotid splits at the angle of the jaw into an internal and an
external branch. The left common carotid, (l.c.c.) arises from the
base of the innominate,* (l.s.cl.a.) the left subclavian, directly from
the aorta. The aorta now curves round to the dorsal middle line, and
runs down as seen in Figure 1, Sheet 1 (d.ao.) and Figure 1, Sheet
2 (d.ao.). Small branches are given off to the ribs, and then comes
the median coeliac (coe.a.) to the stomach and spleen, the median
superior mesenteric (s.mes.a.) to the main portion of the intestine,
and the inferior mesenteric (p.m.a.) to the rectum. Note that no veins
to the inferior vena cava correspond to these arteries-- the blood they
supply going back by the portal vein (p.v.). The paired renal arteries
(r.a.) supply the kidneys, and the common iliacs (c.il.a.) the hind
legs, splitting into the internal iliacs (i.il.a.) and the femoral (f.).
{Lines from Second Edition only.}
[The student should note that the only arteries in the middle line
are those supplying the alimentary canal.]
{Lines from First Edition only.}
* -The figure is inaccurate, and represents the left common carotid
as arising from the aortic arch.-
Section 46. The distribution of the veins of the rabbit has only a
superficial parallelism with arteries. The chief factors of vena cava
inferior are the hepatic vein (h.v.), which receives the liver blood,
the renal veins (r.v.), from the kidneys, the ilaeo-lumbar, from the
abdominal wall, and the external (e.il.v.) and internal ilias (i.il.v.);
with the exception of the renal veins none of these run side by side
with arteries. The superior cavae (r. and l.v.c.s.) are formed by the
union of internal (i.j.) and external jugular (e.j.) veins with a
subclavian (s.cl.v.) from the fore limb. The term pre-caval vein is
sometimes used for superior cava. The attention, of the student is called
to the small azygos vein (az.) running into the right vena cava superior,
and forming the only asymmetrical (not-balancing) feature of the veins
in front of the heart; it brings blood back from the ribs of the thorax
wall, and is of interest mainly because it answers to an enormous
main vessel, the right post-cardinal sinus, in fishes. There are
spermatic arteries and veins (s.v. and a.) to the genital organs. All
these vessels should be patiently dissected out by the student, and
drawn.
Section 47. Between the final branches of the arteries and the first
fine factors of the veins, and joining them, come the systemic
capillaries. These smallest and ultimate ramifications of the
circulation penetrate every living part of the animal, so that if we
could isolate the vascular system we should have the complete form
of the rabbit in a closely-meshed network. It is in the capillaries that
the exchange of gases occurs and that nutritive material passes out
to the tissues and katastases in from them; they are the essential
factor in the circulatory system of the mammal-- veins, arteries, and
heart simply exist to remove and replace their contents. The details of
the branching of the pulmonary artery and the pulmonary veins need
not detain us now.
Section 48. Summarising the course of the circulation, starting from
the right ventricle, we have-- pulmonary artery, pulmonary capillaries,
pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic
capillaries. After this, from all parts except the spleen and alimentary
canal, the blood returns to systemic veins, superior or inferior cavae,
right auricle, and right ventricle. The blood from the stomach spleen,
and intestines however, passes via {through} the portal vein to the liver
capillaries and then through the hepatic vein to inferior cava, and so
on. Material leaves the blood to be excreted in lungs, kidneys, by the
skin (as perspiration), and elsewhere. New material enters most
conspicuously;
(a) by the portal veins portal veins and
(b) by the thoracic duct and left superior cava.
Section 49. The following table summarises what we have learnt up to
the present of the physiology of the Rabbit, considered as a
mechanism using up food and oxygen and disengaging energy:--
-Air_ {Nitrogen... returned unchanged.}
{Oxygen... through Pulmonary Vein to--} {see 3.}
-Food_ {Carbo-Hydrates (Starch, Sugar, Cellulose.)} Sugar.
{Protein.} {Peptones.}
{Fat (little in Rabbit.)} {Glycerine, and fatty acids in soups.}
{Rejected matter got rid of in Defaecation.}
1a. {Chyle in Lacteals going via {through} Thoracic Duct and Left
Superior Cava to--} {see 2.}
1b. {Veins of Villi--}
{Portal Vein--}
{Liver--}
{Hepatic Vein and Inferior Cava to--} {see 2.}
2. {Right side of heart; then to lungs, and then to--} {see 3.}
3. {Left side of heart; whence to Systemic Arteries and Capillaries.}
4. {The tissues and -Kataboly_.}
5. {Urea (?Liver) Kidney and Sweat Glands}
{CO2} {Lungs}
{H2O} {Lungs, Kidney, Sweat Glands}
{Other Substances} {Mainly by [Kidney,] Liver and Alimentary Canal}
4. _The Amoeba. Cells, and Tissue_
Section 50. We have thus seen how the nutritive material is taken into
the animal's system and distributed over its body, and incidentally, we
have noted how the resultant products of the creature's activity are
removed. The essence of the whole process, as we have already
stated, is the decomposition and partial oxydation of certain complex
chemical compounds to water, carbon dioxide, a low nitrogenous
body, which finally takes the form of urea, and other substances. We
may now go on to a more detailed study, the microscopic study, or
histology, of the tissues in which metaboly and kataboly occur, but
before we do this it will be convenient to glance for a moment at
another of our animal types-- the Amoeba, the lowest as the rabbit is
the highest, in our series.
Section 51. This is shown in Figure III., Sheet 3, as it would appear
under the low power of the microscope. We have a mass of a clear,
transparent, greyish substance called protoplasm, granular in
places, and with a clearer border; within this is a denser portion called
the nucleus, or endoplast (n.), which, under the microscope, by
transmitted light, appear brighter, and within that a still denser spot,
the nucleolus (ns.) or endoplastule. The protoplasm is more or less
extensively excavated by fluid spaces, vacuoles; one clearer circular
space or vacuole, which is invariably present, appears at intervals,
enlarges gradually, and then vanishes abruptly, to reappear after a
brief interval; this is called the contractile vacuole (c.v.). The
amoeba is constantly changing its shape, whence its older name of
the Proteus animalcule, thrusting out masses of its substance in
one direction, and withdrawing from another, and hence slowly
creeping about. These thrust-out parts, in its outline, are called
pseudopodia (ps.). By means of them it gradually creeps round and
encloses its food. Little particles of nutritive matter are usually to be
detected in the homogeneous protoplasm of its body; commonly
these are surrounded by a drop of water taken in with them, and the
drop of water is then called a food vacuole. The process of taking in
food is called ingestion. The amoeba, in all probability, performs
essentially the same chemical process as we have summarised in
Sections 10, 11, 12; it ingests food, digests it in the food vacuoles
and builds it up into its body protoplasm, to undergo kataboly and
furnish the force of its motion-- the contractile vacuole, is probably
respiratory and perhaps excretory, accumulating and then, by its
"systole" (compare Section 44), forcing out of its body, the water,
carbon dioxide, urea, and other katastases, which are formed
concomitantly with its activity. The amoeba reproduces itself in the
simplest way; the nucleus occasionally divides into two portions and
a widening fissure in the protoplasm of the animal's body separates
one from the other. It is impossible to say that one is the parent cell,
and the other the offspring; the amoeba we merely perceive, was one
and is now two. It is curious to note, therefore, that the amoeba is,
in a sense, immortal-- that the living nucleus of one of these minute
creatures that we examine to-day under a microscope may have
conceivably drawn, out an unbroken thread of life since the remotest
epochs of the world's history. Although no sexual intercourse can be
observed, there is reason to believe that a process of supposed
"cannabalism," in which a larger amoeba may occasionally engulph
a smaller one, is really a conjugative reproductive process, and
followed by increased vitality and division.
Section 52. Now if the student will compare Section 35, he will see
that in the white blood corpuscles we have a very remarkable
resemblance to the amoeba; the contractile vacuole is absent, but we
have the protoplasmic body, the nucleus and nucleolus, and those
creeping fluctuations of shape through the thrusting out and
withdrawal of pseudopodia, which constitute "amoeboid" motion. They
also multiply, in the same way, by division.
Section 53. It is not only in the white corpuscle of the blood that we
find this resemblance; in all the firmer parts of the body we find, on
microscopic examination, similar little blebs of protoplasm, and at an
early stage of development the young rabbit is simply one mass of
these protoplasmic bodies. Their division and multiplication is an
essential condition, of growth. Through an unfortunate accident, these
protoplasmic blebs, which constitute the living basis of the animal
body, have come to be styled "cells," though the term "corpuscles" is
far more appropriate.
Section 54. The word is "cell" suggests something enclosed by firm
and definite walls, and it was first employed in vegetable histology.
Unlike the typical cells of animals, the cells of most plants are not
naked protoplasm, but protoplasm enclosed in a wall of substance
(cell wall) called cellulose. The presence of this cellulose cell wall,
and the consequent necessity of feeding entirely upon liquids and
gases that soak through it instead of being able to ingest a portion of
solid food is indeed, the primary distinction between the
vegetable and the animal kingdoms, as ordinarily considered.
Section 55. Throughout life, millions of these cells retain their
primary characters, and constitute the white corpuscles of blood,
"phagocytes," and connective tissue corpuscles; others again,
engage in the formation of material round themselves, and lie, in
such cases, as gristle and bone, embedded in the substance they
have formed; others again, undergo great changes in form and internal
structure, and become permanently modified into, for instance, nerve
fibres and muscle substance. The various substances arising in this
way through the activity of cells are called tissues, the building
materials of that complex thing, the animal body. Since such a
creature as the rabbit is formed through the co-operation of a vast
multitude of cells, it is called multicellular; the amoeba, on the other
hand, is unicellular. The rabbit may be thus regarded as a vast
community of amoeboid creatures and their products.
Section 56. Figure IV., Sheet 3 represents, diagrammatically,
embryonic tissue, of which, to begin with, the whole animal
consists. The cells are all living, capable of dividing and similar, but as
development proceeds, they differentiate, some take on one kind of
duty (function), and some another, like boys taking to different
trades on leaving school, and wide differences in structure and
interdependence become apparent.
Section 57. It is convenient to divide tissues into three classes,
though the divisions are by no means clearly marked, nor have they
any scientific value. The first of these comprises tissues composed
wholly, or with the exception of an almost imperceptible cementing
substance, of cells; the second division includes the skeletal tissues,
the tissue of mesentery, and the connective and basement tissue of
most of the organs, tissues which, generally speaking, consist of a
matrix or embedding substance, formed by the cells and outside of
them, as well as the cells themselves; and, thirdly, muscular and
nervous tissue. We shall study the former two in this chapter, and
defer the third division until later.
Section 58. The outer layer of the skin (the epidermis), the inmost
lining of the alimentary canal, the lining of the body cavity, and the
inner linings of blood-vessels, glands, and various ducts constitute our
first division. The general name for such tissues is epithelium. When
the cells are more or less flattened, they form squamous epithelium
(Figure VI.) such as we find lining the inside of a man's cheek (from
which the cells sq.ep. were taken) or covering the mesentery of
various types-- sq.end. are from the mesentery (Section 16) of a frog.
A short cylindroidal form of cell makes up columnar epithelium, seen
typically in the cells covering the villi of the duodenum (Figure V.).
This epithelium of the villi has the outer border curiously striated, and
this is usually spoken of as leading towards "ciliated" epithelium, to
be described immediately. The epithelium of the epididermis is
stratified-- that is to say, has many thicknesses of cells; the deeper
layers are alive and dividing (stratum mucosum), while the more
superficial are increasingly flattened and drier as the surface is
approached (stratum corneum) and are continually being rubbed off
and replaced from below.
Section 59. In the branching air-tubes of the lung, the central canal of
the spinal cord, and in the ureters of the rabbit, and in most other
types, in various organs, we find ciliated epithelium (Figure VII.).
This is columnar or cubical in form, and with the free edge curiously
modified and beset with a number of hair-like processes, the cilia, by
which, during the life of the cell, a waving motion is sustained in one
direction. This motion assists in maintaining a current in the contents
of ducts which are lined with this tissue. The motion is independent of
the general life of the animal, so long as the constituent cell still
lives, and so it is easy for the student to witness it himself with a
microscope having a 1/4-inch or 1/6-inch objective. Very fine cilia may
be seen by gently scraping the roof of a frog's mouth (the cells figured
are from this source), or the gill of a recently killed mussel, and
mounting at once in water, or, better, in a very weak solution of
common salt.
Section 60. The lining of glands is secretory epithelium; the cells
are usually cubical or polygonal (8, g.ep.), and they display in the
most characteristic form what is called metabolism. Anaboly (see
Section 14) we have defined, as a chemical change in an upward
direction-- less stable and more complex compounds are built up in
the processes of vegetable and animal activity towards protoplasm;
kataboly is a chemical running down; metaboly is a more general
term, covering all vital chemical changes. The products of the action
of a glandular epithelium are metabolic products, material derived from
the blood is worked, up within the cell, not necessarily with
conspicuous gain or loss of energy, and discharged into the gland
space. The most striking case of this action is in the "goblet cells"
that are found among the villi; these are simply glands of one cell,
unicellular glands, and in Figure V. we see three stages in their
action: at g.c.1 material (secretion) is seen forming in the cell, at
g.c.2 it approaches the outer border, and at g.c.3 it has been
discharged, leaving a hollowed cell. Usually however, the escape of
secreted matter is not so conspicuous, and the gland-cells are
collected as the lining of pits, simple, as in the gastric, pyloric, and
Lieberkuhnian glands (Figure VIII., Sections 23, 29), or branching
like a tree or a bunch of grapes (Figure r.g.), as in Brunner's glands
(Section 29) the pancreas, and the salivary glands. The salivary
glands, we may mention, are a pair internal to the posterior ventral
angle of the jaw, the sub-maxillary; a pair anterior to these, the
sub-lingual; a pair posterior to the jaw beneath the ear, the parotid,
and a pair beneath the eye, the infra orbital.