Part 4 (1/2)

_Experiments._ 1st. To show the earthy without the animal matter, burn a bone in a clear fire for about fifteen minutes, and it becomes white and brittle, because the gelatin, or animal matter of the bone, has been destroyed.

2d. To show the animal without the earthy matter of the bones, immerse a slender bone for a few days in a weak acid, (one part muriatic acid and six parts water,) and it can then be bent in any direction. In this experiment, the acid has removed the earthy matter, (carbonate and phosphate of lime,) yet the form of the bone is unchanged.

69. The bones are formed from the blood, and are subjected to several changes before they are perfected. At their early formative stage, they are cartilaginous. The vessels of the cartilage, at this period, convey only the _lymph_, or white portion of the blood; subsequently, they convey red blood. At this time, true ossification (the deposition of phosphate and carbonate of lime) commences at certain points, which are called _the points of ossification_.

70. Most of the bones are formed of several pieces, or centres of ossification. This is seen in the long bones which have their extremities separated from the body by a thin part.i.tion of cartilage.

It is some time before these separate pieces are united to form one bone.

71. When the process of ossification is completed, there is still a constant change in the bones. They increase in bulk, and become less vascular, until middle age. In advanced life, the elevations upon their surface and near the extremities become more prominent, particularly in individuals accustomed to labor. As a person advances in years, the vitality diminishes, and in extreme old age, the earthy substance predominates; consequently, the bones are extremely brittle.

How can the earthy matter of the bones be shown? The animal? 69.

What is the appearance of the bones in their early formative stage?

When does true ossification commence? 70. How are most of the bones formed? 71. What is said of the various changes of the bones after ossification?

72. The fibrous membrane that invests the bones is called _per-i-oste-um_; that which covers the cartilages is called _per-i-chondri-um_. When this membrane invests the skull, it is called _per-i-crani-um_.

[Ill.u.s.tration: Fig. 6. A section of the knee-joint. The lower part of the femur, (thigh-bone,) and upper part of the tibia, (leg-bone,) are seen ossified at 1, 1. The cartilaginous extremities of the two bones are seen at _d_, _d_. The points of ossification of the extremities, are seen at 2, 2. The patella, or knee-pan, is seen at _c_. 3, A point, or centre of ossification.]

73. The PERIOSTEUM is a firm membrane immediately investing the bones, except where they are tipped with cartilage, and the crowns of the teeth, which are protected by enamel. This membrane has minute nerves, and when healthy, possesses but little sensibility. It is the nutrient membrane of the bone, endowing its exterior with vitality; it also gives insertion to the tendons and connecting ligaments of the joints.

72. What is the membrane called that invests the bones? That covers the cartilage? That invests the skull? Explain fig. 6. 73. Describe the periosteum.

74. There are two hundred and eight[3] bones in the human body, beside the teeth. These, for convenience, are divided into four parts: 1st. The bones of the _Head_. 2d. The bones of the _Trunk_.

3d. The bones of the _Upper Extremities_. 4th. The bones of the _Lower Extremities_.

[3] Some anatomists reckon more than this number, others less, for the reason that, at different periods of life, the number of pieces of which one bone is formed, varies. _Example._ The breast-bone, in infancy, has _eight_ pieces; in youth, _three_; in old age, but _one_.

75. The bones of the HEAD are divided into those of the _Skull_, _Ear_, and _Face_.

76. The SKULL is composed of eight bones. They are formed of two plates, or tablets of bony matter, united by a porous portion of bone.

The external tablet is fibrous and tough; the internal plate is dense and hard, and is called the _vitre-ous_, or gla.s.sy table. These tough, hard plates are adapted to resist the penetration of sharp instruments, while the different degrees of density possessed by the two tablets, and the intervening spongy bone, serve to diminish the vibrations that would occur in falls or blows.

77. The skull is convex externally, and at the base much thicker than at the top or sides. The most important part of the brain is placed here, completely out of the way of injury, unless of a very serious nature. The base of the cranium, or skull, has many projections, depressions, and apertures; the latter affording pa.s.sages for the nerves and blood-vessels.

74. How many bones in the human body? How are they divided? 75-81.

_Give the anatomy of the bones of the head._ 75. How are the bones of the head divided? 76. Describe the bones of the skull. 77. What is the form of the skull? What does the base of the skull present?