of the Vertebrate Skeleton. 271 



bones is also due to the vertical pressure causing lateral 

 overgrowth at the joints to be growth in the direction of 

 least resistance, as pointed out by Prof. Humphrj. 



The bones normally present are dialjzed by the degeneration 

 of the surrounding connective tissue called periosteum, or from 

 the terminal articular or interosseous cartilage. To this peri- 

 osteum, or to the bone, muscles are for the most part attached, 

 and usually so attached that there is at least one joint between 

 the bones along which they extend. Now the property of a 

 muscle is, that the fibres which constitute it contract and ex- 

 tend. Therefore the very circumstance of their attachment 

 on the bones where there is a condition of yielding implies 

 that when they contract the muscles experience tension 5 and 

 if the bone does not yield, it experiences pressure or tension 

 from the pull of the muscles. Consequently the attached 

 muscles can undergo no movement without bringing their 

 modifying mechanical influence to bear upon the bones, which 

 is done partly by enabling them to act upon each other, and 

 partly by the intermittent pressure which the periosteum thus 

 is caused to exercise. The same action causes the bones or 

 skin to press against the muscles, and one muscle to press 

 against another. Thus in their exercise the muscles themselves 

 experience this same mechanical condition, which, resulting in 

 a pump-like action, sustains growth. 



Of the nerves, only the cerebro-spinal system is sufficiently 

 largely developed to exhibit any visible results of pressure. 

 And here the growth of the brain extends the bones of the 

 brain-case to cover the nervous substance ; and when the brain 

 contracts as in old age, the tension of the dura mater upon the 

 bones causes them to thicken, and adapt their inner surface to its 

 reduced size. Similarly the growth of the spinal cord forms the 

 perforation between the neural arch of the vertebra and its cen- 

 trum ; and the perforation enlarges by growth of the vertebral 

 elements with the increase in size of the spinal cord. 



The lungs, too, by inspiration and expiration, exert a con- 

 tinuous intermittent pressure upon the ribs ; and it may be ob- 

 served that the ribs are stretched and lifted up at each inspi- 

 ration. For a considerable period of life this is done with 

 increasing vigour ; and during that time the articular cartilages 

 grow. But just as hair, when it has passed through its cycle 

 of growth, grows no more, and dies, and as the particles of 

 the muscles and nerves and other organs which have by exer- 

 cise undergone the molecular change which has rendered them 

 effete die, and go through new conditions, so a time comes in 

 the life of cartilage when, in normal health, it can no longer 

 form new cartilage-cells ; and then there is no further growth 



