SKELETON. 377 



beyond its own internal economy, when one part is to act upon and to 

 sustain another, then rigidity becomes requisite, and bone begins to be 

 formed in preparation for this period. This is similar to what goes on 

 in every other part which is to come into action after birth, with a new 

 mode of the continuance of life. Tbus the lungs, stomach, intestines, 

 with every thing relative to them, the brain, nerves, senses, and ex- 

 tremities are forming, from an early period, so far as to be useful when 

 the animal comes, as it is calculated, into the world. The substitutes 

 for bone at those early periods are cartilage and membrane, but carti- 

 lage chiefly. And this varies in consistence according to the age. It 

 is, at first, almost a jelly, and becomes firmer and firmer as the foetus 

 may move upon itself. But as it is not wholly changed for bone at 

 birth, what cartilage remains is become, by this time, fit to act as bone. 

 "Where the least power of resistance is wanted, where no action can 

 take place, there we have only membrane ; and this membrane is only 

 to contain, and to allow itself to be changed for bone ; such as what 

 covers the brain ; for, while in the state of a foetus, the brain requires 

 only to be covered, similar to the abdominal viscera ; but, by the time 

 of birth, it wants something more. 



I shall here observe that the cartilage of those animals where there 

 is bone, is of two kinds : the one that is changed for bone is a uniform 

 mass, but that which does not is fibrous, and is placed on the ends of 

 bones, forming the cavity of the joint ; and as it is for the purpose of 

 the motion of the joint, it remains a cartilage through the life of the 

 animal. The cartilage that is changed for bone is of two kinds respect- 

 ing the time of change : one is where the change is complete when the 

 animal has arrived at its fall growth, forming what are commonly called 

 the ' bones.' The other is where the period of change is uncertain, 

 seldom until considerably after that of full growth. Such commonly 

 retain their original name of cartilage, and when they do become bone, 

 are said to be ossified : such are the cartilages of the larynx, and of 

 the ribs or sternum. The earth in bones is not to be considered as a 

 part of the animal ; it is like the fat ; for it can be diminished or in- 

 creased by simple absorption or deposition. It is a secreted substance, 

 and, as it were, thrown out of the machine simply for a mechanical pur- 

 pose. It is not required to be of the nature of the machine, for it can 

 be made up of parts, e. g. madder, that are not altered by the stomachic 

 process. 



The stone in the bladder, which cannot be considered as part of the 

 animal, is coloured in the same manner as bone ; the urine being the 

 carrier of the madder out of the blood. 



As those cells that contain oil are called ■' adipose membrane/ 



