Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/882

Rh 832 ANATOMY [JOINTS AND of a bone ossifies from a centre distinct from the centre from which the shaft arises, it is called an epiphysis. The epiphysis is united to the shaft of the growing bone by an intermediate plate of cartilage, and so long as any of this cartilage remains unossified the bone can continue to grow in length. The ossification is not completed in the different bones until from the twentieth to the twenty-fifth year. In the case of the long bones, the epiphysis situated at the &amp;lt;md of the bone, towards which the canal in the shaft which trans mits the nutrient artery is directed, ossifies to the shaft before the epiphysis at the other end. In the humerus, tibia, and fibula, where the canal is directed downwards, the epiphyses at the lower ends of the bones first unite with the shaft ; whilst in the femur, radius, and ulna, where the canal is directed upwards, the ossifica tion first takes place between the upper epiphysis and the shaft. All anatomists hold that the bones of the shaft and distal part of a limb belong to the appendicular part of the skeleton, but there is a difference of opinion as to the place in the skeleton to which the bones of the shoulder girdle and haunch are to be referred. Owen considers that the scapular and pelvic arches belong to the axial skeleton, and are homologous with the ribs ; the scapula and coracoid as the visceral or rib-arch of the occipital vertebra, the clavicle of the atlas, and the innominate bone of the upper sacral vertebra?. Goodsir objected to this conclusion of Owen s on the ground that the shoulder girdle was not in series with the visceral arches, but was developed outside the visceral wall, at the junction of the cervical and thoracic regions, from which region the upper limb receives its nerves, and not from the occipito-atlantal region, whence they would have proceeded had it been an appendage of the rib-arches of those segments. Owen s chief argument for regarding the scapula and coracoid as the costal arch of the occi pital vertebra is because in fish the scapula is attached to the occipital bone by a bone which Cuvier called the supra-scapula, and which he believed to be homologous with the supra-scapular cartilage of many other vertebrates. Parker, however, has recently pointed out that the so-called supra-scapula of a fish is not homolo gous with the supra-scapula of a reptile or mammal, that it is not a cartilage bone, but is a splint or scale-like bone, developed as a part of the dermo-skeleton. Between the scapula and coracoid and the innominate bone, anatomists have long recognised homologies to exist ; the scapula is generally admitted to be the homotype of the ilium and the coracoid of the ischium, so that if these elements of the shoulder girdle be not a costal arch, neither can those of the pelvic girdle. The clavicle has by some been re garded as the homotype of the pubis ; but in all probability the pubis is homologous with the procoracoid bone which is found in the amphibia and some reptiles, but is absent in crocodiles, birds, and mammals ; whilst the clavicle is represented in the pelvic girdle, not by a bone, but by a fibrous band called Poupart s ligament. Between the bones of the shafts of the limbs homologies exist : the humerus is the homotype of the femur, the radius of the tibia, the ulna of the fibula ; whilst the patella has no representative in the human upper limb. The scaphoid and semilunar bones in the carpus are homotypes of the astragalus in the tarsus, the cunei form is the homotype of the os calcis, the cuboid of the unciform ; the trapezium of the ento-cuneiform, the trapezoid of the meso-, and the os magnum of the ecto-cuneiform. The tarsal scaphoid is not, as a rule, represented in the human carpus, but its homotype is the os intermedium, found in many mammals. The carpal piaciform is a sesamoid bone developed in the tendon of a muscle. The metacarpal bones and phalanges are homologous with the metatarsal bones and phalanges ; the thumb with the great toe, and the fingers with the four outer toes. During the growth of the limbs outward, and their change from the simple lappet-like form to their elongated condition, a rotation of the proximal segment of the shaft takes place that of the upper limb a quarter of a circle backward, that of the lower limb a quarter of a circle forward to produce in the former case a supine position of the fore-arm and hand, with the thumb as the outermost digit ; in the latter case, a prone condition of the leg and foot, with the great toe as the inner most digit. The range of movement at the radio-ulnar joints enables us, however, to pronate the hand and fore-arm by throwing the radius across the ulna, so as to make the thumb the innermost digit. In many quadrupeds the fore-leg is fixed in this position, so that these animals walk on the soles of both the fore and hind feet. GENERAL OBSERVATIONS ON THE ARTICTJLATORY AND MUSCULAR SYSTEMS. A JOINT or ARTICULATION is the junction or union of any two adjacent parts of the body. Most usually the term is employed to signify the connection established between contiguous bones. It is by the joints that the various bones are knit together to form the skeleton. Joints may be either immovable or movable. The immovable joints are divided into the synclwndroses the fibrous membrane, or peri osteum, passing between the two bones, which plays the part of a ligament, and which is continuous with the Interposed fibrous membrane. In old two bones ; Sc, the interposed cartil age ; 7, the fibrous membrane which plays the part of a ligament. and the sutures. A synchondrosis is the junction of two Syndic bones by the interposition of an intermediate plate of droees. cartilage, the fibrous membrane or periosteum which invests the bones being prolonged from one bone to the other over the surface of the cartilage. A suture is the Suture: connection of two bones by the interlocking of adjacent toothed margins; the periosteal fibrous membrane is prolonged from one bone to the other, and is also FIG. 12. vertic:Uscction through intpmnsprl bptwprm tlipir irlinfonf a cranial suture, b. b, the two interposed oetwee tnar adjacent toncs. ^ opl, osite tllc sutu rc; margins. In a young skull the basi-occipital and basi-sphenoid are united by synchondrosis, but junction by sutures is the mode of union which prevails in the bones of the head. persons the sutures become obliterated by the ossification of the intermediate fibrous membrane, and the bones are permanently fused, together. The cranial sutures may con- 1 veniently be arranged in three groups : a. Median longitudinal, consisting of FIG. 13. vertical sec. A. {. r i. i 1 i, , i tion through a syn- tne trontal suture, which connects the choncirosis. b, b, the two halves of the frontal bone, and the sagittal suture, between the two parietal bones ; b, Lateral longitudinal, consist ing on each side of the head of the fronto-nasal, fronto-maxillary, fronto-lachrymal, fronto- ethmoidal, fronto-malar, fronto-sphenoidal, parieto-sphe- noidal, parieto-squamous, parieto-mastoid sutures ; c, Verti cal transverse, consisting of the coronal or fronto-parietal, the lambdoidal or paricto-occipital, the sphenoido-malar, sphenoido-squamous and occipito-mastoid sutures. As the skull grows by ossification of the cartilage of the base and the membranous vault, the direction of growth is perpendicular to the margins of the bones and the sutures and synchondroses which connect them to gether. The growth of the skull in length is perpen dicular, therefore, to the basi-cranial syn chondrosis and the vertical transverse group of sutures; its growth, in breadth, to the median longi tudinal group, and in height to the lateral longitudinal group. So long as any of the cartilage or membrane be tween the margins of the bones remains unossified, bone may continue to form, and the skull may in crease in size. It sometimes happens that the cartilage or membrane is pre maturely ossified in a particular locality, and the further , . ,, i 11 Fio. 14. Vertex view of a boat-shaped or seapho- grOWth 01 the SKUll cephalic skull, showing the complete disappear- put a stop to in that ance of tho ea e ittal suture - region ; if the brain is still growing, the skull must increase in other directions to permit of the expansion of the cranial cavity, and deformities of the skull are thereby occasioned. One of the most usual of these deformities ia duo to premature closure of the sagittal suture, causing