ARTICULATION. 



255 



ambulacral plates which form the shell-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. 



f>. Schindylesis (a-^iv^v^yn;, Jissio, 0~%iw, 

 diffmdo). 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 (yoptpos, ctavus. 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 walrus are 

 inserted. 



d. Amphiarthrosis (apCpt, utrinque, agOgon, 

 urticultis, i. e. a mixed form of articulation. 

 Articulatio dubia, Bartholin. Synarthrosis diar- 

 tlirodicu). This is a form of articulation where 

 two plane or mutually adapted surfaces are held 

 together by a cartilaginous or fibro-cartilaginous 

 lamina of considerable thickness, as well as 

 by external ligaments. In virtue of the elasti- 

 city of the interposed cartilaginous or fibro- 

 cartilaginous lamina, the amphiarthrosis pos- 

 sesses a manifest, although certainly a very 

 limited degree of motion, and hence most 

 systematic writers class it with the diarthrodial 

 articulations. To me it appears much more 

 consistent to place it among the synarthrodial 

 joints, for, 1. its anatomical characters agree 

 precisely with those of synarthrosis ; 2. the 

 surfaces in amphiarthrosis being continuous, it 

 would make an exception in diarthrosis were 



* Meckel, Anat. Comp. (Fr. transl.) t. ii. p. 43. 



we to place it there; and, 3. its degree of 

 motion is greater than that of suture, only 

 because of the greater development of the in- 

 terosseous substance. These points of similarity 

 led some anatomists to call it Diurthrosis syn- 

 arthrodica ; for the reasons above stated, as 

 well as because it has one point of resemblance 

 to diarthrosis in its greater latitude of motion, 

 I propose the appellation Synarthrosis diar- 

 throdica. 



The examples of this form of joint in the 

 human body are the articulation between the 

 bodies of the vertebrae, that between the two 

 ossa pubis at what is called the symphysis, and 

 that between the ilium and sacrum. We 

 may also, I think, place here the articulation 

 of the ribs with the sternum by means of the 

 costal cartilages.* The bodies of the vertebra? 

 in most of the mammalia are articulated in 

 the same way ; so are they in fishes also ; but 

 in these last there is a peculiarity already re- 

 ferred to, which increases the degree of motion 

 of which the joint is susceptible. f Like the 

 sutures, the amphiarthrosis is liable to become 

 obliterated by age, and from the same cause, 

 namely, the ossification of the interosseous la- 

 mina. This is very common in the costo-sternal 

 joints, less so in the interpubic, and still more 

 rare in the inter-vertebral and sacro-iliac. 



Diarthrosis. Evident mobility is the dis- 

 tinguishing characteristic of this class of joints; 

 the articular surfaces are contiguous, each co- 

 vered by a lamina of cartilage (diarthrodial 

 cartilage), having a synovial sac, and in 

 some cases two synovial sacs interposed, 

 which are separated by a meniscus. The in- 

 tegrity of the articulation is maintained by liga- 

 ments which pass from the one bone to the 

 other. Their mechanism is much more com- 

 plicated than that of synarthrodial joints, being 

 intended not only for security, but also to give 

 a certain direction to the motions of which 

 they are the centre. 



Before proceeding to the enumeration of the 

 varieties of joints that come under this head, it 

 will not be amiss to describe briefly the various 

 motions which may take place between any two 

 segments of a limb, and which it is the object 

 of these joints to admit of. It is obvious that 

 the most simple kind of motion which can exist 

 between two plane or contiguous surfaces, is that 

 of gliding : one surface glides over the other, 

 limited by the ligaments which extend be- 

 tween the bones. This motion, however, is not 

 confined to plane surfaces, it may exist evidently 

 between contiguous surfaces whatever their form. 

 When two segments of a limb, placed in a 

 direct line or nearly so, can be brought to form 



* It may be objected to this arrangement that 

 at the sternal extremity of each cartilage there is 

 a synovial membrane between it and the sternal 

 depression. All anatomists agree in denying its 

 existence at the articulation of the first cartilage, 

 and all admit the great difficulty of fully demon- 

 strating its existence in the others. For my own 

 part I do not believe that it exists in any. 



t The articulation of the lower jaw in the whale- 

 bone whale, above referred to, is a joint of this 

 kind. 



