Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/876

Rh 826 ANATOMY [SKELETON and sphenoid, called spJieno-maxillary fossa, from which the pteryfjo-maxiihiry fissure extends downwards between the pterygoid and superior maxillary; and the spheno- maxillary fissure extends upwards into the orbit. The orbit is a four- walled pyramidal cavity, with the. bass directed forward to the face, and the apex backward to the brain cavity. At the apex are the foramina in the sphenoid, through which the nerve of sight and other nerves pass from the brain to the eyeball, muscles, and other soft structures within the orbit. The nostrils open on the front of the face by a large opening situated between the two superior maxillie, and bounded above by the two nasals. The sides of the opening pass down almost vertically to join the floor, and are not rounded off as in the ape s skull ; from the centre of the floor a sharp process, the nasal spine of the superior maxilla? projects forwards, and forms a characteristic feature of the human skull. Attached to the sides of the opening are the lateral cartilages of the nose, which form the wings of the nostrils, and so modify the position of their openings that in the face they look downwards. The nostrils are separated from each other by a vertical mesial partition composed of the mes-ethmoid, vomer, and triangular nasal cartilage, the last-named of which projects forward beyond the anterior surface of the upper jaw, and con tributes materially to the prominence of the nose. The outer wall of each nostril presents the convoluted turbinals, which are separated from each other by horizontal passages extending antero-posteriorly ; the superior passage or meatus lies between the superior and middle turbinals of the ethmoid, and is continued into the sphenoidal and posterior ethmoidal air-sinuses ; the middle meat us lies between the middle and inferior turbinals, and is continued into the frontal, anterior ethmoidal, and maxillary air sinuses. These sinuses are therefore extensions of the nasal chamber or respiratoiy passage, and correspond with the air cavities which exist in so many of the bones of birds ; the inferior meatus lies between the inferior turbinal and floor of the nose ; into its anterior part opens the nasal duct which conveys the tears from the front of the eyeball. The posterior openings of the nose are separated from each other by the hinder edge of the vomer, and are placed between the internal pterygoid plates of the sphenoid. The skull varies in appearance at different periods of life. In infancy the face is small, about -Jth of the size of the entire head, for the teeth are still rudimentary and the jaws are feeble ; the centres of ossification of the cranial bones are prominent; the forehead projects; the skull is widest at the parietal eminences ; the air-sinuses, and bony ridges corresponding to them, have not formed. In the adult the face is about half the size of the head, and its vertical diameter greatly elongated, from the growth of the antrum, the nose, and the dental borders of the jaws ; and the angle of the lower jaw is almost a right angle. In old age the teeth fall out, the jaws shrink in, their dental borders become absorbed, the angle of the lower jaw, as in infancy, is obtuse ; the vertex and floor of the skull also become flattened, and the sides bulge outwards, changes due to gravitation and the subsidence of the bones by their own weight. The skull of a woman is smaller and lighter, with the muscular ridges and projections due to the air sinuses less strongly marked than in a man, but with the eminences or centres of ossification more prominent. The more feeble air sinuses imply a more restricted respiratory activity and a less active mode of life than in a man. The internal capacity is about 10 per cent, less than that of the male. The face is smaller in proportion to the cranium ; the cranium is more flattened at the vertex, and the height Is consequently not so great in proportion to the length as in the man. In the female skull, therefore, the infantile characters are less departed from than is the case in the male. Turning now to the APPENDICULAR SKELETON, we shall consider first that of the SUPERIOR or THORACIC or PECTORAL EXTREMITY, or UPPER LIMB. The Upper Limb Upper may be subdivided into a proximal part or shoulder, a Limb, distal part or hand, and ah intermediate shaft, which con sists of an upper arm or brachium, and a fore -arm or anti- brachium. In each of these subdivisions certain bones are found : in the shoulder, the clavicle and scapula ; in the upper arm, the humerus ; in the fore-arm, the radius and ulna, the bone of the upper arm in man being longer than the bones of the fore-arm ; in the hand, the carpal and metacarpal bones and the phalanges. The scapula and clavicle together form an imperfect bony arch, the Scapular Arch or Shoulder Girdle ; the shaft and hand form a f rea divergent Appendage. The shoulder girdle is the direct medium of connection between the axial skeleton and the- divergent part of the limb; its anterior segment, the clavicle, articulates with the upper end of the sternum, whilst its posterior segment, the scapula, approaches, but does not reach, the dorsal spines. FIG. 8. Diagrammatic section to represent the relations of the shoulder girdle to the trunk. V, a Dorsal Vertebra ; C, a Rib ; Sr, the Sternum ; So, the Scapula ; Cr, the Coracoid ; Cl, the Clavicle ; M, the Meniscus at its sternal end ; H, the Humerus. The Clavicle, or Collar Bone (Fig. 9), is an elongated Clavicli bone which extends from the upper end of the sternum horizontally outwards, to articulate with the acromion process of the scapula. It presents a strong sigmoida) curve, which is associated with the transverse and horizontal direction of the axis of the human shoulder. It is slender in the female, but powerful in muscular males ; its sternal end thick and somewhat triangular ; its acromial end, flattened from above downwards, has an oval articular surface for the acromion. Its shaft has four surfaces for the attachment of muscles; and a strong ligament, con necting it with the coracoid, is attached to the under surface, near the outer end, whilst near the inner a strong ligament passes between it and the first rib. The clavicle is absent in the hoofed quadrupeds, in the seals and whales, and is feeble in the carnivora; but is well formed, not only in man, but in apes, bats, and in many rodents and insectivora. The Scapula, or Shoulder Blade (Fig. 9), is the most Scapul, important bone of the shoulder girdle, and is present in all mammals. It lies at the upper and back part of the wall of the chest, reaching from the second to the seventh rib. Its form is plate-like and triangular, with three surfaces, three borders, and three angles. The funda mental form of the scapula, as seen in the mole, is that of a three-sided prismatic rod, and its assumption of the plate or blade-like character in man is in connection with the great development of the muscles which rotate the humerus at the shoulder joint. Its costal or ventral surface is in relation to the ribs, from which it is separated by certain muscles : one, called subscapularis, arises from the surface itself, which is often termed subscapular fossa. The dorsum or back of the scapula is traversed from behind forwards by a prominent spine (PI. XIV., fig. 1, S), which lies in the proper axis of the scapula, and subdivides this aspect of the bone into a surface above the spine, the