S2ANATOMY 6 19 
_ In long bones, as those of the arm and leg, a central ring is form- 
ed in the body of the bone, the head and extremities being cartilage, 
in the centre of which the formation of bone afterwards begins.— 
The central ring of the body shoots its long fibres towards the head 
and extremities, which extend towards the body of the bone. . The 
head and extremities at length come so close to the body as to be 
merely separated by a cartilage, which becomes gradually thinner — 
until the twentieth year. ots 
Thick and round bones, as those of the ancle, wrists, breast and 
knee-pan, are at first all cartilage ; ossification begins in the centre 
of each. : 
At birth, the bones of the child are very imperfect. The extremi- 
_ ties and processes of almost all the long bones are connected with 
the body of the bone by cartilage. The skull has no s i 
membrane. On the fore part of the skull, between the side bones 
and the forehead bone, is a considerable membranous space ; and a 
similar but smaller one between the side bones and the hinder bone 
ofthe head. The forehead consists of two bones, and the hinder boné 
offour. The teeth are partly formed, especially the enamel, and are 
placed in adouble row. The outer ear-hole is surrounded bya bony 
circle, in which there is a groove for the attachment of the membrane 
of the drum; this circle gradually elongates into the hearing canal. 
ae joint cavities in all the bones are much shallower than in the 
adult. _ - far : 
~The ch consists ofthree bones, which are connected together by 
very firm cartilage, as 1¢ and its processes. 
CONNEXION OF BONES, 
Bones are connected with each other, so as to admit of motion; — 
r, So as to admit of no motion; or, with an intervening substance 
between them. There are several species of each of these. 
The different species of the moveable connexion, are : the ball and 
socket, where the round head of one bone is received into the deep 
cavity of another, so as to admit of motion in every direction, asthe __ 
thigh bone with the hip. ‘The second species is when the round 
head of one bone is received into a superficial cavity of another, 
no sutures; its 
F 7 cnt aaaio 8 
so as to admit of motion in every direction, as the head of the arm. 
The third is the hinge joint, which admits only of opening and shut- 
ting, or extension and flexion, as the knee-joint. The fourth, is 
where one bone rotates upon another, as the second neck joint, and 
the outer-arm wpon the fore-arm. The filth and last is, where there 
is motion, but that very obséure, as the long bones of the hand and as 
__ The species of the immoveable connexion are three ; first, when . 
_ the union is by means of tooth-like, or dove-tailed processes, as in the 
bones of the skull; or, second, by agreement, where the connexion 
e: is by rough margins, not tooth-like; as the bones of the face. a: ‘hird, 
= hen one bone is fixed to another like.a nail in a board, as the 
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