results in which pores exist only in certain parts, especially in the granular layer of the dentine.

In all cases the formed material is impregnated with calcareous matter from without inwards. The necessity for the existence of bone at a very early period of development, the gradual alteration in the size of the bones during the growth of the body, and the changes in their form seem to be incompatible with a mode of development like that by which dentine is produced.

While Virchow has been led by his researches to the conclusion that nutrient tubes exist in connexion with cells in soft tissues, the results of the observations I have recorded in this lecture, have compelled me to conclude that such tubes do not exist in soft tissues as tubes for the transmission of nutrient material, and that even the so-called dentinal tubes are not nutrient canals; while it appears that the canaliculi of bone which are of this nature do not correspond to the dentinal tubes, or to the communicating system of channels in a tissue composed of stellate branching cells, but are mere spaces left during the rapid formation of a tissue impermeable to fluids.

K

LECTURE VII.

Connective Tissue, continued.-Intercellular Substance (formed Material). — Cells or Nuclei (Germinal matter).- — Areolar or Connective Tissue Corpuscles and the system of communicating nutrient channels.-Areolar Tissue.-Nerves in Skin of Mouse.-Mucous Membrane of Fauces.-Pericardium, its nerves and ganglia.-Voluntary Muscle.-General remarks on Areolar Tissue.-Conclusion.

MR. PRESIDENT AND GENTLEMEN,

IN my last lecture I endeavoured to demonstrate that the important group of textures included under the head of connective tissues formed no exception to the general statements I had advanced with reference to the structure and growth of other tissues of the human body, the tissues of animals generally, and those of plants.

Although these connective tissues are generally described as composed of cells and intercellular substance, I brought forward a number of facts which seem strongly opposed to the view generally entertained, and the preparations which were passed round have, I trust, at least, convinced you that the explanation I have advanced is not incompatible with the facts which were observed. I hold that the so-called intercellular substance exactly corresponds to the cell wall, and like it was formed from germinal matter.

I have tried to show that at an early period of the development of all these tissues there is no proper intercellular substance; and by examining the same tissue at various stages of its growth one cannot but come to the conclusion that the structure, which in the fully developed tissue seems to lie between the cells, formed at an earlier period part of the substance of the cells; that an elementary part of adult cartilage or fibrous tissue, for instance, consists of germinal matter, and formed material upon which the peculiar characters of the tissue entirely depend; and that the proportion of formed material gradually increases as the age of each elementary part advances, so that the masses of germinal matter become separated from each other by greater distances. As a general rule the formed material of the constituent elementary parts of connective tissues is continuous, and it is not possible to isolate each elementary part as in epithelium and such structures; but in certain cases even this can be effected, and the analogy becomes so distinct that it seems impossible to separate the class of connective tissues from epithelial structures on the ground of any mere differences of structure or mode of growth.

I propose now to consider the bearing of the anatomical points I endeavoured to prove in my last lecture upon the question of connective tissue generally. I shall first allude to the intercellular substance, and afterwards consider the nature of the "cells" or their representatives in this series of tissues.

INTERCELLULAR SUBSTANCE (FORMED MATERIAL).

Many observers in the present day seem to consider that the intercellular substance is of far higher importance than the cells or nuclei embedded in it. This matrix or intercellular substance has been supposed, too, to play a most important part in morbid changes. It is considered that the intercellular

substance possesses active powers. It may be at first composed of a soft and perfectly clear and homogeneous substance, but it subsequently undergoes important changes in its properties. By some observers it is supposed to become differentiated into various substances by virtue of its own inherent powers, while others attribute the changes which occur to the action of the cells. It is asserted that in all connective tissues, cells and an intercellular substance can be satisfactorily demonstrated. In white fibrous tissue the yellow elastic tissue is said to be developed from, and is the representative of, the cellular element, while the white fibrillated tissue is regarded as the intercellular substance which by many is supposed to be formed independently of cells. This part of the question has already been considered in page 115.

It appears, then, that some observers consider that the intercellular substance is simply deposited from the blood, and perhaps somewhat modified by the action of the cells between which it is formed, while by other authorities the intercellular substance itself is believed to possess inherent powers of growth, assimilation, and conversion.

If the intercellular substance of white fibrous tissue and cartilage is merely deposited from the blood by a process akin to crystalization, some substances from which gelatine or chondrin could be obtained should exist in that fluid. But no such substance has ever been detected.

If, on the other hand, this intercellular substance possesses formative power, and by its own inherent powers can convert certain nutrient materials into matter possessing the same properties as itself, what end is served by the germinal matter which is so constant? And why are these masses so much more numerous in young than in fully developed cartilage? The object fulfilled by the growth of such textures as fibrous tissue and cartilage is the formation of the so-called intercelJular substance upon which all the properties of the tissue

depend. It is in this that the peculiar properties of firmness, strength, or elasticity reside; for the cells (masses of germinal matter) certainly do not possess any such characters. If, then, this substance can increase itself, wherefore are the cells present at all? The universality of the presence of germinal matter is meaningless, and its abundance in all rapidly-growing tissues, its gradual diminution as the formed material increases, and the deterioration in the properties of the tissue constantly associated with its abnormal increase or with its death, are facts which do not receive explanation.

If both cells and intercellular substance require to be nourished, what regulates the exact supply so that neither increases upon the domains of the other? And how is it that the selected powers of each are exactly balanced?

The investing membrane of a spore or a portion of the stem of mildew, or other simple plant, corresponds to the so-called intercellular matrix of tendon, cartilage, &c. If the latter grows by selecting substances from the nutrient fluid which bathes it, the former must be produced by selecting substances from the fluid which surrounds it. But I have shown that the inner germinal matter may diminish as the cell-wall increases in thickness, and that while there are many examples of germinal matter which is not surrounded with a distinct cellwall or intercellular substance, there is not an instance of the latter existing in a growing state without the former. When the mildew grows, so far from the outer material or cell-wall increasing in thickness, it becomes thinner (Plate II, fig. 10). It is the germinal matter within which increases. The germinal matter alone grows, and the more rapidly it increases the thinner is the external membrane (formed material) found to be. A small portion of the germinal matter of the mildew placed under favourable conditions will grow and will produce the formed material or cell-wall, but the cell-wall from which the germinal matter has been removed will not grow under any circumstances whatever. The envelope or cell-wall exhibits the