The vertebrate skeleton is a support and locomotion device, special to vertebrates, in which it forms an internal framework providing strong supports on which attach the muscles. The bones that compose it are united to one another by articulations, and from this assemblage derive the general form of the body. It is essentially constituted by a central stem, the vertebral column or rachis, at the top of which the skull blossoms; on which, in the dorsal region, is fixed the thorax; at the bottom of which are articulated the coxal bones to form the pelvis, and finally on the thoracic and coxal belts of which are suspended the thoracic and pelvic limbs. The vertebral column and the skull, consisting of the union of pure or modified vertebrae, form the axial or craniospinal skeleton; the bones of the limbs, including the scapula-clavicular and pelvic belts, constitute the appendicular skeleton.

More About the Vertebrate Skeleton

The name Vertebrates has been given and remains applied to certain animals. It is Cuvier who, in his Animal Kingdom, brings together for the first time the classes of Mammals, Birds, Reptiles, and Linnaeus Pisces and has recreated the great general plan of organization that characterizes this great group. Vertebrates are the animals whose functions have the most variety and extent, of which the organism offers the greatest complication. Their nervous system is twofold; that is to say, it comprises two series of nervous centers, the cerebrospinal nervous axis and the double ganglionic chain, called the great sympathetic.

The Cerebrospinal Axis

In the cerebrospinal axis, we distinguish as main parts, first a brain including olfactory lobes, brain, optic lobes, and cerebellum, then a spinal cord. The cerebrospinal axis is entirely situated on the same side of the digestive canal, towards the dorsal surface of the body; he is withdrawn. in a kind of bone or rarely cartilaginous case, composed of the skull, or encephalic box, and of the spinal column, the bone sheath of the spinal cord. The whole body is supported by a series of bony parts on which the fleshy parts are arranged and which are articulated between them to allow movements. This is what is called the vertebrate skeleton, and it has the axis of the spine. Finally, this column, in the majority of vertebrates, is formed of similar small bones arranged in series and which one names vertebrae.

The presence of the vertebral column and vertebrae is a very salient feature of the plan of organization that occupies us; we understand that this character has become the origin of the name used to designate this type.

Red Blood

Blood circulating in channels of two kinds is set in motion by two hearts attached to each other. Each is formed of an atrium and a ventricle (mammals and birds), sometimes by only one of these organs (fish) or even by an incompletely double heart (reptiles), which is caused by the fusion of the two ventricles into one. All vertebrates have capillaries intermediate in all parts of their body between the peripheral end of the arteries and the beginning of the veins. They also have lymphatic vessels and chyliferous vessels.

Breathing

Breathing is done by lungs (mammals, birds, and reptiles) or by gills (fish and many young amphibians breathing fishlike air that is dissolved in water); sometimes: simultaneously by lungs and gills (perennial batrachians); in this last case, they can breathe indifferently in the air out in the water and are amphibious in all the meaning of this word.

The Intestinal Canal

The intestinal canal of these animals is always complete, that is to say, provided with two openings, one oral, the other anal, and it is equipped with conglomerated glands more or less voluminous (salivary glands, liver, pancreas). Many species have the anterior orifice of the intestinal canal, lined with teeth, kinds of bones of a special nature, whose principal use is to be used to grind food.

Two Pairs of Limbs

The body of vertebrates affects very different forms according to families and even genera; or species. He never has more than two pairs of limbs, the jaws move vertically, and the organs, special senses, acquire a complication far superior to that which they have in the rest of the animals.

The integuments are different according to the groups: they consist of hairs in mammals, feathers in birds, a scaly epidermis in ordinary reptiles, and usually in scales among the fish, the skin is sometimes solidified by a true ossification of its dermal tissue.

The Four Classes of Vertebrates

G. Cuvier recognized in his branch of vertebrates four classes, mammals; birds; Reptiles, and Pisces; Blainville was the first to show that ordinary reptiles and amphibians should form two classes instead of one. He remarked that the former are more like birds in their basic characters, and the latter is closer to the fish. The discoveries of researchers who have dealt with the development of animals have shown interest in this distinction. They have shown that scaly reptiles (turtles, crocodiles, lizards, and snakes) have the same mode of development as birds and mammals, while amphibians follow the development plan of fish. Other distinctions and groupings followed. Thus the Reptiles appeared as a rather heterogeneous assemblage where the Turtles are distinguished and where the Crocodilians must be brought together, not only between them but also with the Birds. Birds and Reptiles “classics” form the group of Sauropsidae.

– The super-class of Gnathostomes corresponds to organisms having a complete jaw. There are Tetrapods, which traditionally form six classes: Mammals, Birds, Reptiles, Batrachians (or Amphibians), and Pisces, which are two classes: Chondrichthyes and Osteichthyes.

– The super-class Agnathes represent organisms formerly ranked among the fish but without a lower jaw (feeding by suction). These are lampreys and hagfishes. (P.G.).

The Formation of the Spine

The spine is formed of 26 short bony bones articulated between them, the vertebrae. It supports a considerable bone box, the skull, which gives shelter to the brain and consists of 8 bones articulated between them. Below, it ends with a series of modified and partially fused vertebrae (false vertebrae) that bear the name of sacrum and coccyx. In this column are appendices:

(1) In front and below the skull, the bones of the face, 14 in number, among which the maxillary upper and lower limit a cavity, the oral cavity;

(2) In front of its middle portion, elastic arches, the ribs with the costal cartilages, which terminate, forward, at another column, the sternopal column, to form the thoracic cage;

(3) At the dividing line of the cervical and thoracic regions, and on each side, the scapular girdle with its prolongation, the thoracic limb, successively composed of the bone of the arm or humerus, the bones of the forearm, ulna, and radius, and bones of the hand, bones of carp, metacarpus, and phalanges of the fingers;

(4) At the level of the sacrum, the pelvic girdle with its prolongation, the pelvic limb, successively composed of the bone of the thigh or femur, the bones of the leg, the tibia, and the fibula, and the bones of the foot, bones of the tarsus, metatarsal, and phalanges of the toes.

The Appearance of the Vertebrate Skeleton

The general appearance of the vertebrate skeleton varies greatly according to the kind of station and locomotion of the animal; it varies with the kind of life of the animal species to which it belongs. That is why this aspect varies so much: in the human, total biped, in other hominoids, imperfect bipeds; in other apes, primates climbers; in terrestrial, aerial, aquatic mammals; in birds, in reptiles, in fish.

The human being is the only one of the animals that constantly stands straight and heads up. His eyes are in front, and his attitude is frankly bipedal. This attitude of the human being owes it essentially to the shape of its skeleton, especially the inclination of its pelvis on the spine, the central position of the occipital hole at the base of the skull, and the straightness of the spine. Even though it is doubly curved alternately, it is no less straight. In quadrupeds, the balance of the skull on the vertebral column is broken by the retreat of this hole and its inclination relative to the horizontal.

The development of the vertebrate skeleton is generally completed only around the age of twenty-five. It is earlier in women. It then comprises at least 210 bones.

The Functions of the Vertebrate Skeleton

The functions of the vertebrate skeleton are: to form a stem that is both strong and flexible to the body; to provide protective cavities to the nervous centers, to the sense organs, to the heart, to the lungs, to the viscera; to offer points of attachment to the muscles, and to determine, by its articulations, the extent and direction of the movements. The weight of the dry skeleton is, for men, 4 to 6 kilograms on average, and for women, from 3 to 4 kilograms; the woman has smaller bones, more slender, with weaker projections.

Possible Changes in the Shape and Proportion, and Size of the Vertebrate Skeleton

The shape and proportions of the vertebrate skeleton and its dimensions change with age, sex, individuals, even social backgrounds, and geographical origins. Thus, to take an example, the proportion of the head to the rest of the body is all the greater as the subject, below adulthood, is younger. The face is also smaller relative to the skull that the subject is younger, and so is the pelvis relative to the thorax. The length ratio between the different segments of the skeleton is fixed enough that with one bone, the size of the subject to which this bone belonged can be restored.

Until puberty, the skeleton of the male is hardly distinguishable from the female skeleton. But in adulthood, it is no longer the case. The bones of the woman are more slender; their contours are more rounded, less protruding. The iliac crests of the woman are more enlarged and more flaring; its entire pelvis is wider and shorter than in humans. This enlargement of the pelvis has the effect of further discarding the cotyloid cavities and making the femurs more oblique. The sternum of the woman is shorter than that of the man; its lumbar vertebrae are higher and narrower, which makes it more slender. The skull of the woman is smaller, lighter, and more slender. Its arches and crests are generally not very marked, and its jaws are less massive. In short, it follows from all these differential characters that in the presence of the vertebrate skeleton, can be pronounced without hesitation on sex nine times out of ten. (Ch. Debierre)