young of a species of this group, which is commonly known as the harvest bug," and which is usually designated as the Acarus autumnalis; this is very common in the autumn upon grass or other herbage, and insinuates itself into the skin at the roots of the hair, producing a painful irritation; like other Acarida, for some time after their emersion from the egg, it possesses only six legs (the other pair being only acquired after the first moult), so that its resemblance to parasitic insects becomes still stronger. It is probable that to this group also belongs the Demodex folliculorum, a creature which is very commonly found parasitic in the sebaceous follicles of the human skin, especially in those of the nose. In order to obtain it, pressure should be made upon any one of these that appears enlarged and whitish, with a terminal black spot; the matter forced out will consist principally of the accumulated sebaceous secretion, having the parasites with their eggs and young mingled with it. These are to be separated by the addition of oil, which will probably soften the sebaceous matter sufficiently to set free the animals, which may be then removed with a pointed brush; but if this mode should not be effectual, the fatty matter may be dissolved away by digestion in a mixture of alcohol and ether. The pustules in the skin of a dog affected with the "mange" have been found by Mr. Topping to contain a Demodex, which seems only to differ from that of the human sebaceous follicles in its somewhat smaller size; and M. Gruby is said to have given to a dog a disease resembling the mange, if not identical with it, by inoculating it with the human parasite. The Acarida are best preserved as microscopic objects, by mounting in glycerine.

401. The number of objects of general interest, furnished to the Microscopist by the Spider tribe, is by no means considerable. Their eyes exhibit a condition intermediate between that of Insects and Crustaceans, and that of Vertebrata; for they are single like the "stemmata" of the former, usually number from six to eight, are sometimes clustered together in one mass, but are sometimes disposed separately, while they present a decided approach in internal structure to the type characteristic of the visual organs of the latter. The structure of the Mouth is always mandibulate, and is less complicated than that of the mandibulate insects. The Respiratory apparatus, which, where developed at all among the Acarida, is tracheary like that of Insects, is here constructed upon a very different plan; for the "stigmata," which are usually four in number on each side, open into a like number of respiratory sacculi, each of which contains a series of leaf-like folds of its lining membrane, upon which the blood is distributed so as to afford a large surface to the air. In the structure of the limbs, the principal point worthy of notice is the peculiar appendage with which they usually terminate; for the strong claws, with a pair of which

the last joint of the foot is furnished, have their edges cut into

FIG. 297.

Foot, with comb-like claws, of the common Spider (Epeira).

FIG. 298.

A B

comb-like teeth (Fig. 297), which seem to be used by the animal as cleansing instruments. One of the most curious parts of the organization of the Spiders is the "spinning apparatus," by means of which they fabricate their elaborately constructed webs. This consists of the "spinnerets," and of the glandular organs in which the fluid that hardens into the thread is elaborated. The usual number of the spinnerets, which are situated at the posterior extremity of the body, is six; they are little teat-like prominences, beset with hairy appendages; and it is through a certain set of these appendages, which are tubular and terminate in fine-drawn points, that the glutinous secretion is forced out in a multitude of streams of extreme minuteness. These streams harden into fibrils, immediately on coming into contact with the air; and the fibrils proceeding from all the apertures of each spinneret, coalesce into a single thread. It is doubtful, however, whether all the spinnerets are in action at once, or whether those of different pairs may not have dissimilar functions; for whilst the radiating threads of a spider's web are simple (Fig. 298, A), those which lie across these, forming its concentric circles or rather polygons, are studded at intervals with viscid globules (B), which appear to give to these threads their peculiarly adhesive character; and it does not seem by any means unlikely, that each kind of thread should be produced by its own pair of spinnerets. The total number of spinning tubes varies greatly, according to the species of the spider, and the sex and age of the individual; being more than 1000 in some cases, and less than 100 in others. The size and complexity of the secreting glandulæ vary in like manner: thus in the Spiders which are most remarkable for the large dimensions and regular (A), and gluticonstruction of their webs, they occupy a large portion of the abdominal cavity, and are composed of slender branching tubes, whose length is increased by numerous convolutions; whilst in those which have only occasional use for their threads, the secreting organs are either short and simple follicles, or are undivided tubes of moderate length.

Ordinary thread

nous thread (B), of the common Spider.

CHAPTER XVIII.

VERTEBRATED ANIMALS.

402. WE are now arrived at that highest division of the Animal Kingdom, in which the bodily fabric attains its greatest development, not only as to completeness, but also as to size; and it is in most striking contrast with the class we have been last considering. Since not only the entire bodies of Vertebrated animals, but, generally speaking, the smallest of their integral parts, are far too large to be viewed as Microscopic objects, we can study their structure only by a separate examination of their component elements; and it seems, therefore, to be a most appropriate course, to give, under this head, a sketch of the microscopic characters of those primary tissues, of which their fabric is made up, and which, although they may be traced with more or less distinctness in the lower tribes of Animals, attain their most complete development in this group. Since the time when Schwann first made public the remarkable results of his researches (p. 56), it has been very generally believed that all the Animal tissues are formed, like those of Plants, by a metamorphosis of Cells; an exception being taken, however, by some Physiologists, in regard to the simple fibrous tissues (§ 417). The tendency of many recent investigations, however, has been to throw further doubt on the generality of this doctrine; since they appear to indicate that many other tissues than the fibrous may be formed (like these) by the consolidation of the plasma or formative fluid, without passing through the intermediate condition of cells. Hence no attempt will here be made to do more than describe the most important of those distinctive characters, which the principal tissues present, when subjected to microscopic examination; and as it is of no essential consequence what order is adopted, we may conveniently begin with the structure of the skeleton,' which gives support and protection to the softer parts of the fabric.

403. Bone.-The Microscopic characters of osseous tissue may

This term is used in its most general sense, as including not only the proper vertebral or internal skeleton, but also the hard parts protecting the exterior of the body, which forms the dermal skeleton.

sometimes be seen in very thin natural plates of bone, such as in that forming the scapula (shoulder-blade) of a Mouse; but they are displayed more perfectly by artificial sections, the details of the arrangement being dependent upon the nature of the specimen selected, and the direction in which the section is made. Thus when the shaft of a "long" bone of a Bird or Mammal is cut across in the middle of its length, we find it to consist of a hollow cylinder of dense bone, surrounding a cavity which is occupied by an oily marrow; but if the section be made nearer its extremity, we find the outside wall gradually becoming thinner, whilst the interior, instead of forming one large cavity, is divided into a vast number of small chambers or cancelli, which communicate with each other

FIG. 299.

and with the cavity of the shaft, and are filled, like it, with marrow. In the bones of Reptiles and Fishes, on the other hand, this "cancellated" structure usually extends throughout the shaft, which is not so completely differentiated into solid bone and medullary cavity, as it is in the higher Vertebrata. In the most developed kinds of "flat" bones, again, such as those of the head, we find the two surfaces to be composed of dense plates of bone, with a "cancellated" structure between them; whilst in the less perfect type presented to us in the lower Vertebrata, the whole thickness is usually more or less "cancellated," that is, burrowed out by medullary cavities. When we examine, under a low magnifying power, a longitudinal section of a "long" bone, or a section of a "flat" bone parallel to its surface, we find it traversed by numerous canals, termed Haversian after their discoverer Havers, which are in connection with the central cavity, and are filled, like it, with marrow: in the shafts of long bones, these canals usually run in the direction of their length, but are connected here and there by cross branches; whilst in the flat bones, they form an irregular network. On applying a higher magnifying power to a thin transverse section of a long bone, we observe that each of the canals whose orifices present themselves in the field of view (Fig. 299), is the centre of a rod of bony tissue (1), usually more or less circular in its form, which is arranged around it in concentric

Minute structure of Bone, as seen in transverse sec

tion:-1, an ossicle surrounding an Haversian canal, 3, showing the concentric arrangement of the lamella; 2, the same, with the lacunæ and canaliculi; 4, portions of the lamellæ parallel with the external surface.

rings, resembling those of an Exogenous Stem. These rings are marked out and divided by circles of little dark spots; which, when closely examined (2), are seen to be minute flattened cavities excavated in the solid substance of the bone, from the two flattened sides of which pass forth a number of extremely minute tubules, one set extending inwards, or in the direction of the centre of the system of rings, and the other outwards, or in the direction of its circumference; and by the inosculation of the tubules (which are termed canaliculi) of the different rings with each other, a continuous communication is established between the central Haversian canal and the outermost part of the bony rod that surrounds it, which doubtless ministers to the nutrition of the texture. Bloodvessels are traceable into the Haversian canals; but the "canaliculi," being far too minute to carry blood-corpuscles, can only convey a nutrient fluid that is separated from the blood for the special service of the bone.

FIG. 300.

b, its ramifications.

404. The minute cavities, or lacunae (sometimes, but erroneously termed "bone-corpuscles," as if they were solid bodies), from which the canaliculi proceed, are highly characteristic of the true osseous structure; being never deficient in the minutest parts of the bones Lacunæ of Osseous substance-a, central cavity, of the higher Vertebrata, although those of Fishes are occasionally destitute of them. The dark appearance which they present is not due to opacity, but is simply an optical effect, dependent (like the blackness of air-bubbles in liquids) upon the dispersion of the rays by the highly-refracting substance that surrounds them (§ 98). The size and form of the lacunæ differ considerably in the several Classes of Vertebrata, and even in some instances in the Orders; so as to allow of the determination of the tribe to which a bone belonged, by the microscopic examination of even a minute fragment of it (§ 453). The following are the average dimensions of the lacunæ, in characteristic examples drawn from the four principal classes, expressed in fractions of an inch :

The lacunæ of Birds are thus distinguished from those of Mammals by their somewhat greater length and smaller breadth; but they differ still more in the remarkable tortuosity of the canaliculi, which wind backwards and forwards in a very irregular manner. There is an extraordinary increase in length in the