A History of Science, vol 4 by Henry Smith Williams (best novels to read for beginners txt) 📖

a purple hue; hence the widely different

appearance of arterial and venous blood, which so puzzled the

early physiologists.

 

This proof of the vitally important role played by the red-blood

corpuscles led, naturally, to renewed studies of these

infinitesimal bodies. It was found that they may vary greatly in

number at different periods in the life of the same individual,

proving that they may be both developed and destroyed in the

adult organism. Indeed, extended observations left no reason to

doubt that the process of corpuscle formation and destruction may

be a perfectly normal one—that, in short, every red-blood

corpuscle runs its course and dies like any more elaborate

organism. They are formed constantly in the red marrow of bones,

and are destroyed in the liver, where they contribute to the

formation of the coloring matter of the bile. Whether there are

other seats of such manufacture and destruction of the corpuscles

is not yet fully determined. Nor are histologists agreed as to

whether the red-blood corpuscles themselves are to be regarded as

true cells, or merely as fragments of cells budded out from a

true cell for a special purpose; but in either case there is not

the slightest doubt that the chief function of the red corpuscle

is to carry oxygen.

 

If the oxygen is taken to the ultimate cells before combining

with the combustibles it is to consume, it goes without saying

that these combustibles themselves must be carried there also.

Nor could it be in doubt that the chiefest of these ultimate

tissues, as regards, quantity of fuel required, are the muscles.

A general and comprehensive view of the organism includes, then,

digestive apparatus and lungs as the channels of fuel-supply;

blood and lymph channels as the transportation system; and muscle

cells, united into muscle fibres, as the consumption furnaces,

where fuel is burned and energy transformed and rendered

available for the purposes of the organism, supplemented by a set

of excretory organs, through which the waste products—the

ashes—are eliminated from the system.

 

But there remain, broadly speaking, two other sets of organs

whose size demonstrates their importance in the economy of the

organism, yet whose functions are not accounted for in this

synopsis. These are those glandlike organs, such as the spleen,

which have no ducts and produce no visible secretions, and the

nervous mechanism, whose central organs are the brain and spinal

cord. What offices do these sets of organs perform in the great

labor-specializing aggregation of cells which we call a living

organism?

 

As regards the ductless glands, the first clew to their function

was given when the great Frenchman Claude Bernard (the man of

whom his admirers loved to say, “He is not a physiologist merely;

he is physiology itself”) discovered what is spoken of as the

glycogenic function of the liver. The liver itself, indeed, is

not a ductless organ, but the quantity of its biliary output

seems utterly disproportionate to its enormous size, particularly

when it is considered that in the case of the human species the

liver contains normally about one-fifth of all the blood in the

entire body. Bernard discovered that the blood undergoes a change

of composition in passing through the liver. The liver cells

(the peculiar forms of which had been described by Purkinje,

Henle, and Dutrochet about 1838) have the power to convert

certain of the substances that come to them into a starchlike

compound called glycogen, and to store this substance away till

it is needed by the organism. This capacity of the liver cells

is quite independent of the bile-making power of the same cells;

hence the discovery of this glycogenic function showed that an

organ may have more than one pronounced and important specific

function. But its chief importance was in giving a clew to those

intermediate processes between digestion and final assimilation

that are now known to be of such vital significance in the

economy of the organism.

 

In the forty odd years that have elapsed since this pioneer

observation of Bernard, numerous facts have come to light showing

the extreme importance of such intermediate alterations of

food-supplies in the blood as that performed by the liver. It has

been shown that the pancreas, the spleen, the thyroid gland, the

suprarenal capsules are absolutely essential, each in its own

way, to the health of the organism, through metabolic changes

which they alone seem capable of performing; and it is suspected

that various other tissues, including even the muscles

themselves, have somewhat similar metabolic capacities in

addition to their recognized functions. But so extremely

intricate is the chemistry of the substances involved that in no

single case has the exact nature of the metabolisms wrought by

these organs been fully made out. Each is in its way a chemical

laboratory indispensable to the right conduct of the organism,

but the precise nature of its operations remains inscrutable. The

vast importance of the operations of these intermediate organs is

unquestioned.

 

A consideration of the functions of that other set of organs

known collectively as the nervous system is reserved for a later

chapter.

 

VI. THEORIES OF ORGANIC EVOLUTION

 

GOETHE AND THE METAMORPHOSIS OF PARTS

 

When Coleridge said of Humphry Davy that he might have been the

greatest poet of his time had he not chosen rather to be the

greatest chemist, it is possible that the enthusiasm of the

friend outweighed the caution of the critic. But however that

may be, it is beyond dispute that the man who actually was the

greatest poet of that time might easily have taken the very

highest rank as a scientist had not the muse distracted his

attention. Indeed, despite these distractions, Johann Wolfgang

von Goethe achieved successes in the field of pure science that

would insure permanent recognition for his name had he never

written a stanza of poetry. Such is the versatility that marks

the highest genius.

 

It was in 1790 that Goethe published the work that laid the

foundations of his scientific reputation—the work on the

Metamorphoses of Plants, in which he advanced the novel doctrine

that all parts of the flower are modified or metamorphosed

leaves.

 

“Every one who observes the growth of plants, even

superficially,” wrote Goethe, “will notice that certain external

parts of them become transformed at times and go over into the

forms of the contiguous parts, now completely, now to a greater

or less degree. Thus, for example, the single flower is

transformed into a double one when, instead of stamens, petals

are developed, which are either exactly like the other petals of

the corolla in form, and color or else still bear visible signs

of their origin.

 

“When we observe that it is possible for a plant in this way to

take a step backward, we shall give so much the more heed to the

regular course of nature and learn the laws of transformation

according to which she produces one part through another, and

displays the most varying forms through the modification of one

single organ.

 

“Let us first direct our attention to the plant at the moment

when it develops out of the seed-kernel. The first organs of its

upward growth are known by the name of cotyledons; they have also

been called seed-leaves.

 

“They often appear shapeless, filled with new matter, and are

just as thick as they are broad. Their vessels are

unrecognizable and are hardly to be distinguished from the mass

of the whole; they bear almost no resemblance to a leaf, and we

could easily be misled into regarding them as special organs.

Occasionally, however, they appear as real leaves, their vessels

are capable of the most minute development, their similarity to

the following leaves does not permit us to take them for special

organs, but we recognize them instead to be the first leaves of

the stalk.

 

“The cotyledons are mostly double, and there is an observation to

be made here which will appear still more important as we

proceed—that is, that the leaves of the first node are often

paired, even when the following leaves of the stalk stand

alternately upon it. Here we see an approximation and a joining

of parts which nature afterwards separates and places at a

distance from one another. It is still more remarkable when the

cotyledons take the form of many little leaves gathered about an

axis, and the stalk which grows gradually from their midst

produces the following leaves arranged around it singly in a

whorl. This may be observed very exactly in the growth of the

pinus species. Here a corolla of needles forms at the same time a

calyx, and we shall have occasion to remember the present case in

connection with similar phenomena later.

 

“On the other hand, we observe that even the cotyledons which are

most like a leaf when compared with the following leaves of the

stalk are always more undeveloped or less developed. This is

chiefly noticeable in their margin which is extremely simple and

shows few traces of indentation.

 

“A few or many of the next following leaves are often already

present in the seed, and lie enclosed between the cotyledons; in

their folded state they are known by the name of plumules. Their

form, as compared with the cotyledons and the following leaves,

varies in different plants. Their chief point of variance,

however, from the cotyledons is that they are flat, delicate, and

formed like real leaves generally. They are wholly green, rest on

a visible node, and can no longer deny their relationship to the

following leaves of the stalk, to which, however, they are

usually still inferior, in so far as that their margin is not

completely developed.

 

“The further development, however, goes on ceaselessly in the

leaf, from node to node; its midrib is elongated, and more or

less additional ribs stretch out from this towards the sides. The

leaves now appear notched, deeply indented, or composed of

several small leaves, in which last case they seem to form

complete little branches. The date-palm furnishes a striking

example of such a successive transformation of the simplest leaf

form. A midrib is elongated through a succession of several

leaves, the single fan-shaped leaf becomes torn and diverted, and

a very complicated leaf is developed, which rivals a branch in

form.

 

“The transition to inflorescence takes place more or less

rapidly. In the latter case we usually observe that the leaves of

the stalk loose their different external divisions, and, on the

other hand, spread out more or less in their lower parts where

they are attached to the stalk. If the transition takes place

rapidly, the stalk, suddenly become thinner and more elongated

since the node of the last-developed leaf, shoots up and collects

several leaves around an axis at its end.

 

“That the petals of the calyx are precisely the same organs which

have hitherto appeared as leaves on the stalk, but now stand

grouped about a common centre in an often very different form,

can, as it seems to me, be most clearly demonstrated. Already in

connection with the cotyledons above, we noticed a similar

working of nature. The first species, while they are developing

out of the seed-kernel, display a radiate crown of unmistakable

needles; and in the first childhood of these plants we see

already indicated that force of nature whereby when they are

older their flowering and fruit-giving state will be produced.

 

“We see this force of nature, which collects several leaves

around an axis, produce a still closer union and make these

approximated, modified leaves still more unrecognizable by

joining them together either wholly or partially. The

bell-shaped or so-called one-petalled calices represent these

cloudy connected leaves, which, being more or less indented from

above, or divided, plainly show their origin.

 

“We can observe the transition from the calyx to the corolla in

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