TEXTURES.] ANATOMY 845 It is not an easy matter to devise a classification of the tissues, based on their structural characters, which shall be in all respects logically perfect ; but a convenient basis of arrangement for descriptive purposes may be found by dividing them into those which consist 1st, of cells sus pended in fluids; 2d, of cells placed en free surfaces ; 3d, of cells imbedded in solid tissues. 1st Group. Cells /Suspended in Fluids. The fluids of the body which have cells or other minute solid particles suspended in them are the blood, the lymph, and the chyle. Sometimes cells are found floating in the secretions of glands. THE BLOOD. The blood is the well-known red fluid which circulates throughout the blood-vascular system. As its composition and general properties will be described in the article PHYSIOLOGY, the solid particles only, which are suspended in the liquor sanguinis, will be considered here. If a drop of human blood be examined under the microscope, crowds of minute bodies, the blood corpuscles, or blood globules, may be seen in it. These present two different appearances, and are distinguished by the names of red and white blood corpuscles. The red corpuscles, which are by far the more numerous, are minute circular discs, slightly concave on both surfaces. Their average diameter is about ^-g^th of an inch, and their thickness about Jth of that measurement; hence they are not spheres, as the old name blood globules would imply. They are non-nucleated. Single corpuscles have a faint fawn-coloured hue, but collectively they give to the blood its characteristic red colour. This colour is due to the presence in the corpuscles of the substance termed haemoglobin. It has been estimated by Yierordt and Welcker that 5,000,000 red corpuscles are present in every cubic millimetre of healthy human Mood. The red corpuscles in the blood of all mammals, except the tribe of camels, are circular bi-concave discs ; but in these exceptional mammals they have an elliptical outline. In all mammals the red corpuscles are non-nucleated, though appearances of nucleation have been seen in exceptional individual cases; for Rolleston saw a nucleated appearance in a small proportion of the dried red blood corpuscles of a two-toed sloth; and Turner observed in a proportion of the red blood discs of a Hoffmann s sloth an appearance of a central nucleus. In all birds, reptiles, and amphibia the red corpuscles are oval or elliptical, and in each corpuscle an oval or elliptical nucleus is situated. In all fishes they are nu cleated and also elliptical in form, except in some of the Cycloatomata, which possess circular discs. In the elliptical nu cleated corpuscles the surfaces are not bi concave, but have central projections, which correspond in position to the nu cleus (2, 4, 5, Fig. 28). The red cor puscles vary mate rially in size in dif- fprpnt vprtpbrita FlG - 2S.1, red coi-puscles of human blood ; 2, red prata, corpuscles of blood of common fowl, seen on and these variations tlle surface and edgeways; 3, red corpuscles of T i . frog; 4, of Squalus squatina; 5, of Lophius nave been especi- piscatorius; 6, corpuscles of the blood of a scor- ally studied by Gul- pion - liver. He has found them to vary in mammals from an average diameter of Tr^th of an inch in the elephant, and ^Vfrffth in Orycteropus capensis, to r^^tli in Traguhis javanicus, and he concludes that the smallest blood discs occur in the small species of an order or family, the largest? in the large species. In birds they are larger than in mammals, and vary in length from an average of T ^V^ " lcn in Casuarius javanicus to ^iV &th in Linaria minor. In reptiles they are still larger, and vary in length from an average of TrVF^ ^ n Anguis fragUis to TeT^ii in Lacerta, viridis. In amphibia the largest corpuscles, according to Gulliver, are about y-J-g- inch in length in Proteus and Siren, though Riddell states that in Amplduma tridactylum they are ^d larger ; whilst the smallest, as in the common frog, average in length -^^-g inch. In cartilaginous fish the corpuscles are larger than in osseous. In Lamna cor- nubica Gulliver found their long diameter to be ^ 3 inch ; while in the Salmonidce, which have the largest blood discs among osseous fish, the long diameter in the salmon and common trout is only about T ^Vff inch. The white or colourless corpuscles are comparatively few White in number in the healthy human blood. Welcker has corpuscle estimated the normal relative number as one white to 335 red ; in pregnant and menstruating women the pro portion is increased to about 1 to 280. In some forms of disease the proportion is so very materially increased that they appear to be almost as numerous as the red. They are rounded in form, finely granulated or mulberry-like in appearance, and nucleated the nucleus becoming more distinct after the addition of acetic acid; moreover, they are larger than the red corpuscles, their average diameter being from Tj-sW^h to -j-gV-oth of an inch. Corpuscles of a similar form are found in the blood of all vertebrata. They do not vary so much in size in different animals as do the red corpuscles. In Triton, according to Gul liver, their average diameter is T^^th, whilst in Herpestes griseiis they are not more than -3-^-$ i nc h- The white blood corpuscles are minute nucleated clumps of proto plasm ; they are therefore minute cells. It is very doubt ful if they possess a cell wall, the evidence being against rather than in favour of its presence. The red blood corpuscles in all vertebrata, except the mammalia, are nucleated clumps of protoplasm ; they are therefore minute cells. In mammals, owing to the ab sence of a nucleus, they do not accord with the definition of a cell adopted in this article, and they are not therefore morphologically identical with the red corpuscles in other vertebrates. What their precise homology may be is some what difficult to say, owing to the obscurity which prevails as to their exact origin. If they are merely clumps of specially modified protoplasm, budded off from the white corpuscles, then they are cytodes. If, as some have sup posed, they are the nuclei of the white corpuscles, specially modified in composition, then they are free nuclei. If, again, they are the white corpuscles, the cell substance of which has undergone a special differentiation, and the nucleus has disappeared, then they are potentially cells, though no nucleus is visible. Whatever may be their exact homology, there can be no doubt that the non-nucleated mammalian red corpuscle, and that part of the nucleated red corpuscle which lies outside the nucleus, are function ally identical with each other ; the protoplasm having undergone a special chemical differentiation into hremo- globin, a proximate principle characterised by containing iron as its essential constituent. The action of water, spirit, acids, alkalies, various gases, heat, cold, and electri cal currents, on the red corpuscles has been studied by several observers, and the conclusion has been reached that the corpuscles consist of a " stroma," with which the colouring matter is blended, but from which it may be separated without the stroma affording any evidence of the presence of an investing envelope or membrane. When blood is drawn from the vessels the red corpuscles, in about
half a minute, run together into piles, like rouleaux of cciua
