Structure of bone Bone is composed partly of ?brous tissue, partly of bone matrix comprising phosphate and carbonate of lime, intimately mixed together. The bones of a child are about two-thirds ?brous tissue, whilst those of the aged contain one-third; the toughness of the former and the brittleness of the latter are therefore evident.
The shafts of the limb bones are composed of dense bone, the bone being a hard tube surrounded by a membrane (the periosteum) and enclosing a fatty substance (the BONE MARROW); and of cancellous bone, which forms the short bones and the ends of long bones, in which a ?ne lace-work of bone ?lls up the whole interior, enclosing marrow in its meshes. The marrow of the smaller bones is of great importance. It is red in colour, and in it red blood corpuscles are formed. Even the densest bone is tunnelled by ?ne canals (Haversian canals) in which run small blood vessels, nerves and lymphatics, for the maintenance and repair of the bone. Around these Haversian canals the bone is arranged in circular plates called lamellae, the lamellae being separated from one another by clefts, known as lacunae, in which single bone-cells are contained. Even the lamellae are pierced by ?ne tubes known as canaliculi lodging processes of these cells. Each lamella is composed of very ?ne interlacing ?bres.
GROWTH OF BONES Bones grow in thickness from the ?brous tissue and lime salts laid down by cells in their substance. The long bones grow in length from a plate of cartilage (epiphyseal cartilage) which runs across the bone about 1·5 cm or more from its ends, and which on one surface is also constantly forming bone until the bone ceases to lengthen at about the age of 16 or 18. Epiphyseal injury in children may lead to diminished growth of the limb.
REPAIR OF BONE is e?ected by cells of microscopic size, some called osteoblasts, elaborating the materials brought by the blood and laying down strands of ?brous tissue, between which bone earth is later deposited; while other cells, known as osteoclasts, dissolve and break up dead or damaged bone. When a fracture has occurred, and the broken ends have been brought into contact, these are surrounded by a mass of blood at ?rst; this is partly absorbed and partly organised by these cells, ?rst into ?brous tissue and later into bone. The mass surrounding the fractured ends is called the callus, and for some months it forms a distinct thickening which is gradually smoothed away, leaving the bone as before the fracture. If the ends have not been brought accurately into contact, a permanent thickening results.
VARIETIES OF BONES Apart from the structural varieties, bones fall into four classes: (a) long bones like those of the limbs; (b) short bones composed of cancellous tissue, like those of the wrist and the ankle; (c) ?at bones like those of the skull; (d) irregular bones like those of the face or the vertebrae of the spinal column (backbone).
The skeleton consists of more than 200 bones. It is divided into an axial part, comprising the skull, the vertebral column, the ribs with their cartilages, and the breastbone; and an appendicular portion comprising the four limbs. The hyoid bone in the neck, together with the cartilages protecting the larynx and windpipe, may be described as the visceral skeleton.
AXIAL SKELETON The skull consists of the cranium, which has eight bones, viz. occipital, two parietal, two temporal, one frontal, ethmoid, and sphenoid; and of the face, which has 14 bones, viz. two maxillae or upper jaw-bones, one mandible or lower jaw-bone, two malar or cheek bones, two nasal, two lacrimal, two turbinal, two palate bones, and one vomer bone. (For further details, see SKULL.) The vertebral column consists of seven vertebrae in the cervical or neck region, 12 dorsal vertebrae, ?ve vertebrae in the lumbar or loin region, the sacrum or sacral bone (a mass formed of ?ve vertebrae fused together and forming the back part of the pelvis, which is closed at the sides by the haunch-bones), and ?nally the coccyx (four small vertebrae representing the tail of lower animals). The vertebral column has four curves: the ?rst forwards in the neck, the second backwards in the dorsal region, the third forwards in the loins, and the lowest, involving the sacrum and coccyx, backwards. These are associated with the erect attitude, develop after a child learns to walk, and have the e?ect of diminishing jars and shocks before these reach internal organs. This is aided still further by discs of cartilage placed between each pair of vertebrae. Each vertebra has a solid part, the body in front, and behind this a ring of bone, the series of rings one above another forming a bony canal up which runs the spinal cord to pass through an opening in the skull at the upper end of the canal and there join the brain. (For further details, see SPINAL COLUMN.) The ribs – 12 in number, on each side – are attached behind to the 12 dorsal vertebrae, while in front they end a few inches away from the breastbone, but are continued forwards by cartilages. Of these the upper seven reach the breastbone, these ribs being called true ribs; the next three are joined each to the cartilage above it, while the last two have their ends free and are called ?oating ribs. The breastbone, or sternum, is shaped something like a short sword, about 15 cm (6 inches) long, and rather over 2·5 cm (1 inch) wide.
APPENDICULAR SKELETON The upper limb consists of the shoulder region and three segments – the upper arm, the forearm, and the wrist with the hand, separated from each other by joints. In the shoulder lie the clavicle or collar-bone (which is immediately beneath the skin, and forms a prominent object on the front of the neck), and the scapula or shoulder-blade behind the chest. In the upper arm is a single bone, the humerus. In the forearm are two bones, the radius and ulna; the radius, in the movements of alternately turning the hand palm up and back up (called supination and pronation respectively), rotating around the ulna, which remains ?xed. In the carpus or wrist are eight small bones: the scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate and hamate. In the hand proper are ?ve bones called metacarpals, upon which are set the four ?ngers, each containing the three bones known as phalanges, and the thumb with two phalanges.
The lower limb consists similarly of the region of the hip-bone and three segments – the thigh, the leg and the foot. The hip-bone is a large ?at bone made up of three – the ilium, the ischium and the pubis – fused together, and forms the side of the pelvis or basin which encloses some of the abdominal organs. The thigh contains the femur, and the leg contains two bones – the tibia and ?bula. In the tarsus are seven bones: the talus (which forms part of the ankle joint); the calcaneus or heel-bone; the navicular; the lateral, intermediate and medial cuneiforms; and the cuboid. These bones are so shaped as to form a distinct arch in the foot both from before back and from side to side. Finally, as in the hand, there are ?ve metatarsals and 14 phalanges, of which the great toe has two, the other toes three each.
Besides these named bones there are others sometimes found in sinews, called sesamoid bones, while the numbers of the regular bones may be increased by extra ribs or diminished by the fusion together of two or more bones.... Medical Dictionary
The contents of the pelvis are the urinary bladder and rectum in both sexes; in addition the male has the seminal vesicles and the prostate gland surrounding the neck of the bladder, whilst the female has the womb, ovaries, and their appendages.
A second meaning is as in renal pelvis – that part of the collecting system proximal to the URETER which collects urine from the renal pyramids (see KIDNEYS).... Medical Dictionary
The human backbone is about 70 cm (28
inches) in length, and varies little in full-grown people; di?erences in height depend mainly upon the length of the lower limbs. The number of vertebrae is 33 in children, although in adult life ?ve of these fuse together to form the sacrum, and the lowest four unite in the coccyx, so that the number of separate bones is reduced to 26. Of these there are seven in the neck, known as cervical vertebrae; 12 with ribs attached, in the region of the thorax known as thoracic or dorsal vertebrae; ?ve in the loins, called lumbar vertebrae; ?ve fused to form the sacrum; and four joined in the coccyx. These numbers are expressed in a formula thus: C7, D12, L5, S5, Coc4=33.
Although the vertebrae in each of these regions have distinguishing features, all the vertebrae are constructed on the same general plan. Each has a thick, rounded, bony part in front, known as the body, and these bodies form the main thickness of the column. Behind the body of each is a ring of bone, the neural ring, these rings placed one above another forming the bony canal which lodges the spinal cord. From each side of the ring a short process of bone known as the transverse process stands out, and from the back of the ring a larger process, the spinous process, projects. These processes give attachment to the strong ligaments and muscles which unite, support, and bend the column. The spines can be seen or felt beneath the skin of the back lying in the centre of a groove between the muscular masses of the two sides, and they give to the column its name of the spinal column. One of these spines, that of the seventh cervical vertebra, is especially large and forms a distinct bony prominence, where the neck joins the back. Between the bodies of the vertebrae lies a series of thick discs of ?brocartilage known as intervertebral discs. Each disc consists of an outer portion, known as the annulus ?brosus, and an inner core, known as the nucleus pulposus. These 23 discs provide the upper part of the spine with pliability and resilience.
The ?rst and second cervical vertebrae are specially modi?ed. The ?rst vertebra, known as the atlas, is devoid of a body, but has a specially large and strong ring with two hollows upon which the skull rests, thus allowing forward and backward movements (nodding). The second vertebra, known as the axis, has a pivot on its body which ?ts into the ?rst vertebra and thus allows free rotation of the head from side to side. The spinal column has four natural curves (see diagram) which help to cushion the shocks of walking and running.
The neural rings of the vertebrae form a canal, which is wide in the neck, smaller and almost round in the dorsal region, and wide again in the lumbar vertebrae. Down the canal runs the spinal cord, and the nerves leaving the cord do so through openings between the vertebrae which are produced by notches on the upper and lower margins of each ring. The intervertebral foramina formed by these notches are so large in comparison with the nerves passing through them that there is no chance of pressure upon the latter, except in very serious injuries which dislocate and fracture the spine.... Medical Dictionary
In its course from the base of the skull to the lumbar region, the cord gives o? 31 nerves on each side, each of which arises by an anterior and a posterior root that join before the nerve emerges from the spinal canal. The openings for the nerves formed by notches on the ring of each vertebra have been mentioned under the entry for spinal column. To reach these openings, the upper nerves pass almost directly outwards, whilst lower down their obliquity increases, until below the point where the cord ends there is a sheaf of nerves, known as the cauda equina, running downwards to leave the spinal canal at their appropriate openings.
The cord is a cylinder, about the thickness of the little ?nger. It has two slightly enlarged portions, one in the lower part of the neck, the other at the last dorsal vertebra; and from these thickenings arise the nerves that pass to the upper and lower limbs. The upper four cervical nerves unite to produce the cervical plexus. From this the muscles and skin of the neck are mainly supplied, and the phrenic nerve, which runs down through the lower part of the neck and the chest to innervate the diaphragm, is given o?. The brachial plexus is formed by the union of the lower four cervical and ?rst dorsal nerves. In addition to nerves to some of the muscles in the shoulder region, and others to the skin about the shoulder and inner side of the arm, the plexus gives o? large nerves that proceed down the arm.
The thoracic or dorsal nerves, with the exception of the ?rst, do not form a plexus, but each runs around the chest along the lower margin of the rib to which it corresponds, whilst the lower six extend on to the abdomen.
The lumbar plexus is formed by the upper four lumbar nerves, and its branches are distributed to the lower part of the abdomen, and front and inner side of the thigh.
The sacral plexus is formed by parts of the fourth and ?fth lumbar nerves, and the upper three and part of the fourth sacral nerves. Much of the plexus is collected into the sciatic nerves, the largest in the body, which go to the legs.
The sympathetic system is joined by a pair of small branches given o? from each spinal nerve, close to the spine. This system consists of two parts, ?rst, a pair of cords running down on the side and front of the spine, and containing on each side three ganglia in the neck, and beneath this a ganglion opposite each vertebra. From these two ganglionated cords numerous branches are given o?, and these unite to form the second part – namely, plexuses connected with various internal organs, and provided with numerous large and irregularly placed ganglia. The chief of these plexuses are the cardiac plexus, the solar or epigastric plexus, the diaphragmatic, suprarenal, renal, spermatic, or ovarian, aortic, hypogastric and pelvic plexuses.
The spinal cord, like the brain, is surrounded by three membranes: the dura mater, arachnoid mater, and pia mater, from without inwards. The arrangement of the dura and arachnoid is much looser in the case of the cord than their application to the brain. The dura especially forms a wide tube which is separated from the cord by ?uid and from the vertebral canal by blood vessels and fat, this arrangement protecting the cord from pressure in any ordinary movements of the spine.
In section the spinal cord consists partly of grey, but mainly of white, matter. It di?ers from the upper parts of the brain in that the white matter (largely) in the cord is arranged on the surface, surrounding a mass of grey matter (largely neurons – see NEURON(E)), while in the brain the grey matter is super?cial. The arrangement of grey matter, as seen in a section across the cord, resembles the letter H. Each half of the cord possesses an anterior and a posterior horn, the masses of the two sides being joined by a wide posterior grey commissure. In the middle of this commissure lies the central canal of the cord, a small tube which is the continuation of the ventricles in the brain. The horns of grey matter reach almost to the surface of the cord, and from their ends arise the roots of the nerves that leave the cord. The white matter is divided almost completely into two halves by a posterior septum and anterior ?ssure and is further split into anterior, lateral and posterior columns.
Functions The cord is, in part, a receiver and originator of nerve impulses, and in part a conductor of such impulses along ?bres which pass through it to and from the brain. The cord contains centres able to receive sensory impressions and initiate motor instructions. These control blood-vessel diameters, eye-pupil size, sweating and breathing. The brain exerts an overall controlling in?uence and, before any incoming sensation can a?ect consciousness, it is usually ‘?ltered’ through the brain.
Many of these centres act autonomously. Other cells of the cord are capable of originating movements in response to impulses brought direct to them through sensory nerves, such activity being known as REFLEX ACTION. (For a fuller description of the activities of the spinal cord, see NEURON(E) – Re?ex action.)
The posterior column of the cord consists of the fasciculus gracilis and the fasciculus cuneatus, both conveying sensory impressions upwards. The lateral column contains the ventral and the dorsal spino-cerebellar tracts passing to the cerebellum, the crossed pyramidal tract of motor ?bres carrying outgoing impulses downwards together with the rubro-spinal, the spino-thalamic, the spino-tectal, and the postero-lateral tracts. And, ?nally, the anterior column contains the direct pyramidal tract of motor ?bres and an anterior mixed zone. The pyramidal tracts have the best-known course. Starting from cells near the central sulcus on the brain, the motor nerve-?bres run down through the internal capsule, pons, and medulla, in the lower part of which many of those coming from the right side of the brain cross to the left side of the spinal cord, and vice versa. Thence the ?bres run down in the crossed pyramidal tract to end beside nerve-cells in the anterior horn of the cord. From these nerve-cells other ?bres pass outwards to form the nerves that go direct to the muscles. Thus the motor nerve path from brain to muscle is divided into two sections of neurons, of which the upper exerts a controlling in?uence upon the lower, while the lower is concerned in maintaining the muscle in a state of health and good nutrition, and in directly calling it into action. (See also NERVE; NERVOUS SYSTEM.)... Medical Dictionary