foramina on the shaft of a long bone, but more easily into the articular ends. A nerve also enters the medullary canal with the nutrient artery of the medulla, and divides like the artery into an ascending and a descending branch. Of all the bones, the tibia presents the largest canal for the nutrient artery of the marrow; in this bone also it is easy to trace the entrance of the nerve with the artery. Though bone in health has but little sensibility, yet, when diseased, it becomes greatly exalted. Mr. Stanley says he has "witnessed the manifestation of pain as acute from the penetration of an inflamed bone by a saw or trephine, as from incisions of the inflamed soft parts." The same author has likewise a chapter on "Neuralgia of Bone." Every surgeon must have witnessed how sensitive are granulations from bone. Indeed, it is probable that the severe pain attendant on the ulceration of articular cartilage is occasioned by the pressure of the cartilage on the bone granulations beneath it.

bone.

ABSORBENTS of The absorbents of bone have not hitherto been actually demonstrated. That bone does possess absorbents, is rendered highly probable from the fact that ivory pegs introduced into bones, for the purpose of consolidating ununited fractures, are in some instances absorbed.

MICROSCOPIC STRUCTURE OF BONE.

This is a most interesting and instructive study. It reveals to us that bones are as minutely provided with blood-vessels and nerves, and in all respects as much cared for, as the softer parts of the body. Being as fully organised as the other parts, we cannot wonder that they are subject to the same diseases. We shall have to investigate how the bones are formed in early life, how they grow to maturity, how their health is maintained, and how their injuries are repaired. Would anyone, looking at a solid bone, expect to find that even its hardest parts are tunnelled out by a network of minute canals for the passage of capillary blood-vessels;

* On Diseases of the Bones. Introduction, p. xi.

and that from these canals other tubes, infinitely more minute, and expanding here and there into reservoirs, radiate in all directions for the purpose of nutrition?

General idea of Let us first get a general idea of the microthe subject. scopic structure of bone, and go into details afterwards. If a transverse section from the shaft of a long bone be ground extremely thin, and examined with a power of about 20 diameters (Plate B, fig. 4), we shall see a number of holes, with dark spots grouped round them, in a series of tolerably concentric circles. These holes are sections of the canals (termed "Haversian," after their discoverer, Clopton Havers *) which transmit blood-vessels into the substance of the bone. The dark spots are minute reservoirs, called "bone corpuscles," "bone cells," or "lacunæ." One would imagine they were solid bodies, but they are really cavities; and their dark appearance in the dry bone is owing to the refraction of the light. As we examine the different parts of the section under the microscope, we notice that the Haversian canals vary considerably in size and shape. They are generally round or oval. Those nearest to the circumference of the bone are very small; but towards the medullary cavity they gradually grow larger, and at last open out into the cells of the cancellous texture.

Let us now examine the same section with a higher power (Plate B, fig. 6), and we shall find that the Haversian canals are surrounded by a series of concentric lines, reminding us of the tranverse section of the branch of a tree. These lines are termed the "laminæ:" they are in truth so many layers or rings of bone that have been developed within the Haversian canal. Understand that even the smallest Haversian canal was, when originally formed, a much wider space, and circumscribed by only a single layer of bone; but in process of growth the canal becomes gradually contracted by the deposit of successive layers of bone. We notice also that the dark spots, before alluded to as the "lacunæ," are situated between the laminæ, and that now, under a higher magnifying power, they look like spiders. The central part of the lacuna, representing the body of the spider, is hollow, and the dark fila

* An English physician of the 17th century.

ments which pass off from it, representing the legs, are minute tubes termed " canaliculi:" these are exceedingly numerous, and radiate from all parts of the "lacuna," through the lamina. Now since the canaliculi of one circle of lacunæ communicate most freely with those of the next circle, and the canaliculi nearest to the Haversian canal open directly into it, we see that by means of this system of radiating tubes a complete communication is established between the Haversian canal in the centre, and the successive circles of bone which surround it. The nutrient juice of the bone proceeds from the blood-vessels in the central canal, and is transmitted through the canaliculi from one lacuna to another.

What is an Ha

Every Haversian canal taken in conjunction versian system? with its concentric layers of bone, lacunæ, and canaliculi, is termed an "Haversian system." It may be compared to the planetary system. As the sun is the centre of light and heat to the planets around it, so is the blood-vessel in the Haversian canal the centre of nutrition to the surrounding circles of bone.

Almost all the compact substance of bone is made up of a multitude of these "Haversian systems." Each system is, to a certain extent, independent of its neighbour, since the lacunæ of one system communicate very sparingly with those of another. In consequence of this isolation, we may sometimes find, in favourable sections, that each system is more or less circumscribed by a tolerably distinct white line, which is transparent bone with but few canaliculi.

terspaces.

Haversian in- As the Haversian systems are for the most part circular, and arranged like sticks in a faggot, it is clear they cannot touch each other in all parts of their circumference; so that here and there we may observe that triangular portions of bone fill up the gaps between them. Such portions are termed "Haversian interspaces (Plate B, fig. 4b). These "outlying" portions of bone are also provided with lacunæ and canaliculi, and they derive their nourishment from the surrounding Haversian systems, of which they are, so to speak, dependencies.