Man is a _vertebrate_ animal.[1] He has (anatomically at least) a
backbone. He belongs to that substantial class of organisms
possessing an articulating internal skeleton—the family of the
fishes, amphibians, reptiles, birds, and mammals. Most animals have
some sort of skeleton, some sort of calcareous contrivance, whose
business it is to give form and protection to the softer parts of
the organism. Some animals, as the starfishes, have plates of lime
scattered throughout the surface parts of the body; others, as the
corals and sponges secrete plant-like frames, upon and among the
branches of which the organisms reside; and still others, as the
clams, crustaceans, and insects, have skeletons consisting of a
shell or sheath on the outside of, and more or less surrounding, the
softer substances of the body. The limbs of insects are tiny tubes
on the inside of which are the miniature muscles with which they
perform their marvels of locomotion. The skeleton of vertebrates,
consisting of levers, beams, columns, and arches, all skilfully
joined together and sunk deep within the muscular tissue, forms a
conspicuous contrast to the rudimentary frames of other animals. The
vertebrate skeleton consists of a hollow axis, divided into segments
and extending along the dorsal region of the body, from the ventral
side of which articulate, by means of awkwardly-constructed girdles,
an anterior and a posterior pair of limbs. This dorsal axis ends in
front in a peculiar bulbous arrangement called the head, which
contains, among other valuables, the brain and buccal cavern. The
thoracic segments of the backbone send off pairs of flat bones,
which, arching ventrally, form the chest for the protection of the
heart and other vitals. The limbs (except in fishes) consist each of
a single long bone, succeeded by two long bones, followed by two
transverse rows of short, irregular wrist or ankle bones, ending
normally in five branching series of bones called digits. This is
essentially the skeleton of all fishes, amphibians, reptiles, birds,
and mammals. In short, it is the universal vertebrate type of frame.
There are minor modifications to suit the various kinds of
environment, adaptations to the necessities of aquatic, terrestrial,
and aerial locomotion and life, some parts being specialised, others
atrophied, and still others omitted, but there is never anywhere,
from fishes to philosophers, any fundamental departure from the
established vertebrate type of skeleton.[2] The pectoral fins of
fishes correspond to the fore-limbs of frogs and reptiles, the wings
of birds, and the arms of men. The pelvic fins of fishes are
homologous with the hind-limbs of frogs, reptiles, and quadrupeds,
and the legs of birds, apes, and men. The foot of the dog and
crocodile, the hand of the orang, and the flipper of the dolphin and
seal, all have the same general structure as the hand of man; and
the wings of the bat and bird, the forelimbs of the lizard and
elephant, and the comical shovels of the mole and ornithorhynchus,
notwithstanding the great differences in their external appearance
and use, contain essentially the same bones and the same arrangement
of the bones as do the arms of men and women. The human body has two
primary cavities in it. So have the bodies of all vertebrates: a
neural cavity containing the brain and spinal cord, and a visceral
cavity containing the heart, liver, lungs, and alimentary canal.
Invertebrates have only one body cavity—the one corresponding to
the visceral cavity of vertebrates—and the main nerve trunk,
instead of extending along the back, as among vertebrates, is in
invertebrates located ventrally. Vertebrates are the only animals on
the earth that have a highly developed circulatory system, a system
entirely shut off from the other systems, and containing a heart,
arteries, veins, and capillaries. In all invertebrates the digestive
and circulatory systems remain to a greater or less extent
connected, the blood and food mingling more or less in the general
cavity of the body. Worms and insects have pulsating tubes instead
of heart and arteries. Crustaceans have hearts with one chamber, and
mollusks have two or three chambered hearts, but the blood, instead
of returning to the heart after its journey through the arteries,
passes into the body cavity. In man and other vertebrates the
circulating current is confined strictly to the bloodvessels, no
particle of it ever escaping into the general body cavity. The heart
of vertebrates is distinguished from that of invertebrates by being
located ventrally. The heart of invertebrates is in the back. The
blood of vertebrates differs from that of invertebrates in
containing both red and white corpuscles. Invertebrates have white
corpuscles only. Worms have yellow, red, or bright green blood. The
blood of crustaceans is bluish, that of mollusks is white, and that
of insects dusky or brown. The blood of all vertebrates, excepting
amphioxus, is red. All backboned beings, whether they dwell in seas
or cities, and whether they build nests or empires, have two eyes,
two ears, nose and mouth, all located in the head, and always
occupying the same relative position to each other. Invertebrates
may have their brains in their abdomen, as do the mites; hear with
their legs or antennae, as many insects do; see with their tunics,
like the scallops; and breathe with their skin, as do the worms. The
crayfish hears with its ‘feelers,’ the cricket and katydid with
their fore-legs, the grasshopper with its abdomen, the clam with its
‘foot,’ and mysis and other low crustaceans have their auditory
organs on their tails.
Man is, then, like the fishes, frogs, reptiles, birds, and
quadrupeds, a vertebrate animal. Excepting in his infancy, when he
is a quadruped going on all fours, he uses his posterior limbs only
for locomotion, and his anterior for prehension and the like. His
spinal axis is erect instead of horizontal, and his tail is
atrophied. But he possesses all of the unmistakable qualities of the
vertebrate type of structure—a two-chambered body cavity, a highly
developed and dorsally located nerve trunk, vertebrate vitals, a
closed circulatory system, a ventral heart, red blood, a head
containing sense organs and brain, and a well-ordered internal
skeleton, consisting of a vertebral column with skull and ribs and
two pairs of limbs, the limbs consisting each of one long bone, two
long bones, two transverse rows of irregular bones, and five
branches at the end.
1. See ‘Classes of Animals,’ at the end of the chapter.
2. Snakes are limbless, and hind-limbs are lacking in whales and
other degenerates; but rudimentary limbs are found in the embryonic
stages of these animals. Frogs, it may be said also, have no ribs.