There are two hundred and four bones in the body:
| The vertebral column contains | 26 |
| The skull—cranium and face | 22 |
| The hyoid bone—bone of the tongue | 1 |
| Ribs and sternum, forming the thorax | 25 |
| The upper limbs—arms and shoulders | 64 |
| The lower limbs | 62 |
| Small bones, including the patella or kneecap, to the number of | 16 |
The organic constituents form about 33.3 per cent of the composition of bone, while the remainder, 66.7 per cent, is inorganic matter; as follows:
| Organic matter (gelatine and blood-vessels) | 33.30 | ||
| Inorganic substances |  | Phosphate of lime | 51.04 |
| Carbonate of lime | 11.30 | ||
| Fluoride of calcium | 2.00 | ||
| Phosphate of magnesia | 1.16 | ||
| Soda and chloride of sodium | 1.20 | ||
The mineral or earthy matter enters very largely into the composition of bone.
A fibrous membrane covers bone externally, and is called periosteum. The hollow bones contain marrow, composed of fat, 96 parts; water, 3; connecting tissue, 1. Bones are supplied with blood-vessels, which carry the nutritious fluid to them.
1. The master tissues. Primarily, it is the tissue, and not the blood, that gets loaded with carbonic acid, the latter simply receiving the gas from the former by diffusion, and the oxygen which passes from the blood into the tissues being at once taken up in some combination.
2. Nearly one-half of the weight of the body consists of the skeletal muscles, and about one-quarter of the total blood in the body is contained in them.
3. The muscles are always producing carbonic acid (CO2), and when they contract there is a sudden and extensive increase of the normal production.
4. Oxygen is necessary for the life of the muscle; it is for the nervous tissue, but for muscular tissue especially.