In the second class of joints, motion is enjoyed freely and fully: this class is designated by the term Diarthrosis (,ku, per, and ocehoy): the segments are interrupted completely in their continuity ; the extremities of the bones can only be said to be contiguous.
Synarthrosis.—The general characters of the articulations belonging to this class are, 1. that they are very limited in their motion, insomuch as to be considered by some as im moveable; 2. that their surfaces are continuous, i. e. without the intervention of a synovial cavity, but with that of some structure different from bone. The following varieties may be noticed among synarthrodial articulations.
a. Suture (Germ. Nath or Naht. Com nzissura cranii, Vesal.).—When the margins of two bones exhibit a series of processes and indentations (dovetailing) which are received and receive reciprocally, with a very thin car tilaginous lamina interposed, this is the ordi nary kind of suture, sutura vera, of which three kinds are distinguished : sutura dentata, where the processes are long and dentiform, as in the interparietal suture of the human skull ; sutura serrata, when the indentations and processes are small and fine like the teeth of a saw, as in the suture between the two por tions of the frontal bone ; sutura limbosa, when there is along with the dentated margins a degree of bevelling of one, so that one bone rests on the other, as in the occipito-parietal suture.
When two bones are in juxta-position by plane but rough surfaces, the articulation is likewise said to be by suture, and this is the false suture, sutura notha, of which there are two kinds : sutura squamosa, where the be velled edge of one bone overlaps and rests upon the other, as in the temporo-parietal suture, and harmonia (ccv, adapto), where there is simple apposition : this last kind of articulation is met with, as Bichat observes, wherever the mechanism of the parts is alone sufficient to maintain them in their proper situation, as may be seen in the union of most of the bones of the face.
It is in the articulation of the bones of the skull and face of animals, as has been already noticed, that we see the best examples of su tures. In the chelonian reptiles, as the tortoise, the bodies, laminae, and spinous processes of the vertebra are united by suture, and the same mode of articulation unites the elements of the sternum of the land-tortoise to each other.t The bones of the head of birds and fishes are united chiefly by the harmonic and squamous sutures. In the lateral parts of the heads of
fishes, and in the opercula of their gills, as between the opercular and subopercular bones, there is a species of articulation, most re sembling the squamous suture, but differing from it in admitting a considerable latitude of motion by which these bones can glide on one another.X To descend still lower in _ the scale, we may observe a mode of joining very similar to suture, between the tubercular and ambulacral plates which form the shcll-like covering of the echinida.* The sutures have the peculiarity of a con siderable tendency to become obliterated by age, the intervening cartilage being ossified ; it rarely happens that the sutures are all ma nifest in a human skull past fifty years of age, and sometimes the obliteration takes place at a much earlier period. The frontal suture is by no means permanent; it is not often found at puberty. In birds and fishes this tendency to the obliteration of the sutures is particularly manifest.
b. Schindylesis (crvavAwric, fissio, crxlev, diftindo).—This form of articulation is where a thin plate of bone is received into a space or cleft formed by the separation of two laminae of another, as is seen in the insertion of the azygos process of the sphenoid bone into the fis sure on the superior margin of the vomer ; and in the articulation of the lacrymal bone with the ascending process of the superior maxillary.
c. Gomphosis (yop,9oc, clavus. Clavatio, conclavatio).—When a bone is inserted into a cavity in another, as a nail is driven into a board, or as a tree is inserted into the earth by its roots, the articulation is by gomphosis. The only example we have of it in the human subject or in quadrupeds is in the insertion of the teeth into the alveoli. In the weapon of offence of the saw-fish we find also an example in the manner in which the strong osseous spines are inserted like teeth into its lateral edges. Cuvier mentions a variety of gomphosis, the only modification of the above : it is where a bony process grows from the bottom of the recipient cavity, and is inserted into a cavity in the base of the received bone or hard part. This is the mode of articulation of the nails with the ungueal phalanges in animals of the cat kind ; the nail is received into an osseous sheath, from the bottom of which the body of the phalanx projects and •fills up the cavity of the nail. A similar pivot grows from the bottom of the alveoli, into which the long canine teeth of the walru's are inserted.