Monthly Microscopical" of plates. The osseous tissue contains well-developed lacunæ, and where the plates are thick they are hollowed by vascular canals, and by medullary spaces enclosing fat-cells. It appears to be evolved out of fibrous tissue. The cartilage is of the hyaline variety. In the track of the sclerotic in many eyes the cartilaginous tissue exceeds the common connective tissue. The optic nerve pierces the sclerotic a little below and at the inner side of the posterior pole of the eye-ball, the nerve appearing at its inner surface, nearly 1" to the nasal side of the fovea centralis retinæ, in the form of a disc, usually circular, sometimes elliptical, and when so the major axis is generally vertical. The common aperture in sclerotic and choroid through which the nerve passes is a canal narrower anteriorly, where it lightly clasps the nerve, and wider posteriorly, where it loosely embraces it. Around this opening the choroid and the sclerotic adhere very intimately, their fibrous tissues intermingling here concentrically around the nerve. Here too the minute recurrent branches of the posterior ciliary arteries distributed to the outer part of the sclerotic effect a slight communication with the capillaries in the nerve-sheaths, and indirectly with those in the nerve itself. Some of these last inosculate with the choroidal blood-vessels in the level of the choroidal opening. Through these collateral channels, where the trunk of the arteria centralis is plugged, a small quantity of blood can enter the retina. This choroidal stroma around the nerve contains the same stellar pigment-cells which occur in it elsewhere. In some eyes, in this situation, these cells are more plentiful and richer in pigment, and in such eyes the connective-tissue corpuscles of the neighbouring sclerotic are also not unfrequently pigmented. This excess of pigment expresses itself in the living eye by an incomplete narrow brown or blackish ring round the optic nerve-disc. In the plane of the choroid and of the inner third of the sclerotic the nerve-opening is crossed by a fibrous web-the lamina cribrosa -which peripherally merges in the connective tissues of these two coats. The anterior surface of this perforated lamina is concave, the posterior convex. In the living eye the lamina reveals itself as a white tendinous spot stippled with minute grey dots, the bundles of nerve-fibres going in its meshes. These details are recognizable in the healthy nerve-disc in a small central area which corresponds to a depression which I shall presently describe, known as the physiological pit. A strongly-defined image of these details of the lamina overstepping this limit, and reaching towards or even touching the edge of the disc, is a sign of atrophy. The nerve-fibres in the trunk of the nerve, behind the lamina cribrosa, are of the opaque or double-bordered sort, while in front of the lamina they are pale and transparent. Behind the lamina each nerve-fibre consists of an axis-cylinder-a delicate external tubular sheath (the homologue of the sarcolemma of a primitive muscular fibre), and of an intermediate cortical substance or medulla-the white substance of Schwann. At the lamina the medulla ceases, the axis-cylinder with perhaps a very attenuated prolongation of the sheath passing forwards into the nerve-disc and retina. The greatly reduced bulk of the nerve in the lamina, and the transparence of the nerve-bundles in the nerve-disc and retina, are due to this change in the constitution of the primitive nerve-fibres. Exceptionally, as a congenital error, some bundles of opaque nervefibres reach the inner surface of the nerve-disc, and are even prolonged for some distance into the retina. This is not a very uncommon defect. The opaque nerve-fibres produce a white particle, which is to be distinguished from similar particles, due to exudations, by its brush-like feathered edge. After emerging from the anterior surface of the lamina cribrosa the bundles of transparent nerve-fibres bend away on all sides, quaquaversally, from a central point, and curving over the edge of the choroidal foramen spread out on the inner surface of the retina. In doing this they leave a central void—a small hollow-the physiological pit. This pit is usually in the centre of the nerve-disc, but not always so, and when excentric the vasa centralia usually also pierce the disc excentrically. A normal physiological pit never, however, is so excentric as to touch the contour of the disc. The physiological pit is then a small funnel-like hollow in the centre of the nerve-disc, perforated by the vasa centralia, appearing as a bright, hollow spot, in which, with an enlargement of twelve or fifteen diameters, the meshes of the lamina cribrosa and the ends of the bundles of opaque nerve-fibres are plainly discernible. Blood-vessels.-At a variable distance from the eye-ball the trunk of the nerve is pierced by a branch of the ophthalmic artery, which soon gains the axis of the nerve, and running forwards through the lamina cribrosa, perforates the nerve-disc in which it divides into two primary branches which bifurcate, and passing across the boundary of the disc are distributed to the retina. In the disc we can distinguish, first, a short vertical piece of the arterial trunk, and next, the branches making a large angle with the trunk and following the surface of the disc. The capillaries of the nerve-trunk and those distributed to its disc-like interocular end are very numerous; they are sufficiently abundant to redden the disc when distended with blood, a thing which no amount of hyperomia of the retinal capillaries ever does in this membrane. The very slight diminution which the arteria centralis undergoes from its origin to its final termination in the disc, shows that it mainly ministers to the nutrition of the retina. It gives, however, in its course small twigs for the nutrition of the nerve behind the eye-ball, and these reinforced by others derived from minute nameless arteries distributed to the sheath, ramify in the septa between the nerve-bundles. The veinlets accompanying the primary branches of the arteria centralis in the retina do not usually coalesce in a single trunk in front of the lamina cribrosa; but they pierce this separately and first unite the trunk of the nerve behind it. In the arrangement of its sheath the optic nerve differs from all the other large nerve-trunks. These have but one tightlyfitting tube of connective tissue-the external neurilemma; but the optic nerve has a double sheath. It has a thin tightly-fitting sheath representing that of the nerves, from the inner surface of which septa are produced inwards, between the nerve-bundles constituting the internal neurilemma or frame which holds the bundles together and carries the nutrient blood-vessels. In front of the lamina cribrosa, in the nerve-disc, the neurilemma is of the very delicate kind to which Virchow has given the name neuroglia. Its fibres have two principal directions; one transverse to that of the nerve-bundles, and therefore parallel to those of the lamina cribrosa, the other direction vertical to the front of the lamina and free surface of the nerve-disc. These last fibres correspond to the radial connective-tissue fibres in the retina. The inner sheath and internal neurilemma consist of common connective tissue-fibrillated bundles with interspersed corpuscles. In front this division of the sheath blends with the lamina cribrosa and with the inner third of the sclerotic. The outer division of the sheath is continuous posteriorly with the dura mater, so that coloured fluids injected into the space between the sheaths soon find their way backwards into the cranial cavity. In front, this sheath blends with the posterior two-thirds of the sclerotic. The inner and outer sheaths are loosely connected by a very open areolated tissue, composed chiefly of curling fibres and of coarser bundles of connective tissue, in which are imbedded large nucleated fusiform corpuscles. These give to the interstitial or areolar spaces the appearance of an epithelial lining. They play an important rôle in neuritis and in the evolution of morbid growths. The space between the sheaths has lately been described as a lymph-space. In severe injuries of the head blood is sometimes poured out into it. PROGRESS OF MICROSCOPICAL SCIENCE. Performance of Vivisection.-Two papers of considerable importance were read at the British Association by Dr. Brown-Sequard; but they were the more so because they led to the following remarks from Professor Huxley, who is especially and very properly desirous of removing experiment from unnecessary bans :-Professor Huxley said, the great theoretical problem they had now to determine was what effect artificial modifications and external conditions had upon living organisms-whether they produced changes which, being transmitted hereditarily, became the basis of new races. Referring to a resolution which had been brought forward at a former meeting, which endeavoured to pledge the association to abstain from making grants of money to persons engaged in experiments which involved vivisection, he said they had before them that day one of the most experienced physiologists and vivisectors of his day, and he had only to ask the audience to form their own judgment as to whether Dr. Brown-Sequard was likely to inflict one particle of pain upon any creature whatever without having a plain and definite purpose in view. For himself he might say that nothing was more grievous to him than to think of the existence of pain in anything whatever. Neither Dr. Brown-Sequard nor himself was indifferent to pain, and he hoped that in no sense were they cruel. He thought that the gentleman who brought forward the resolution to which he had referred, and the gentleman who supported him, hardly knew what they were dealing with. If his friend Dr. Brown-Sequard would pardon his referring to a matter personal to him, he would remind the meeting that that great experimental physiologist, and that accomplished vivisector, who had, he supposed, performed as many vivisections as any man in the world, some years ago thought it advisable to turn the vast knowledge of the diagnosis of disease which he had obtained by this means into actual practice, and he (Professor Huxley) could assure them, from what he knew, that before long his wonderful mastery over symptoms caused his consulting rooms to be absolutely crowded by human beings suffering under multiform varieties of nervous disorders, who sought at his hands and from his knowledge that relief which they could not obtain elsewhere. The prevention of cruelty to animals, when understood in its proper sense, was as good an object as men could devote themselves to, but when they confounded the brutal violence of the carter or the wife-beater with an experiment carried out by a man of science, gently and for the purpose of relieving misery, the enthusiasts in that cause should change their name, and convert themselves into a society for the promotion of cruelty to mankind. If that question came before the Association again, and he hoped it would, he trusted they would recollect that the order of nature was such that certain kinds of truth were only attainable by experiments upon living animals, and that when they might result to the welfare of thousands and thousands of untold human beings who might other wise be suffering unimaginable misery, those experiments were perfectly justifiable. Is there such a thing as Spontaneous Generation?—This is a very sérious question, and one which we fear cannot be answered definitely in the present state of science. Nevertheless Professor Huxley, in his opening address to the British Association at Liverpool, attempted to answer it, and gave a very able and remarkably clear account of the history of the whole process, and of the various efforts that have been made in its support and against it to within the last few years. We cannot, however, see that he has done justice to some who have laboured well in defence of spontaneous generation, or who have at all events lately done much to oppose the view which Professor Huxley has taken. We hope to have further information in time for our next number. Meanwhile we may mention that a paper supporting spontaneous generation was read at Liverpool by Dr. G. W. Child, and that a paper against was read at the same meeting by Mr. Samuelson, but that neither of them dealt fully or fairly with the matter. A New Mode of Studying Capillary Circulation.-The views of Professor Burdon Sanderson and Herr S. Stricker on this subject were communicated to the British Association at the Liverpool meeting. We regret that no report of the paper was given in the public press, so far as we have been able to see. The British Association in Edinburgh in 1871.-We are glad to see that Edinburgh has been chosen for the meeting of the Association next year. Sir Roderick Murchison said it was the third time the Association would meet in Edinburgh. He did not believe there was any place in the world in which the sciences to which he was attached -geography and geology-could be so admirably illustrated as in Scotland, and it gave him great satisfaction to think that they would have at Edinburgh a number of eminent men of science. Mr. Cowan, late M.P. for Edinburgh, seconded the motion, which was carried unanimously. On the motion of Lord Houghton, seconded by Mr. Gassiot, it was then unanimously decided to hold the meeting of the Association in 1872 at Brighton. On the motion of Professor Stokes, seconded by Mr. Spottiswoode, Sir William Thompson, F.R.S., London, and professor in the University of Glasgow, was unanimously appointed president-elect. The vice-presidents and other officers having been elected, it was resolved that the next meeting of the Association be held about the middle of August, on such days as the council may agree to fix. The Structure of Fossil Corals.-A very valuable paper, entitled the "Report of Committee on Fossil Corals," was read at Liverpool. The author, Mr. J. Thomson, exhibited several specimens and slices and sections fully illustrating his views. The Facts of Succession in relation to a Theory of Continuity.-The Department of Anatomy and Physiology was occupied at the British Association with the paper by Dr. Spencer Cobbold, F.R.S., on this subject. After dealing at some length with the opinions of men like VOL. IV. R |