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APPENDICES TO THE SECTION ON OSTEOLOGY.
A. Architecture of the Bones of the Skeleton.
B. Variations in the Skeleton.
C. Serial Homologies of the Vertebræ.
D. Measurements and Indices employed in Physical Anthropology.
ARCHITECTURE OF THE BONES OF THE SKELETON.
The Vertebræ.-The vertebræ are formed of spongy bone confined within a thin and dense envelope. In the bodies the arrangement of the spongy tissue, which is traversed by venous channels, is such as to display a vertical striation with lamellæ arranged horizontally. The lateral, superior, and inferior walls are very thin- that directed to the vertebral canal being usually thicker and denser than the others. In the roots of the vertebral arches and roots of the transverse processes the spongy tissue is much more open. The outer envelope is much thicker where it bounds the vertebral canal, and where it forms the bottom of the superior and inferior intervertebral notches. In the lamina the spongy tissue is confined between two compact layers, of which that directed to the vertebral canal is the thicker. In the spinous processes the upper edge is always the more compact.
The Sternum consists of large-celled spongy bone, which is highly vascular, and is contained between two layers of thin compact tissue.
The Ribs. Each rib consists of a curved and compressed bar of bone, the interior of which consists of highly vascular spongy tissue with an external envelope of compact bone. The inner table is much the stronger, attaining its maximum thickness opposite the angle-in front and behind which it becomes gradually reduced. The outer table, much thinner, is stoutest opposite the angle; on the posterior surface of the tubercle and neck it forms but a thin layer. The compact layers forming the upper and lower borders are not so thick as those forming the inner and outer surfaces. The spongy tissue, loose and open in the body, is most compact in the region of the head and towards the anterior extremity.
The Frontal Bone. -The frontal bone is composed, like the other bones of the cranial vault, of two layers of compact tissue, enclosing between them a layer of spongy cancellous texture-the diploë. In certain definite situations, owing to the absorption of this intermediate layer, the bone is hollow, forming the frontal air-sinuses. The position and extent of these is to some extent indicated by the degree of projection of the superciliary ridges, though this must not be taken as an absolutely reliable guide, for cases are recorded where the ridges were low and the sinuses large, and vice versa. Of much surgical importance, these air-spaces only attain their full development after the age of puberty, being of larger size in the male than in the female, a circumstance which accounts for the more vertical appearance of the forehead in woman as contrasted with man. Usually two in number, they are placed one on either side of the median plane, and communicate by means of the infundibulum with the nasal cavity of the same side. It is exceptional to find the sinuses of opposite sides in communication with each other, as they are generally separated by a complete partition which, however, is occasionally much deflected to one or other side. Logan Turner ("On the Illumination of the Air Sinuses of the Skull, with some Observations upon the Surgical Anatomy of the Frontal Sinus," Edin. Med. Journ., May 1898) gives the average dimensions of these sinuses as follows:-Height, 31 mm., .e. from the frontonasal aperture upwards; breadth, 30 mm., i.e. from the septum horizontally in a lateral direction; depth, 17 mm., from the anterior wall at the level of fronto-nasal suture backwards along the orbital roof. Exceptionally large sinuses are sometimes met with extending backwards over the orbit so as to form a double roof to that space. There is a specimen in the Oxford collection in which the sinus is so large, and extends so far back, that the optic nerve is transmitted through it in a bony tube. Another point of some practical importance is that the sinuses are hardly ever symmetrical. It is rare to meet with cases of their complete absence, although sometimes the sinus on one or other side may be wanting.
The zygomatic process, from the arrangement of its surfaces and the density of its structure, is particularly well adapted to resist the pressure to which it is subjected when the jaws are firmly closed.
The Parietal Bone.-Thin towards its lower part, where it enters into the formation of the temporal fossa, it is thickest along the superior border and in the neighbourhood of the posterior superior angle.
The Occipital Bone. The squamous part displays thickenings in the position of the various ridge and crests, the stoutest part corresponding to the internal and external occipital protuberances,
though it should be noted that the two protuberances do not necessarily coincide, the internal being, as a rule, placed at a higher level than the external. If the bone be held up to the light it will be at once apparent that it is much thinner where it forms the floor of the inferior fossæ than in the upper part. The basilar portion consists of a spongy core surrounded by a more compact outer envelope, thickest on its lower surface. In the condyles the spongy tissue is arranged radially to their convex articular surfaces, the hypoglossal canal being surrounded by particularly dense and compact bone, which assists in strengthening this naturally weak part of the bone.
The Temporal Bone.-The temporal bone is remarkable for the hardness and density of its petrous part, wherein is lodged the osseous labyrinth, which contains the delicate organs associated with the senses of hearing and equilibration. The middle ear or tympanum is a cavity which contains the small auditory ossicles, and is separated from the external acoustic meatus by the membrana tympani. In front it communicates with the pharynx by the auditory tube; behind, it opens into the tympanic antrum and mastoid air-cells by the aditus ad antrum. Superiorly, it is parated from the middle cranial fossa by a thin plate of bone called the tegmen tympani. Inferiorly, its floor is formed in part by the roof of the jugular fossa and the carotid canal. Medially, it is related to the structures which form the inner ear, notably the cochlea and vestibule, in front of which it is separated by a thin plate of bone from the carotid canal. Owing to the disposition of the internal and external acoustic meatus the weakest part of the bone corresponds to a line connecting these two channels, the only parts intervening being the cochlea and tympanum. It is usually in this position that fracture of the bone occurs. Curving over the cavity of the tympanum is the canalis facialis, the thin walls of which are Occasionally deficient. These details, together with an account of the tympanic ossicles, will be further dealt with in the section devoted to the Organs of Sense.
The Sphenoid Bone.In the adult the body of the bone is hollow and encloses the sphenoidal air-sinuses, usually two in number, separated by a septum. The arrangement and extent of these air-sinuses vary; sometimes they are multilocular, at other times simple, while occasionally they extend backwards into the basi-occipital and laterally and downwards into the roots of the great wings and pterygoid processes. Cases are on record in which in the adult the body of the bone was not pneumatic.
The Lacrimal Bone.-The bone consists of a thin papery translucent lamina, somewhat strengthened by the addition of the vertical crest.
The Vomer.-The bone is composed of two compact layers fused below, but separated above by the groove for the lodgment of the rostrum of the sphenoid behind, and the septal cartilage in front. The lamellæ are also separated from each other by a canal which runs horizontally from behind forwards in the substance of the bone, and which transmits the nutrient vessel of the bone. The Nasal Bone. Formed of dense and compact bone; the strength of the nasal bones is increased by their mode of union and the formation of a median crest posteriorly.
The Maxilla. The disposition of the maxillary sinus within the body of the bone has been already referred to. In union with its fellow, the vaulted arrangement of the hard palate is well displayed, and the arched outline of the alveolar processes is obvious. It is in these latter processes around the sockets for the reception of the teeth that the spongy tissue of the bone is seen; elsewhere its walls are formed by thin and dense bone.
The Zygomatic Bone. In structure the bone is compact, with little spongy tissue. Together with the zygomatic process of the temporal bone it forms the buttress which supports the maxilla and the lateral wall of the orbit. Additional strength is imparted to the bone by the angular mode of union of its orbital and facial parts.
The Mandible.-The mandible is remarkable for the density and thickness of its medial and lateral walls. Where these coalesce below at the base of the body, the bone is particularly stout. Superiorly, where they form the walls of the alveoli, they gradually thin, being thicker, however, on the medial than the lateral side, except in the region of the last molar tooth, where the medial wall is the thinner. The spongy substance is open-meshed below, finer and more condensed where it surrounds the alveoli. The mandibular canal is large and has no very definite wall; it is prolonged beyond the mental foramen to reach the incisor teeth. From it numerous channels pass upwards to the sockets of the teeth, and it communicates freely with the surrounding spongy tissue. Above the canal the substance of the bone is broken up by the alveoli for the reception of the roots of the teeth. In the substance of the condyle the spongy tissue is more compact, with a general striation vertical to the articular surface.
The mental protuberance is an essentially human characteristic; by some it is associated with the development of speech in man, others regard it as due to the reduction in the size of the teeth. The Clavicle. The body consists of an outer layer of compact bone, thickest towards the middle of the shaft, but gradually thinning towards the extremities, the investing envelope of which consists merely of a thin shell. Within the body the spongy tissue displays a longitudinal striation, which internally assumes a more cellular appearance. At the acromial end the general arrangement of the fibres resembles the appearance of the sides of a Gothic arch. The curves of the bone impart an elasticity to it, which is of much service in reducing the effects of the shocks to which it is so frequently subjected.
The Scapula. For so light and thin a bone, the scapula possesses a remarkable rigidity. This is owing to the arrangement of its parts. Stout and thick where it supports the glenoid cavity
and coracoid process, the rest of the bone is thin, except along the axillary margin; but strength is imparted to the body by the manner in which the spine is fused at right angles to its dorsal surface.
The Humerus.-The body consists of a layer of compact bone surrounding a long medullary cavity. The outer shell, thickest in the distal third of the bone, gradually thins until it reaches the proximal epiphysial line, where it forms a layer no thicker than stout paper. Distally the external shell is thicker and stouter than it is proximally, until it reaches the epicondyles, distal to which the articular surfaces are formed of a layer of compact spongy bone. The proximal end of the medullary cavity is surrounded by loose spongy tissue, the fibres of which arch inwards from the inner surface of the compact outer layer, whilst at the distal end the spongy tissue which springs from the outer shell sweeps distally in a radiating fashion on either side of the olecranon fossa towards the epicondyles. Proximal to the olecranon fossa there is a number of laminae of dense bone which arch across from one side to the other, the convexity of the arches being directed distally. The proximal epiphysis, formed of spongy bone, is united to the body by a wavy line, concave laterally and convex medially, leading from the base of the greater tuberosity on the lateral side to the distal articular edge on the medial side. The mass above this includes the head and the two tubercles. The spongy tissue of the head is fine, and is arranged generally in lines radial to its surface; that of the greater tubercle is more open, and often displays large spaces towards its interior, which in old bones communicate freely with the medullary cavity of the body. The general direction of the fibres is parallel to the lateral surface of the tuberosity. The distal articular end is formed of fine spongy tissue, more compact towards the surface, and arranged in lines more or less at right angles to its articular planes. In the adult the principal nutrient canal, viz., that which opens on the surface near the insertion of the coraco-brachialis, traverses the outer compact wall of the body obliquely distally for a distance of two and a quarter inches before it opens into the medullary cavity.
The Ulna. The weakest parts of the bone are the constricted portion of the semilunar notch, and the body in its distal third, the bone being most liable to fracture at these points. On section the medullary cavity is seen to extend proximally as high as the base of the coronoid process; distally it reaches the proximal end of the distal fifth of the bone. The walls of the body, which are formed of dense bone, are much thicker on the dorsal surface than on the volar. Proximally they are continuous with the volar surface of the coronoid process and the dorsal surface of the olecranon, where they are composed of layers of looser texture, which, however, gradually become thinner as the points of these processes are reached. Distally they gradually taper until the head and styloid process are reached, round which they form a thin shell, considerably thickened, however, in the region of the groove for the extensor carpi ulnaris muscle. The bulk of the proximal extremity is formed of loose spongy bone, arranged in a series of arcades, stretching from the interior to the dorsal wall over the proximal end of the medullary canal. Proximal to the constricted part of the semilunar notch the bone displays a different structure; here it is formed of spongy bone, of closer texture, arranged generally in lines radiating from the articular surface. At the point of constriction of the semilunar notch the layer immediately subjacent is much denser and more compact.
The distal fifth of the bone is formed of loose spongy bone, the fibres of which have a general longitudinal arrangement; towards its extremity the meshes become smaller.
The Radius. The neck is the narrowest part of the bone; here fracture may occur, though not commonly. The point at which the bone is usually broken is about one inch proximal to the distal extremity. This is accounted for by the fact that the radius supports the hand at the radio-carpal articulation, and the shocks to which the latter is subjected, as in endeavouring to save oneself from falling, are naturally transmitted to the radius. On section, the medullary cavity is seen to extend to the neck; distally, it reaches to the level of the distal fifth of the bone. Its walls are thick as compared with the diameters of the bone, particularly along the interosseous border, thus imparting rigidity to the curve of the body; these walls thin out proximally and distally. Proximally, the surface of the tuberosity is formed of a thin shell of bone, which, however, thickens again where it passes on to the neck. The proximal extremity is formed of spongy bone arranged in the form of arcades, reaching distal to the level of the tuberosity medially, but not extending distal to the level of the neck laterally. Beneath the capitular articular surface there is a dense layer, thickest in the centre, and thinning towards the circumference; this is overlain by a very thin layer of less compact bone.
The distal fifth of the body and distal extremity are formed of loose spongy bone arranged more or less longitudinally. Immediately subjacent to the carpal articular surface the tissue is more compact, and displays a striation parallel to the articular plane. The nutrient canal of the shaft pierces the volar wall of the proximal part of the medullary cavity obliquely, running proximally for half an inch.
The Carpus. The bones are formed of fairly compact spongy tissue, surrounded by a thin shell of denser bone. They are very vascular, and their non-articular surfaces are pierced by many foramina.
The Metacarpus.-Similar in arrangement to that of long bones generally, though it may be noted that the compact walls of the body are thicker in proportion to the length of the bone than in the other long bones of the upper extremity.
The Phalanges. Each phalanx has a medullary cavity, the walls of the body being formed of dense compact bone, especially thick along the dorsal aspect. The extremities are made up of spongy bone within a thin dense shell.
Interior of tro
The Hip Bone. As a flat bone the os coxae consists of spongy tissue between two compact external layers. These latter vary much in thickness, being exceptionally stout along the ilio-pectineal line and the floor of the iliac fossa immediately above it. The gluteal aspect of the ilium is also formed by a layer of considerable thickness. The spongy tissue is loose and cellular in the thick part of the ilium and in the body of the ischium; absent where the floor of the iliac fossa is formed by the coalescence of the thin dense confining layers; fine grained and more compact in the tuberosity of the ischium, the iliac crest, and the floor of the acetabulum, in which latter situation it is striated by fibres which are directed radi
Base of trochanter minor
Space containing loose spongy
ally to the surface of that hollow, these
Compact tissue -with dense core forming the calcar femorale
FIG. 278.-DISSECTION SHOWING THE CALCAR
A slice of bone has been removed from the pos-
ANTERIOR fossa, where absorption of the thin bony plate has taken place.
The Femur.- The body has a medullary cavity which reaches the root of the lesser trochanter proximally. Distally it extends to within 3 inches of the distal articular surface. In the proximal half the outer compact wall is very thick, but distal to the middle of the body it gradually thins until it reaches the condyles, over which it passes as a thin, hardly definable external layer. Proximally, it is especially thick along the line of the linea aspera, and here the large nutrient canal may be seen passing obliquely towards the proximal end in the substance of the dense bone for the space of two inches. In the proximal end of the body the osseous lamellæ springing from the sides of the medullary cavity arch inwards towards the centre, intersecting each other in a manner comparable to the tracery of a Gothic window. The lower wall of the neck is thick distally, near the trochanter minor, but thins rapidly before it reaches the head. From this aspect of the neck there spring a series of oblique lamella which pass proximally and upwards, spreading in fan-shaped manner into the under surface of the head. These are intersected above by lamella which arch medially from the lateral side of the
FIG. 279.-SECTION THROUGH HEAD AND NECK OF
shaft distal to the greater trochanter, as well as from the inner surface of the thin but compact outer shell of the upper surface of the neck, the whole forming a bracket-like arrangement which assists materially in adding to the strength of the neck. Further support is afforded by the addition of a vertical layer of more compact bone within the spongy tissue of the neck. Distally, as may be seen in Fig. 278, this is continuous with the dense posterior wall of the body below; whilst proximally it sweeps up beneath the lesser trochanter, from which it is separated by a quantity of loose spongy tissue, to fuse proximally with the posterior dense wall of the neck above and medial to the intertrochanteric crest. It may be regarded as a continuation proximally of the posterior wall of the body beneath the trochanteric epiphysis. When studied in section (see Fig. 279), the central dense core of this partition exhibits a spur-like appearance: hence the name calcar femorale applied to it. It is of surgical importance in cases of fracture of the neck of the femur. (R. Thompson, Journ. Anat. and Physiol., vol. xlii. p. 60.) From it, stout lamellæ having a vertical direction arise. The spongy tissue of the head and greater trochanter is finely reticulated, that of the distal part of the neck and proximal part of the shaft being more open in its texture. Passing vertically through this tissue there is a vascular canal, the orifice of which opens externally on the floor of the trochanteric fossa.
The spongy tissue of the distal part of the body is more delicate and uniform in its arrangement, displaying a more or less parallel striation in a longitudinal direction. Subjacent to the articular surface the tissue is rendered more compact by the addition of lamellæ disposed in curves in harmony with the external aspect of the bone.
The Patella. The bone consists of a thick dense layer anteriorly, which thins towards the edges on either side and distally; proximally, it corresponds to the area of insertion of the quadriceps. The femoral articular surface is composed of a layer of compact bone, thickest in correspondence with the vertical elevation. Sandwiched between these two layers is a varying thickness of spongy tissue of fairly close grain, the striation of which on cross section runs in parallel lines from back to front; on vertical section the tissue appears to be arranged in lines passing radially from the deep surface of the femoral area to the more extensive anterior dense plate.
The Tibia. The body of the bone is remarkable for the thickness and density of the osseous tissue which underlies the anterior crest. The posterior wall is stout, but the medial and lateral walls are thinner. The several walls are thickest opposite the middle of the body, and thin out proximally and distally where the body unites with the epiphyses. The medullary cavity, narrow and circular in the middle of the bone, increases in all its diameters proximally and distally, and reaches to within 2 to 3 inches of either extremity. Proximally the arrangement of the lamellæ of the spongy tissue resembles a series of arches springing from the dense outer walls. These form a platform on which the proximal epiphysis rests, the spongy tissue of which displays a more or less vertical striation. This is much more compact under the condylic surfaces, the superficial aspect of which is formed by a thin layer of dense bone. The intercondyloid eminence and the tuberosity are also formed of compact tissue, whilst the circumference of the condyles is covered by a thinner and less dense wall. In the distal end of the body the spongy tissue, of a loose and cellular character, is arranged in vertical fibres, blending with the closer tissue of the distal epiphysis, the articular surface of which is covered by a thin but dense layer. In the adult bone the nutrient canal for the body is embedded in the dense posterior wall for the space of two inches.
The Fibula.-A medullary cavity runs throughout the length of the body, reaching the neck proximally, and extending to a point about 2 inches from the distal extremity of the lateral malleolus. The lateral wall of the body is usually considerably thicker than the medial. The head is formed of loose spongy bone, enclosed within a very thin dense envelope. The spongy tissue of the distal extremity is more compact, and acquires considerable density on the surfaces underlying the articular area and the pit behind it. The canal for the nutrient artery of the body opens into the medullary cavity about an inch distal to its external aperture.
The Bones of the Foot.-A longitudinal section through the articulated bones of the foot reveals the fact that the structure of the spongy substance of each individual bone is determined by the stress to which it is habitually subjected. In this connexion it is necessary to refer to the arched arrangement of the bones of the foot, a subject which is also treated in the section which deals with the Joints. The summit of the arch is formed by the talus, on which rests the tibia. Subjected as the talus is to a crushing strain, it is obvious that this load must be distributed throughout the arch, of which the calcaneus is the posterior pillar, whilst the heads of the metatarsal bones constitute the anterior pillar. It is found, consequently, that the lamella of the spongy tissue of the talus are arranged in two directions, which intercross and terminate below the dorsal articular surface. Of these fibres, some sweep backwards and downwards towards the posterior calcanean facet, beyond which they are carried in the substance of the calcaneus in a curved and wavy manner in the direction of the heel, where they terminate; whilst others, curving downwards and forwards from the trochlea of the talus, pass through the neck to reach the articular surface of the head, through which in like manner they may be regarded as passing onwards through the several bones which constitute the anterior part of the arch, thus accounting for the longitudinal striation as displayed in the structure of the navicular, cuneiform, and metatarsal bones. In the calcaneus, in addition to the foregoing arrangement, another set of curving fibres sweep from back to front of the bone beneath the more compact tissue which forms its under shell. These are obviously of advantage to prevent the spread of the bone when subjected to the crushing strain. In the sustentaculum tali a bracket-like