48 PHYSIOLOGY. 



enclosed in membranous bags, and in health the fluid which 

 moistens them is of just the right quantity and quality ; but 

 if we do not take proper exercise, or if we take food or drink 

 that is too heating or irritating, the joints grow stiff, or 

 rheumatism or gout may be the consequence. 



22. In those parts where bones are to be formed, a mould of 

 gristle or cartilage is first deposited, of the exact shape that 

 the future bone is to take ; and as bony particles are secret, 

 ed by the blood, the cartilage is taken up by the absorbent 

 vessels. In the long round bones, this process begins in the 

 middle, and in the flat bones, like the shoulder blade, or those 

 of the skull, it begins in the centre and extends gradually 

 towards the circumference^ But in the skull, all the bones 

 are not completely formed, till several months after birth. 



23. The bones are variously connected by joints or arti- 

 culations, which admit of different degrees of motion, both 

 in extent and variety. Some of these connections allow 

 free, easy, and conspicuous motion, as the shoulder and hip 

 joints, which are called ball and socket joints ; in others, 

 there is motion in only two directions, as the knee, elbow, 

 wrist, and ankle. These are called hinge-joints, from their 

 resemblance to a hinge. Some bones, like those of the skull, 

 though connected by a kind of articulation, are nevertheless 

 immoveable. 



24. Another form of animal matter, differing essentially 

 from those we have been considering is, muscular tissue. 

 This is familiar to all under the name of flesh. It is a sub- 

 stance of a peculiar nature, arranged in fibres of extreme 

 delicacy. |It is distinguished from every other texture in 

 the body, by an innate power of contraction j on examin- 

 ing it with microscopes of great magnifying power, it is 

 found to b^[ composed of filaments so fine as the one forty 

 thousandth part of an inch in diameter] These filaments 

 collected together, form fibres, which are plainly perceptible 

 in boiled flesh v {A collection of these fibres form a bundle, 

 and these bundles collectively constitute a muscle ; and 



