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Cartilage, Ligaments, and Tendons.

between each two an elastic bed of intervertebral substance, are light and spongy, while their articulating surfaces and processes are very hard. In the joints we see the tough elastic smooth substances, called cartilage, covering the rubbing ends of the bones, defending and padding them, and destroying friction. In infants we find all the bones soft or cartilaginous, and therefore calculated to bear without fracture, the falls and blows incidental to early age; and we see in certain parts, where elasticity is necessary or useful, the cartilage retains its character for life, as at the anterior extremities of the ribs. About the joints we have to remark the ligaments which bind the bones together, possessing a tenacity scarcely equalled in any other known substance; and we see that the muscular fibres, whose contractions move the bones and thereby the body, because they would have rendered the limbs clumsy, even to deformity, had they passed unchanged over the joints to the parts which they have to pull,-attach themselves at convenient distances from the joint to a strong cord called a tendon, by means of which, like a hundred sailors at one rope, they make their effort effective at any distance. The tendons are remarkable for the great strength which resides in their slender forms, and for the lubricated smoothness of their surfaces. Many other striking particulars might be enumerated, but these may suffice.

SECTION II-ANIMAL HYDRAULICS AND PNEUMATICS.

ANALYSIS OF THE SECTION.

1. The Circulation of the Blood.-There is constantly streaming from the heart to all parts of the animal body a red opaque fluid, the blood, carrying fresh nourishment to the various tissues and crgans, and taking from them the results of waste, or old material which has served its purpose in the body, and has to be carried away. The motion is kept up chiefly by the pumping action of the heart, forcing the blood along the tubes called arteries, which gradually ramify to every spot, through the extreme branches, called from their minuteness capillaries, into a corresponding tubular system called veins, which carry it back to the lungs to be purified and renewed.

2 Respiration or Breathing.-The chest is a cavity which alternately expands and contracts like a pair of bellows, thereby taking in and again expelling a certain volume of atmospheric air. The air comes nearly into contact with every particle of the circulating blood as this passes at every revolution through the spongy lobes of the lungs which occupy the chest. These lobes consist chiefly of delicate air-cells and minute capillaries, so thin that air can immediately act through their substance. Great changes are produced in the blood by the air, and it is again rendered fit to support life.

The Circulation of the Blood.

1088. There are few things more remarkable in the history of the progress by which man has arrived at his present knowledge of nature, than that, until within a comparatively recent period, he was ignorant of the fact that the blood in his own and in other animal bodies, is constantly travelling from the heart to all other parts, and back again. This truth was at variance with strong appearances, and the most fixed prejudices. It fell to the lot of our countryman, Dr. Harvey, to make this grand discovery, and he was probably led to it from having a more extensive knowledge of mechanical philosophy than was common among his professional brethren at that time. He published his proofs in the year 1628.* A person who

* Exercitatio de motu cordis et sanguinis.

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Plan of the Circulation.

tries to imagine what the science of medicine could have been while it took no account of this fact, on which, as a basis, nearly all correct reasoning about the phenomena of life and disease must rest, is prepared for what old medical books exhibit of the writhings of human reason, in attempts to account for numerous facts, or to form theories, while a fatal error was mixed with every supposition. The chief circumstance which prevented the earlier discovery of the circulation was, that, on examining dead bodies, the arteries were always found to be without blood in them, while the veins were charged with it; which was the reason, also, of the first-named vessels being called arteries or air-tubes.

We now know, that, as water from a central source spreads over a large city in pipes, to supply the inhabitants generally, so in the human body, does the blood spread from one centre, the heart, through the arteries, to nourish all the parts, and to supply to the liver, kidneys, stomach, and other organs, the materials for secretion and excretion. It then returns by the veins to the heart, and thence to the lungs, to be purified and to have its waste so replenished that it may again renew its course through the body.

In the water-works of a great city, the motion is given by a pumpbarrel and piston, worked by steam power. In the human body the

pump of the blood is the heart, a strong muscular bag, which relaxes to let bloed enter through a valve at one side, and then, by contracting, forces it out again through a valve on the other side into the arterial tube, which carries it forward. This kind of action is well illustrated by the common caoutchouc-bag syringe, A, worked by the force of a hand squeezing it. The bag, A, is of the size of an orange. Its entrancetube, b, if immersed in water, admits the charge through the valve at b, which then closes to prevent any return. On then squeezing the bag with the hand, the water in it is forced out through the valve, c, and jets from the end of the tube, d. On ceasing to squeeze, the bag is refilled from b, while the valve, c, prevents the return of what had already passed through. If an elastic tube, as sketched here by the dotted lines, d, e, f, b, be added to form an open communication between the two orifices of outlet and inlet, and if the hand

A

Fig. 3.5.

Constituents of the Blood.

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then contract at intervals like the beats of the heart, a strong current or circulation of the water through the bags and tubes will be produced, like that of blood in the living body,-with this difference, that the channels of departure and return in the syringe are single tubes, but in the body are tubes with innumerable branches, of which the extremities, of capillary size, open or inosculate into one another. It is further to be noticed, with respect to the circulation in warm-blooded animals, that there are really two distinct hearts, although so connected as to appear one: the first, on the left side, serves the purposes of the general circulation; the other, on the right side, receives all the dark-coloured and impure blood returning from the general circulation, and sends it through the shorter circulation of the lungs, where it is depurated or purified, and acquires a bright red colour from the oxygen of the air breathed, before it again enters the left side of the heart for general circulation through the body.

The pulse is merely the sudden gush of blood driven into the great trunk of the arterial tree, the aorta, by the sudden contraction of the heart, causing an undulation over the whole system. It takes place visibly in all arteries above a certain size, and it can be felt in those which are superficially covered with skin or which lie over bone, as in the radial artery at the wrist.

1089. Among the facts in nature offered to man's observation, there is perhaps nothing more marvellous than that, out of the same red opaque fluid, the blood, which, if drawn from a vein and allowed to stand at rest, is quickly turned into a soft coagulum and a strawcoloured liquid (serum), the living powers in the body should find and separate from the mass the materials of which all other solids and liquids in and about the body are formed.* How strange, that these powers can produce from this liquid the pure watery tear

*The blood in all warm-blooded animals owes its red colour to the presence of a number of minute cells or bladders containing red colouring matter. These are mechanically diffused, and float in the liquid portion or serum. They are of larger size in man than in most other animals. They have an average diameter of the 3500th of an inch. A drop of blood owes its intense colour to the aggregation of these small bodies. The number contained in the blood of the human adult cannot be less than sixty-one billions. At a late scientific meeting at South Kensington (1876), Dr. J. B. Sanderson described the method of microscopical measurement, and stated that the normal standard is four millions of red corpuscles in a cubic millimetre, i.e., a cube of the twenty-fifth part of an inch.

Products of the Blood.

854 which constantly keeps the eyeballs moist and clean—the colourless saliva, the milk, and the deadly poison of the cobra and rattlesnake, as well as the curved teeth or fangs through which this poison is ejected! These liquids and solids are not only formed by and from the blood, but each is deposited in its due place and proper proportion for its intended future use. In reference to human beings, it may be observed, that all the varied secretions of the body-the milk, bile, and gastric juice, as well as the materials of solid flesh, skin, hair, nails, the hard bones, and the enamel of the teeth, are derived from this wonderful fluid. It is stranger still that, after finding the fit materials, these living powers are able therewith to construct such curious and complex organs as the eye, the ear, the brain, and the heart. Then we see in all the inferior races of animals the like phenomena going on. Out of the blood of the creatures, are formed the teeth and claws of the tiger, the proboscis and tusks of the elephant, the shell of the tortoise, the fur of the beaver, and the feathers of the peacock with their radiant colours and symmetrical arrangement. Neither physics nor chemistry can reasonably furnish an explanation of these phenomena. They are referable only to the exercise of powers of a special kind acting within the living body and entirely independent of the will or consciousness of the living animal.

1090. In order to complete this series of wonders it may be noticed that each species of animal, in search of the food which is to make its blood, errs not as to the kind of food which will yield the ingredients required. Thus the elephant lives upon vegetable substances. These not only supply what is necessary for the growth and nutrition of the animal, but also those mineral matters which build up its defensive ivory tusks. Ivory, like bone, consists of twothirds mineral matter-the phosphate and carbonate of lime. These mineral ingredients exist only in small proportion in the vegetable food of the animal, but they are extracted, eliminated, and deposited as ivory in the enormous tusks of the male elephant. Sir Samuel Baker describes a pair of tusks of a large African elephant as weighing 150 pounds (the weight of a full-grown man). These would contain at least 100 pounds of mineral calcareous matter transferred from the vegetable food to the blood of the animal, and by it fashioned under the vital force into that most beautiful substance, ivory. The large amount of food required to supply the mineral matter for building up the tusks of an elephant may well excite wonder, but that wonder is greatly increased when it is con

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