The nerve cells sometimes present numerous processes, as shown below. Fig. 142. Two nerve cells from the human retina, magnified 350 times. The smaller The Crystalline Lens.-In perfectly fresh specimens the delicate capsule may be readily separated from the lens, and its epithelial lining demonstrated; but the structure of the lens itself is best studied in specimens that have been boiled, or otherwise hardened by immersion in alcohol or chromic acid. A very good plan is to allow a fresh lens to remain on a slip of glass exposed to the air for a day or two, until it becomes sufficiently firm to be cut with a razor. In this way the transparency of the structure is in a great degree preserved. From the hardened lens the concentric laminæ may be stripped off layer by layer, until we arrive at the central point. In order to examine the structure of the laminæ, a small portion of one of them should be placed in a drop of water on the slide, and gently separated into fibres in a longitudinal direction by means of needles, then covered and examined with a high power. A number of flattened bands will be seen; some of them quite separate, and others united together by their edges, which are denticulated as shown in the next wood-cut. Transverse sections, which are easily made with a razor from a lens previously hardened, prove the bands or fibres to be hexagonal in form, as indicated in the drawing. Fig. 143 Lenticular tubes or fibres. 1. Of the ox, with slightly dentated margins. 2. Transverse section of the lenticular fibres of man.-Magnified 350 The crystalline lens has been, not inaptly, compared to an onion, consisting of a series of lamina or layers, which, in the hardened structure, may readily be stripped off, a layer at a time. The bands of which the laminæ are composed are not of the same breadth throughout, but taper towards the extremities in the same way as the spaces between the meridian lines of a geographical globe, which are broad at the equator and gradually become narrower as they converge to the two opposite poles. This arrangement is seen in the most simple form in the lenses of birds and some fishes as depicted in the next drawing. M A more complicated plan is found among some mammalia, as the rabbit, hare, &c., as well as in certain fishes and reptiles, the centres or poles on each side being crossed by a transverse line or septum, as in the next figure. Among mammalia in general there are three lines or septa diverging from the poles on each side; these may be distinguished very easily in a lens that is just beginning to lose its transparency, and appear exactly as represented beneath. The central portion of the lens, that part which remains after all the laminæ are stripped off, is depicted in the next drawing, from the eye of the Salamander maculata. Fig. 147. Central point of the crystalline lens of the Salamander maculata. A small The vitreous body, which occupies the space between the lens and the retina, is enveloped in a fine membrane, the hyaloid, which, according to Kölliker, proceeds to the border of the lens, in order to coalesce with it. During this course it divides into two lamella; the anterior division extends forward to become attached to the lens a little in front of its border, and forms the zonule of Zinn. The posterior lamella unites with the capsule of the lens, behind its border. The space between the two lamella is the canal of Petit. The vitreous body is perfectly transparent and colourless, and contains no microscopic elements of importance. THE INTERNAL EAR. THE parts composing the external ear, consisting, as they do, of integument, cartilage, muscle, bone and mucous membrane, present no special points for microscopical study, nor will the method of examination differ from that previously recommended for the study of the same structures in other parts of the body. The tissues contained in the labyrinth of the ear, however, are exceedingly delicate and beautiful, but especially difficult to Transverse section through the spiral lamella of the first turn of |