the outer layers of this pulp become successively calcified and added to the substance of the dentine. In young growing teeth the pulp-cavity remains widely open, but in mammals the general rule is that as a tooth gets older and the crown becomes fully formed, the remainder of the pulp becomes converted into one or more tapering roots which are imbedded in the alveolar cavities of the jaws. The opening of the pulp-cavity is then reduced to a minute perforation at the base of each root. A tooth of this kind is called a rooted tooth.
But it is not only in young teeth that the pulp-cavity sometimes remains widely open; for some teeth, such as the tusks of Elephants and the incisor teeth of Rodents, form no roots and continue to grow throughout the animal's life. Such teeth are said to be rootless or to have persistent pulps.
An intermediate condition is seen in some teeth, such as the grinding teeth of Horses. These teeth grow for a very long time, their crowns wearing away as fast as their bases are produced; finally however definite roots are formed and growth ceases.
I. Incisor or tusk of elephant, with pulp-cavity persistently open at base. II. Human incisor during development with root imperfectly formed, and pulp-cavity widely open at base. III. Completely formed human incisor, with pulp-cavity contracted to a small aperture at the end of the root. IV. Human molar with broad crown and two roots. V. Molar of Ox, with the enamel covering the crown, deeply folded and the depressions filled with cement. The surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cement by dots.
The teeth of any animal may be homodont, that is, all having the same general character, or heterodont, that is, having different forms adapted to different functions. The dentition is heterodont in a few reptiles and the majority of mammals.
Succession of teeth. In most fishes, and many amphibians and reptiles the teeth can be renewed indefinitely. In sharks, for example, numerous rows of reserve teeth are to be seen folded back behind those in use (see fig. 15). The majority of mammals have only two sets of teeth, and are said to be diphyodont; some have only a single series (monophyodont).
Development of teeth. A brief sketch of the method in which development of teeth takes place in the higher vertebrates may here be given. Along the surface of the jaws a thickening of the epiblastic epithelium takes place, giving rise to a ridge, which sinks inwards into the tissue of the jaw, and it is known as the primary enamel organ. At the points where teeth are to be developed special ingrowths of this primary enamel organ take place, and into each there projects a vascular dental papilla from the surrounding mesoblast of the jaw. Each ingrowth of the enamel organ forms an enamel cap, which gradually embraces the dental papilla, and at the same time appears to be pushed on one side, owing to the growth not being uniform. The external layer of the dental papilla is composed of long nucleated cells, the odontoblasts, and it is by these that the dentine is formed. Similarly the internal layer of the enamel organ is formed of columnar enamel cells, which give rise to the enamel. The mesoblastic cells surrounding the base of the tooth give rise to the cement.
Bone is in many cases exoskeletal, but it will be most conveniently described with the endoskeleton.
The scales of fish are wholly or in part mesoblastic in origin, being totally different from those of reptiles. The cycloid and ctenoid scales of Teleosteans (see p. 105) are thin plates coated with epidermis. They are sometimes bony, but as a rule are simply calcified. Ganoid scales are flat plates of bone coated with an enamel-like substance, and articulating together with a peg and socket arrangement; they are probably identical with enlarged and flattened placoid scales.
The armour plates of fossil Ganoids, Labyrinthodonts, and Dinosaurs, and of living Crocodiles, some Lizards and Armadillos, are composed of bone. They are always covered by a layer of epidermis.
The antlers of deer are also composed of bone; they will be more fully described in the chapter on mammals. It may perhaps be well to mention them here, though they really belong to the endoskeleton, being outgrowths from the frontal bones.
B. Endoskeletal structures.
I. Hypoblastic.
(a) The notochord is an elastic rod formed of large vacuolated cells, and is surrounded by a membranous sheath of mesoblastic origin. It is the primitive endoskeleton in the Chordata, all of which possess it at some period of their existence; while in many of the lower forms it persists throughout life. Even in the highest Chordata it is the sole representative of the axial skeleton for a considerable part of the early embryonic life. A simple unsegmented notochord persists throughout life in the Cephalochordata, Cyclostomata, and some Pisces, such as Sturgeons and Chimaeroids.
(b) The enamel of the pharyngeal teeth of the Salmon and many other Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum, in which the other teeth are developed, passes into the hypoblast of the mesenteron in which these pharyngeal teeth are formed.
II. Mesoblastic.
The most primitive type of a mesoblastic endoskeleton consists of a membranous sheath surrounding the notochord, as in Myxine and its allies. The first stage of complication is by the development of cartilage in the notochordal sheath, as in Petromyzon. Often the cartilage becomes calcified in places, as in the vertebral centra of Scyllium and other Elasmobranchs. Lastly, the formation of bone takes place; it generally constitutes the most important of the endoskeletal structures.
Bone may be formed in two ways:—
(1) by the direct ossification of pre-existing cartilage, when it is known as cartilage bone or endochondral bone;
(2) by independent ossification in connective tissue; it is then known as membrane or dermal or periosteal bone.
With the exception of the clavicle[4] all the bones of the trunk and limbs, together with a large proportion of those of the skull, are preformed in the embryo in cartilage, and are grouped as cartilage bones; while the clavicle and most of the roofing and jaw-bones of the skull are not preformed in cartilage, being developed simply in connection with a membrane. Hence it is customary to draw a very strong line of distinction between these two kinds of bone; in reality however this distinction is often exaggerated, and the two kinds pass into one another, and as will be shown immediately, the permanent osseous tissue of many of those which are generally regarded as typical cartilage bones, is really to a great extent of periosteal origin. The palatine bone, for instance, of the higher vertebrates in general is preceded by a cartilaginous bar, but is itself almost entirely a membrane bone.
Before describing the development of bone it will be well to briefly describe the structure of adult bone and cartilage.
The commonest kind of cartilage, and that which preforms so many of the bones of the embryo, is hyaline cartilage. It consists of oval nucleated cells occupying cavities (lacunae) in a clear intercellular semitransparent matrix, which is probably secreted by the cells. Sometimes one cell is seen in each lacuna, sometimes shortly after cell-division a lacuna may contain two or more cells. The free surface of the cartilage is invested by a fibrous membrane, the perichondrium.
Bone consists of a series of lamellae of ossified substance between which are oval spaces, the lacunae, giving rise to numerous fine channels, the canaliculi, which radiate off in all directions. The lacunae are occupied by the bone cells which correspond to cartilage cells, from which if the bone is young, processes pass off into the canaliculi. It is obvious that the ossified substance