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DEVELOPMENT OF BONE: OSSIFICATION.

65

for the blood-vessels to pass along them. And it is through their complete blocking up, by a continuance of the same growth, that the supply of blood is cut off from the interior of the bone which forms the antlers of the deer, so that they die and fall off; their shedding and renewal being an annual process.1 .1-Whilst the formation of the Haversian canals and cancelli is being effected by the partial removal of the first formed partitions, a complete cavity is formed in the centre of the shaft of every long bone (at least in Mammals and Birds), by the entire removal of the solid tissue. This cavity is at first not much larger than one of the Haversian canals; but as the bone grows in diameter by additions to the exterior of its shaft, so is the cavity in its interior augmented by the removal (by absorption) of the first-formed bone; and this double process continues until the bone has attained its full diameter. The formation of new bone on the exterior of the shaft seems to be the result of the consolidation-of the fibrous tissue of the periosteum (or membrane covering the bone) by calcareous deposit; the lacunæ being probably the cavities of cells which were entangled in the fibres, and the canaliculi being outgrowths from these; and new fibrous tissue being formed on the outside of the periosteum, to replace that which has been taken into the bone. Thus it comes to pass, that after a time none of the bone first formed in its cartilaginous mould any longer remains, the whole of it having been removed by absorption; since the central cavity of the perfect bone is much larger than the entire cartilaginous shaft in which it originated. And thus it also comes to pass, that (as gelatin is the basis of fibrous tissue) bones yield gelatin, not chondrin, upon being long boiled.— The increase of the shaft in length, however, is the result of a different process. In all bones of any considerable dimensions, the process of ossification commences in more than one point at a time. In the long bones, there are usually three such points; one for the shaft, and the others for the two

1 It is commonly stated that the death of the antlers is due to the formation of a bony ring at their base, which cuts off the supply of blood from the "velvet" which covers them; but though this may contribute to produce the effect, it is by no means the sole cause, as the interior of the antlers is supplied with blood from the vessels of the bone from which they sprout, and not from those of the "velvet" only.

F

66

DEVELOPMENT OF BONE: OSSIFICATION.

extremities. Long after the ossification of the shaft and of the extremities has been completed, these parts remain separated from each other by the interposition of a thin layer of unconsolidated cartilage; so that, although the bone appears firm and complete, its three portions fall apart, if it be macerated sufficiently long in water for the cartilage to decay. Now it is by the progressive consolidation of the cartilage at these two junctions, and by the continual formation of new cartilage as the old is taken into the bone, that the length of the shaft continues to increase up to adult age; and then, its full size having been attained, the whole thickness of the intervening layer of cartilage is replaced by bone, so that the shaft and extremities become firmly consolidated. The general history of the formation of the flat bones is nearly the same. In these, when they are large, or have projecting out-growths, there are several centres of ossification; and although the first ossification takes place in the substance of cartilage, yet the subsequent growth seems to be effected mainly by the consolidation of fibrous membrane.

53. The foregoing description applies chiefly to those higher and more complete forms of Bone, which are found in Birds and Mammals. In Reptiles and Fishes, the process of ossification is stopped short, as it were, at an early period; and thus the texture of their bones resembles that which we find the skeleton to present in the earlier life of the higher animals. The long bones of Reptiles (with one remarkable exception in the Pterodactylus, § 669, which is adapted to the life of a Bird) have no one central cavity, but are penetrated by numerous large Haversian canals, like those of very young bone ; and various pieces remain separate in them throughout life, which, originating in distinct centres of ossification, subsequently coalesce in Birds and Mammals. This permanent separation is still more remarkable in the bones of Fishes; and it is consequently in them that we can best study the real composition of the skeleton,-every piece which originates in a distinct centre of ossification, being, in the eye of the philosophical anatomist, a separate bone. Further, there is a large group of Fishes in which the skeleton retains the cartilaginous character through life; a certain quantity of mineral matter being deposited in the

BONES OF FISHES:-TEETH.

67 cartilage, but its conversion into true bony structure never taking place. In a few, not even a firm cartilage is produced; and all the trace of a skeleton is a cylinder formed of hexagonal cells, resembling those of the pith of plants, which takes the place that is generally occupied by the "bodies" of the vertebræ (§ 71). Such a cylinder, which is termed the chorda dorsalis, precedes the formation of the vertebral column in other vertebrated animals (§ 757). In the curious Amphioxus (ZooL. § 642), even this is wanting; and the only rudiment of the bony skeleton is to be found in the fibrous sheath that surrounds the nervous centres, and sends off prolongations between the successive transverse bands of muscles, which are attached to these, as they are in other fishes to the ribs and the spines of the vertebræ.

54. In connexion with the structure of Bone, it will be
convenient to describe that of Teeth, although the general
description of the form and development of these organs will
be more appropriately given in connexion with the account
of their instrumental uses (§§ 181-183). The principal part
of the substance of all teeth is made up of a solid tissue,
which has been appropriately called Dentine. Of this sub-
stance, one variety, which is peculiarly close in texture, and
susceptible of a high polish, is familiarly known as ivory.
The more perfect forms of dentine, such as present them-
selves in Man and the Mammalia generally, consist of a hard
transparent substance formed by the union of animal matter
and calcareous salts (chiefly phos-

phate of lime), in the proportion
of about 28 of the former to 72 of
the latter; the mineral matter thus
bearing a somewhat larger ratio
to the organic, than it does in
bone. This dentinal substance is
traversed by minute tubuli of
about 1-10,000th of an inch in
diameter, which appear as dark
lines, generally very close to-
gether; these pass in a radiating PORTION OF DENTINE (highly magni-
manner from the central cavity fied), showing its tubular structure.
of the tooth, diverging from each other as they approach
its exterior; but when seen in only a small part of their

[graphic]

Fig. 18.

68

STRUCTURE OF TEETH.

course, they appear to be nearly parallel (fig. 18), though usually more or less wavy. They occasionally divide into two branches, which continue to run, at a little distance from one another, in the same parallel direction; and they also frequently give off small lateral branches, which again send off smaller ones. In some animals the tubuli may be traced at their extremities into minute cavities analogous to the lacunæ of bone; and the lateral branchlets also occasionally terminate in similar cavities. Thus the whole tooth may be likened, in some degree, to a single Haversian system in bone; the central cavity, which is lined by a vascular membrane, representing the Haversian canal, while the radiating tubuli of the former correspond with the radiating canaliculi of the latter; the chief difference lying in the absence of lacunæ along the course of the radiating tubes. In a large proportion of Fishes, however, there is no single central cavity, but the whole tooth is traversed by a system of medullary canals, not only resembling the Haversian, but actually continuous with those of the bone on which the tooth is implanted; and as each of these is the centre of a distinct system of radiating tubuli, the resemblance of their dentine to bone is very close. A somewhat similar condition of the dentine (obviously a lower or less specialized form of this substance) presents itself in certain Reptiles and Mammals.In the Teeth of Man and most other Mammals, and in those of many Reptiles and some Fishes, we find two other sub

stances, one of them harder and the other softer than dentine. The former, which is called Enamel, consists of long prismatic cells, which pass from one surface to the other of the thin layer formed by this substance over the crown, or sometimes in the interior of the tooth (§ 182). These prisms are usually hexagonal in form, as is seen in PORTION OF ENAMEL (highly magni- transverse section (fig. 19); and their course is usually more or less wavy. In teeth which have to sustain an extraordinary amount of compression (as is especially the case with those of

[graphic]

Fig. 19.

fied), showing its component prisms.

TEETH.-MUSCLE AND NERVE.

69.

the Rodentia), the enamel-prisms cross and interlace with one another, in such a manner as to prevent that separation which would readily occur if the direction of all of them were the same. Of all the tissues of the animal body, the Enamel is the most remarkable for the predominance of mineral ingredients; these amount to no fewer than 98 parts in 100, leaving when removed only 2 per cent. of organic matter. The softer component of Teeth, known as the Cementum, or Crusta petrosa, possesses the essential characters of true bone; but when only a thin layer of it is present, we do not find it traversed by medullary canals, its system of lacunæ and canaliculi being then in relation to the nearest vascular surface, as is the case also with very thin laminæ of ordinary bone, such as we find in the scapula (blade-bone) of a Mouse.

55. We come, lastly, to the two tissues which are of the highest importance in the Animal fabric, and to which all the rest are merely subsidiary; namely, the Muscular and the Nervous. It is through the instrumentality of these, that all the actions are performed which essentially constitute Animal life; for the nervous apparatus is the medium by which the consciousness of the individual is affected by what takes place around him, or within his own body, and by which, in his turn, he originates movements in his body, and through it in things external to it; whilst the muscles are, so to speak, the servants of the nerves, doing, with a force of their own, the work which the nerves direct. The relation between the two may be likened to that of the rider and his horse, or of the engine-driver and his locomotive; for the nerves can put forth no motor power by themselves;.. whilst, on the other hand, the muscles (with certain exceptions) remain inert except when stimulated to contract by the agency of the nerves. The muscles use the tendons and the framework of bones, joints, &c., for the mechanical application of their power, as will be shown hereafter (Chap. XII.); but these parts of the fabric have not the slightest power of originating motion by themselves. Hence, all Animal Force takes its rise in one or other of these two tissues; and we shall find that the special purpose of the whole apparatus of Organic life, is, by providing materials for their nutrition and renovation, to build them up in the first instance, and then

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