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a membrane, called the periosteum. The long, roundish bones of the legs and arms have this peculiarity, that they are hollow in the middle, and are filled with a soft, fatty substance, called marrow.
The skeleton is divided into the head, the trunk, and the extremities.
1. The Head.—The bony framework of the head, called the skull, is divided into two parts, the cranium and the face. THE CRANIUM 2 is the shell which contains the brain, and is composed of eight bones, of which the following six form its top and sides : the frontal or coronal bone, so called from Latin corona, a crown, because it forms the crown of the head,
marked 1 on fig. 41 ; the two
Latin paries, a wall, because
they form the walls or sides
Greek sphēn, a wedge, is so
called because it wedges in and locks together all the bones of the head and face, being attached to fourteen distinct bones ; and the ethmoid, from Greek ēthmos, a sieve, from its being perforated with a large number of holes, for the passage of nerves from the brain to the face, and of blood vessels into the brain. It will be observed that the bony plates forming the upper part of the cranium are joined by ragged edges. These joinings are an elaborate system of dove-tailing, binding together the different pieces as firmly as if the whole were one, while certain purposes are also served by the cranium consisting of separate pieces. At the sides of the skull, some of the bones overlap each other, those above being supported by those beneath. Altogether, the bones of the skull form an arched or vaulted covering of extraordinary strength for the protection of the brain.
THE FACE is made up of fourteen bones, all of which, except the lower jawbone, are immovably fixed to each other, and to the bones of the
1 From Greek peri, about, and os, a bone. 2 Latin cranium, Greek kranion, from kara, the head.
cranium. By their union these bones form five cavities, which contain and protect the organs of sight, smell, and taste. The two principal bones are the upper maxillaryl or jaw bones, marked 8 on fig. 41, Overlapping, and joined to them at the sides, are the two malar2 or cheek bones, marked 6. These are joined behind to the two palate 3 bones, which form the back part of the roof of the mouth. The inside walls of the cavities for the eyes are partly formed by two small bones, called the lachrymal 4 bones, because there are holes through them for the passage of the ducts or canals which convey the tears from the eyes to the nose. The greater part of the nose is formed of cartilage or gristle, so that the bony part, formed of the two nasal bones, marked 7, is not very prominent. The nasal cavity is divided into two by a partition, which is partly formed by a bone called the vomer, from its resemblance to a ploughshare ; and its outer walls are formed of two small turbinated bones, which are thin bony plates, in the form of a scroll or horn, the use of which will be explained when we treat of the organ of smell. Last of all, comes the lower maxillary or lower jawbone, marked 9.
2. The Trunk.—The most important part of the trunk is the SPINE? or backbone, so called from its spikes or points. It consists of a large number of small pieces so jointed together as to make it exceedingly flexible. Each of the pieces is called a vertebra (Latin, 'a joint'], and is attached to the two between which it lies by strong elastic ligaments ;8 while between each pair is a cushion of cartilage, which is thickest in the lower part of the spine, and serves a very important purpose. When the body is jolted in any way, for example, in jumping from a height, these cushions act like the buffers of a railway train, and neutralise the shock. From its being composed of these vertebræ, the spine is called the vertebral column. Each vertebra is perforated with a round hole ; and from the manner in which they are joined together, these holes form a continuous canal, which contains and protects the spinal cord. This substance, although sometimes erroneously called the spinal marrow, is quite distinct from the real marrow found in the long bones of the legs and arms. It is, in fact, next to the brain, the most important part of the nervous system, in connection with which it will afterwards be described. Behind the backbone are three rows of projections ; one in the centre, forming the ridge felt along the back, and seen farthest to the right in the figure, called the spinous processes ; and one on each side of these, called the transverse processes. In the whole
1 From Latin maxilla, diminutive of mala, a jaw.
2 From Latin mala, a jaw.
19T CERDICAL OR ATLAS
2ND CERVICAL OR AXIS
column there are 33 vertebræ. At the top are 7 cervical 1 vertebræ, or
vertebræ of the neck. The topmost, or first cervical, is termed atlas, because by it the head is properly borne up, as the earth was by the fabled god Atlas. The second cervical is called the axis, because it is the proper axis or joint of the neck, the joint between it and the atlas, which is a pivot-joint, being that which enables the head to turn round. Next to the cervical are the 12 dorsal2 vertebræ, or vertebræ of the back; after which come 5 lumbar 3 vertebræ, or vertebræ of the loins. All the above mentioned may be separated from each other, and are called true vertebrce; the rest, namely, the 5 that are ossified together, and form the sacrum, or sacred bone, and the 4, also united, that form the coccyx,4 so named from its resemblance to the beak of the cuckoo, are called false vertebrce.
It is interesting to know that the skull is merely a prolongation of the backbone, and consists of four of the segments or vertebræ described above. The hollow space which contains the spinal cord is in these four expanded into a capacious chamber (the cranium) for containing the enlargement of the nerve-substance called the brain. The four segments of the spinal cord which correspond to the four vertebræ, and which form the basis of the brain, give off the nerves of smell, sight, taste, and hearing.
The bony part of the trunk is completed by THE RIBS, which are 24 elastic arches of bone, 12 on each side, attached behind to the dorsal vertebræ, and in front to the sternum, or breastbone, by a cartilaginous ligament. It will be seen from fig. 43 that the sternum does not extend far enough down to allow of all the ribs being directly attached to it; there are, in fact, only seven thus attached, which are therefore called true ribs. The other five are called false ribs. Of these, the ends of three are indirectly attached to each other and to the sternum by a
1 From Latin cervex, cervicis, the neck.
2 From Latin dorsum, the back.
cartilaginous band ; the other two are unattached to anything in front, and are therefore called floating ribs. The ribs, along with the sternum,
form the bony framework of the chest, in which are placed the heart and lungs.
3. The Extremities.—The extremities may be divided into upper and lower, or the arms and the legs. Each member consists of a set of extended
movable bones, which are worked by means of muscles, like so many levers, and of a bony framework to which the limbs proper are attached, and which is fixed to the trunk.
(1) THE UPPER EXTREMITIES—THE ARMS.—The bone in which is the socket for the first bone of the arm is a triangular, flat bone, lying on the back of the dorsal ribs, and called the scapula 1 (1 in fig. 43). It lies quite freely on the ribs, and so tends to give freedom of motion to the arm ; but it is very firmly supported by the clavicle2 or collar-bone (2 in fig. 43), which is attached to a projection on the scapula. The socket in the scapula for the reception of the head of the arm-bone, is a shallow cup-like cavity, into which the round head of the bone fits; and in consequence of the shallowness of this socket, the arm has perfect freedom to move in every direction. Yet this arrangement has its disadvantage, from the greater danger there is of the head of the bone being dislodged from its place, as not unfrequently happens, security being thus slightly sacrificed for freedom of motion. It will be seen that in the case of the hip-joint, on the other hand, through the greater depth of the socket, freedom of motion is sacrificed for security and strength. THE ARM itself is divided into three parts—the arm proper (from the shoulder to the elbow), the forearm (between the elbow and the wrist), and the hand. The bony part of the arm proper is a single hollow bone, called the humerus 3 or shoulder (3 in fig. 43). The shoulder-joint has already been described ; at the lower end the bone is flattish, to allow of its being jointed at the elbow to the two bones of the fore-arm-namely, the radius 4 and the ulna 14 and 5 in fig. 43). The ulna is connected with the humerus by a common hinge-joint ; while the radius has a pivot-joint, with a cavity in the end to receive a rounded knob on the end of the humerus, so that it is capable of a rotatory motion.
To these two bones at the wrist is attached THE HAND, which consists altogether of 27 bones. First come the 8 carpal 6 or wrist bones, which are arranged in two rows—3 to 10 in fig. 44, 1 and 2 being the ends of the radius and ulna. The
surface of the first row is convex, and fits into a cup-like socket in the lower end of the radius. The first carpal bone, 3, of the first row supports, by means of 7 and 8 in the second row, the bones of the thumb and forefinger (I and II); 5 in the first row, in like manner, supports, through 10 in the second row, the bones of the little finger and of the one next to it ; 4 and 9 in the first and second rows, support the bones of the middle finger. These 8 carpal
i Latin scapula = spatula, diminutive of spatha, a spade.
2 From Latin clavicula, diminutive of clavis, a key, so called from its resemblance to a Roman key. 3 Latin, 'the shoulder.'
4 Latin, 'a spoke of a wheel.' 6 Latin, 'the elbow.'
6 From low Latin carpus, the wrist.