can squeeze out of their excretories a mucilaginous liquor, which somewhat resembles the white of an egg, or serum of the blood; but it is manifestly salt to the taste. It does not coagulate by acids or by heat, as the serum does, but by the latter turns first thinner, and, when evaporated, leaves only a thin salt film.
The vessels which supply liquors for making the secretion of this mucilage, and the veins which bring back the blood remaining after the secretion, are to be seen without any preparation; and, after a tolerable injection of the arteries, the glands are covered with them.
In a sound state, we are not conscious of any sensibility in those glands; but, in some cases, when they inflame and suppurate, the most racking pain is felt in them: a melancholy, though a sure proof that they have nerves.
These mucilaginous glands are commonly lodged in a cellular substance; which is also to be observed in other parts of the bag formed by the ligaments of the articulation; and contains a fatty matter, that must necessarily be attenuated, and forced through the including membranes into the cavity of the joint, by the pressure which it suffers from the moving bones.
After the liquor of the articulations becomes too thin and unserviceable, by being constantly pounded and rubbed between the moving bones, it is reassumed into the mass of blood by the absorbent vessels.
Sect. II. Of the Skeleton.
Among anatomists, Skeleton is universally understood to signify the bones of animals connected together, after the teguments, muscles, bowels, glands, nerves, and vessels are taken away.
A skeleton is said to be a natural one, when the bones are kept together by their own ligaments; and it is called artificial, when the bones are joined with wire, or any other substance which is not part of the creature to which they belonged.
The human skeleton is generally divided into the Head, the Trunk, the Superior and the Inferior Extremities.
OF THE HEAD.
By the Head is meant all that spheroidal part which is placed above the first bone of the neck. It therefore comprehends the cranium and bones of the face.
The cranium, helmet, or brain-case, consists of several pieces, which form a vaulted cavity, for lodging and defending the brain and cerebellum, with their membranes, vessels, and nerves.
The cavity of the cranium is proportioned to its contents. Hence such a variety of its size is observed in different subjects; and hence it is neither so broad nor so deep at its fore-part, in which the anterior lobes of the brain are lodged, as it is behind, where the large posterior lobes of the brain, and the whole cerebellum, are contained.
The external surface of the upper part of the cranium is very smooth, and equal, being only covered with the periosteum, (common to all the bones; but in the skull, distinguished by the name of pericranium), the thin fronttal and occipital muscles, their tendinous aponeurosis, and with the common teguments of the body; while the external surface of its lower part has numerous risings, depressions, and holes, which afford convenient origin and insertion to the muscles that are connected to it, and allow safe passage for the vessels and nerves that run through and near it.
The internal surface of the upper part of the skull is commonly smooth, except where the vessels of the dura mater have made furrows in it, while the bones were soft.Surgeons should be cautious when they trepan here, lest, in sawing or raising the bone where such furrows are, they wound these vessels.In the upper part of the internal surface of several skulls, there are likewise pits of different magnitudes and figures, which seem to be formed by some parts of the brain being more luxuriant and prominent than others. Where these pits are, the skull is so much thinner than any where else, that it is often rendered diaphanous, the two tables being closely compacted without a diploe; the want of which is supplied by vessels going from the dura mater into a great many small holes observable in the pits.The knowledge of these pits should teach surgeons to saw cautiously and slowly through the external table of the skull, when they are performing the operation of the trepan; since, in a patient whose cranium has these pits, the dura mater and brain may be injured, before the instrument has pierced near the ordinary thickness of a table of the skull.The internal base of the skull is extremely unequal for lodging the several parts and appendices of the brain and cerebellum, and allowing passage and defence to the vessels and nerves that go into, or come out from these parts.
The bones of the cranium are composed of two tables, and intermediate cancelli, commonly called their diploe. The external table is thickest; the inner, from its thinness and consequent brittleness, has got the name of vitrea.
The diploe has much the same texture and uses in the skull, as the cancelli have in other bones.
The diploe of several old subjects is so obliterated, that scarce any vestige of it can be seen; neither is it observable in some of the hard craggy bones at the base of the skull. Hence an useful caution to surgeons who trust to the bleeding, want of resistance, and change of sound, as certain marks, in the operation of the trepan, for knowing when their instrument has sawed through the first table, and reached the diploe.
The cranium consists of eight bones, six of which are said to be proper, and the other two are reckoned common to it and to the face.—The six proper are, the os frontis, two offa parietalia, two offa temporum, and the os occipitis.The common are, the os ethmoides, and sphenoides.
The os frontis forms the whole fore-part of the vault; the two ossa parietalia form the upper and middle part of it; the ossa temporum compose the lower part of the
