266 



HARD WJCKE 'S S CIENCE - G OS SIP. 



internal skeleton consists of a chain of bones, called 

 vertebras, in the axis of the body, and of appendages 

 called limbs, never more than four. Each vertebra 

 has a central mass, the "body," from which, in 

 a typical vertebra, spi-ing two arches — a dorsal arch, 

 which, witli the corresponding arches of the other verte- 

 bra;, forms a canal for the lodgment of the principal 

 mass of the nervous system, the brain and spinal cord, 

 — and a larger ventral arch, which encloses the principal 

 blood-vessels of the body and the alimentary canal. 

 Three, or perhaps four, vertebrae at the anterior end of 

 the body are expanded and modified to form the skull, 

 which contains the brain. The vertebrse from which 

 the hinder limbs spring, are often fused together into 

 a mass, called " os sacrum," and the vertebr^-e of the 

 tail are often reduced to a body, and nothing else, the 

 arches being absent, — as, indeed, the ventral arches 

 are in most vertebrae. Except in the skull and 

 sacrum, the vertebrae are jointed together by means of 

 interlocking processes and an elastic fibro-cartilaginous 

 pad, allowing a little motion between each vertebra and 

 the next. They vary in number, from eight in some of 

 the frogs to many hundreds in snakes. The limbs 

 are not attached directly to the vertebral column, but 

 to intermediate bones ; they consist of long bones so 

 jointed together as to allow of a large amount of 

 movement. 



The materials of which the internal skeleton is 

 composed are cartilage and bone. 



Cartilage is the substance of which the skeleton 

 consists in the early stages of development of all 

 vertebrate animals, and permanently in some of the 

 fishes, as the sturgeon and lamprey. It is also met 

 with throughout life in certain parts of the body, 

 where firmness is required, yet with a certain amount 

 of flexibility, as in the ribs, covering the ends of the 

 bones where they form joints, in the larynx, ear, &c. 

 Cartilage consists, chemically, of chondrin, a nitro- 

 genous substance allied to gelatine, but scarcely dis- 

 solved on boiling. Microscopically, it consists of 

 cells embedded in a matrix. In what is called simple 

 cartilage, as that of the mouse's ear, the matrix is 

 absent, or only just sufficient to bind the cells together, 

 so that the microscope shows a mass of cells very like 

 vegetable pith. In hyaline cartilage, as that of the 

 ribs, the matrix is clear and structureless ; in fibro- 

 cartilage, the matrix is made up of white" or yellow 

 elastic fibres. When cartilage undergoes conversion 

 into bone, the cells multiply and increase in size ; they 

 then arrange themselves in rows perpendicular to the 

 surface of the bone. Particles of phosphate of lime 

 then become deposited in the matrix, while the cells 

 are believed to be converted into the "lacunas" of 

 bone. The flat bones of the skull are not formed 

 from cartilage, but commence as a radiating net-work 

 of bony fibres between two layers of meml^rane. 



Bone consists, chemically, of^by weight of gelatine 

 and § of earthy matter, mostly phosphate of lime. 

 If we burn a bone in an open fire, we get a white, 



brittle, earthy residue retaining the shape of the 

 bone. If, on the other hand, we soak a bone in acid, 

 we dissolve out the earthy matter and find the animal 

 matter left, so that the bone becomes soft and flexible, 

 like cartilage. The animal matter of bone, unlike 

 that of cartilage, is readily soluble in water, especially 

 when digested with it at a high temperature, as in 

 making soup and glue. Cartilage contains no blood- 

 vessels ; bone, on the other hand, is highly vascular. 

 The blood-vessels in the compact parts of bone iixn in 

 channels termed Haversian canals, around which the 

 laminae, of which the bone is made up, are arranged 

 concentrically. Throughout the substance of the 

 bone are scattered small cavities, called lacunre, from 

 which proceed extremely fine branching-tubes, called 

 canaliculi. The lacunse are arranged in concentric 

 circles around the Haversian canals, with which the 

 canaliculi are connected at their inner ends, while 

 externally they anastomose with those of the next 

 circle ; thus a system of channels is formed for the 

 nutrition of the bone. 



Bones are of three classes, long bones, flat bones, 

 and short bones. The long bones are found in the 

 limbs ; they have a more or less cylindrical hollow 

 shaft of compact bone, and an expanded head of loose 

 spongy bone at either end, the surface of which, 

 where it enters into the joint, is covered with a layer 

 of smooth cartilage, to break shocks, and enable the 

 bones to glide easily one upon another. The flat 

 bones, as those of the skull, have two layers of com- 

 pact bone, with an intermediate spongj' layer. The 

 short square bones are composed of spongy tissue. 



The nutrient blood-vessels enter the bone in three 

 ways. 1st. Through a single large oblique canal near 

 the middle of the shaft of long bones. 2nd. Through 

 a series of holes at the articular ends, which convey 

 blood-vessels into the spongy tissue. 3rd. Through 

 a tough vascular membrane which lines the whole 

 surface of the bone, and from which minute blood- 

 vessels pass into its substance. 



Bone has no power of interstitial growth, but can 

 only increase in size by additions to its exterior. 

 Consequently, in order to allow a bone to increase in 

 length, the articular ends of long bones are ossified 

 from independent centres, a layer of cartilage inter- 

 vening between the bony ends and the bony shaft ; 

 by the growth of this layer the bone is increased in 

 length, and it is not until adult age is reached and 

 growth has ceased that the bone becomes welded 

 into a solid piece. 



The dermal skeleton or skin is, in different classes 

 of animals, of very different degrees of density, from 

 the hard bony scales of the ganoid fishes and the 

 crocodile to the soft moist naked skin of the frog ; 

 but we almost always find it, in some parts of the body 

 at least, modified into hard appendages, as nails and 

 hoofs, scales, horns, and, what you will perhaps be 

 surprised to hear mentioned as parts of the skin, 

 teeth. 



