[Sidenote: 30. What are the arteries? Their walls? Their membrane?]

30. THE ARTERIES.--The tube-like ca.n.a.ls which carry the blood away from the heart are the arteries. Their walls are made of tough, fibrous materials, so that they sustain the mighty impulse of the heart, and are not ruptured.

In common with the heart, the arteries have a {115} delicately smooth lining membrane. They are also elastic, and thus re-enforce the action of the heart: they always remain open when cut across, and after death are always found empty.

[Sidenote: 31. Early anatomists? The service of the ill.u.s.tration?]

31. The early anatomists observed this phenomenon, and supposing that the same condition existed during life, came to the conclusion that these tubes were designed to act as air-vessels, hence the name artery, from a Greek word which signifies containing air. This circ.u.mstance affords us an ill.u.s.tration of the confused notions of the ancients in reference to the internal operations of the body. Cicero speaks of the arteries as "conveying the breath to all parts of the body."

[Sidenote: 32. The arterial system? The branches and sub-branches of the arteries?]

32. The arterial system springs from the heart by a single trunk, like a minute and hollow tree, with numberless branches. As these branches leave the heart they divide and subdivide, continually growing smaller and smaller, until they can no longer be traced with the naked eye. If, then, we continue the examination by the aid of a microscope, we see these small branches sending off still smaller ones, until all the organs of the body are penetrated by arteries.

[Sidenote: 33. Successive undulations from the heart? Course of the arteries? Protection of the arteries? General location of the arteries?]

33. THE PULSE.--With each contraction of the left heart, the impulse causes a wave-like motion to traverse the entire arterial system. If the arteries were exposed to view, we might see successive undulations speeding from the heart to the smallest of the branches, in about one-sixth part of a second.

The general course of the arteries is as far as possible from the surface.

This arrangement is certainly wise, as it renders them less liable to injury, the wounding of an artery being especially dangerous. It also protects the arteries from external and unequal pressure, by which the force of the heart would be {116} counteracted and wasted. Accordingly, we generally find these vessels hugging close to the bones, or hiding behind the muscles and within the cavities of the body.

[Sidenote: 34. Where do the arteries lie? If we apply the finger? Pulse?

Where felt?]

34. In a few situations, however, the arteries lie near the surface; and if we apply the finger to any of these parts, we will distinctly feel the movement described, taking place in harmony with the heart-beat. This is part of the wave-motion just mentioned, and is known as the pulse. All are more familiar with the pulse at the wrist, in the _radial_ artery; but the pulse is not peculiar to that position, for it may be felt in the _carotid_ of the neck, in the _temporal_ at the temple, and elsewhere, especially near the joints.

[Ill.u.s.tration: FIG. 30.--THE FORM OF THE PULSE.]

[Sidenote: 35. The pulse as an index? Of what does it inform the physician?

Instrument for recording pulsation?]

35. Since the heart-beat makes the pulse, whatever affects the former affects the latter also. Accordingly, the pulse is a good index of the state of the health, so far as the health depends upon the action of the heart. It informs the physician of the condition of the circulation in four particulars: its rate, regularity, force, and fullness; and nearly every disease modifies in some respect the condition of the pulse. A very ingenious instrument, known as the sphygmograph, or pulse-writer, has recently been invented, by the aid of which the pulse is made to write upon paper its own signature, or rather to sketch its own profile. This instrument shows with great accuracy the difference between the pulses of health and those of disease. In Fig. 30 is traced the form of the pulse in health, which should be read from left to right. That part of the trace {117} which is nearly perpendicular coincides with the contraction of the ventricles; while the wavy portion marks their dilatation.

[Sidenote: 36. What are the veins? How do they form? What do they resemble?]

36. THE VEINS.--The vessels which convey the blood on its return to the heart are the veins. They begin in the several organs of the body, and at first are extremely small; but uniting together as they advance, they constantly increase in size, reminding us of the way in which the fine rootlets of the plant join together to form the large roots, or of the rills and rivulets that flow together to form the large streams and rivers.

In structure, the veins resemble the arteries, but their walls are comparatively inelastic. They are more numerous, and communicate with each other freely in their course, by means of interlacing branches.

[Ill.u.s.tration: FIG. 31.--THE VALVES OF THE VEINS.]

[Sidenote: 37. Valves in the veins? What are they? Their position?

Experiment with the cord?]

37. But the chief point of distinction is in the presence of the valves in the veins. These are little folds of membrane, disposed in such a way, that they only open to receive blood flowing toward the heart, and close against a current in the opposite direction. Their position in the veins on the back of the hand may be readily observed, if we first obstruct the return of blood by a cord tied around the forearm or wrist. In a few minutes the veins will appear swollen, and upon them will be seen certain prominences, about an inch apart. These latter indicate the location of the valves, or, rather, they show that the vessels in front of the valves are distended by the blood, which cannot force a pa.s.sage back through them.

[Sidenote: 38. What will be proved by the experiment? What inference is drawn?]

38. This simple experiment proves that the true direction of the venous blood is toward the heart. That the color {118} of the blood is dark, will be evident, if we compare the hand thus bound by a cord with the hand not so bound. It also proves that the veins lie superficially, while the arteries are beneath the muscles, well protected from pressure; and that free communication exists from one vein to another. If now we test the temperature of the constricted member by means of a thermometer, we will find that it is colder than natural, although the amount of blood is larger than usual. From this fact we infer, that whatever impedes the venous circulation tends to diminish vitality; and hence, articles of clothing or constrained postures, that confine the body or limbs, and hinder the circulation of the blood, are to be avoided as injurious to the health.

[Sidenote: 39. Capillaries? How regarded? Harvey?]

39. THE CAPILLARIES.--A third set of vessels completes the list of the organs of the circulation, namely, the _capillary_ vessels, so called (from the Latin word _capillaris_, hair-like), because of their extreme fineness.

They are, however, smaller than any hair, having a diameter of about 1/3000 of an inch, and can only be observed by the use of the microscope. These vessels may be regarded as the connecting link between the last of the arteries and the first of the veins. The existence of these vessels was unknown to Harvey, and was the one step wanting to complete his great work.

The capillaries were not discovered until 1661, a short time after the invention of the microscope.

[Sidenote: 40. The circulation of the blood in the web of a frog's foot?

Describe it. How general is the existence of the tissues?]

40. The circulation of the blood, as seen under the microscope, in the transparent web of a frog's foot, is a spectacle of rare beauty, possessing more than ordinary interest, when we consider that something very similar is taking place in our own bodies, on a most magnificent scale. It is like opening a secret page in the history of our own frames. We there see distinctly the three cla.s.ses of vessels with their moving contents; first, the artery, {119} with its torrent of blood rushing down from the heart, secondly, the vein, with its slow, steady stream flowing in the opposite direction; and between them lies the network of capillaries, so fine that the corpuscles can only pa.s.s through "in single file." The current has here an uncertain or swaying motion, hurrying first in one direction, then hesitating, and then turning back in the opposite direction, and sometimes the capillaries contract so as to be entirely empty. Certain of the tissues are dest.i.tute of capillaries; such are cartilage, hair, and a few others on the exterior of the body. In all other structures, networks of these vessels are spread out in countless numbers: so abundant is the supply, that it is almost impossible to puncture any part with the point of a needle without lacerating tens, or even hundreds of these vessels.

[Ill.u.s.tration: FIG. 32.--WEB OF A FROG'S FOOT, slightly magnified.]

[Ill.u.s.tration: FIG. 33.--MARGIN OF FROG'S WEB magnified 30 diameters.]

[Sidenote: 41. Elasticity of the capillaries? Grain of sand in the eye?

Blush? Other cases?]

41. The capillaries are elastic, and may so expand as to produce an effect visible to the naked eye. If a grain of sand, or some other foreign particle, lodge in the eye, it will become irritated, and in a short time the white of the eye will be "blood-shot." This appearance is due to an {120} increase in the size of these vessels. A blush is another example of this, but the excitement comes through the nervous system, and the cause is some transient emotion, either of pleasure or pain. Another example is sometimes seen in purplish faces of men addicted to drinking brandy; in them the condition is a congestion of the capillary circulation, and is permanent, the vessels having lost their power of elastic contraction.

[Sidenote: 42. Show what time is required for a given portion of blood to travel once around the body.]

42. RAPIDITY OF THE CIRCULATION.--That the blood moves with great rapidity is evident from the almost instant effects of certain poisons, as prussic acid, which act through the blood. Experiments upon the horse, dog, and other inferior animals, have been made to measure its velocity. If a substance, which is capable of a distinct chemical reaction (as _pota.s.sium ferrocyanide_, or _barium nitrate_), be introduced into a vein of a horse on one side, and blood be taken from a distant vein on the other side, its presence may be detected at the end of twenty or thirty-two seconds. In man, the blood moves with greater speed, and the circuit is completed in twenty-four seconds.

[Sidenote: 43. Time required for all the blood to circulate completely around?]

43. What length of time is required for all the blood of the body to make a complete round of the circulation? This question cannot be answered with absolute accuracy, since the amount of the blood is subject to continual variations. But, if we a.s.sume this to be one-eighth of the weight of the body, about eighteen pounds, it will be sufficiently correct for our purpose. Now to complete the circuit, this blood must pa.s.s once through the left ventricle, the capacity of which is two ounces. Accordingly, we find that, under ordinary circ.u.mstances, all the blood makes one complete rotation every two minutes; pa.s.sing successively through the heart, the capillaries of the lungs, the arteries, the capillaries of the extremities, and through the veins. {121}

[Sidenote: 44. What is meant by a.s.similation? What can you say of its use, etc.? Time?]

44. a.s.sIMILATION.--The crowning act of the circulation, the furnishing of supplies to the different parts of the body, is effected by means of the capillaries. The organs have been wasted by use; the blood has been enriched by the products of digestion. Here, within the meshes of the capillary network, the needy tissues and the needed nutriment are brought together. By some mysterious chemistry, each tissue selects and withdraws from the blood the materials it requires, and converts them into a substance like itself. This conversion of lifeless food into living tissue is called a.s.similation. The process probably takes place at all times, but the period especially favorable for it is during sleep. Then the circulation is slower, and more regular, and most of the functions are at rest. The body is then like some trusty ship, which after a long voyage is "hauled up for repairs."

[Sidenote: 45. What is stated of the injuries to the blood-vessels?]

45. INJURIES TO THE BLOOD-VESSELS.--It is important to be able to discriminate between an artery and a vein, in the case of a wound, and if we remember the physiology of the circulation we may readily do so. For, as we have already seen, haemorrhage from an artery is much more dangerous than that from a vein. The latter tends to cease spontaneously after a short time. The arterial blood flows away from the heart with considerable force, in jets; its color being bright scarlet. The venous blood flows toward the heart from that side of the wound furthest from the heart; its stream being continuous and sluggish; its color dark. In an injury to an artery, pressure should be made between the heart and the wound; and in the case of a vein that persistently bleeds, it should be made upon the vessel beyond its point of injury. {122}

QUESTIONS FOR TOPICAL REVIEW.

PAGE 1. In what organisms is the so-called circulatory fluid found? 101 2. How is it designated in the different organisms? 101 3. What can you state of the importance of blood to the body? 101, 105 4. Of its great abundance, color, and composition? 101, 102, 107 5. Describe the corpuscles of the human blood. 102, 103, 104 6. What is said of them in comparison with those of the lower animals? 103 7. Of the importance of sometimes detecting human from other blood? 103 8. What means have we of detecting blood in spots or stains? 103, 104 9. What is meant by coagulation of the blood? 104 10. What wisdom is there in the law of the blood's coagulation? 104, 105 11. How is this wisdom made manifest? 105 12. In what cases is the aid of the surgeon required? 105 13. What are the two great uses of the blood? 105 14. Through what mediums is the blood provided with new material and relieved of the old material? 105 15. What do you understand by the operation called transfusion? 106 16. What cases of transfusion are reported of the lower animals? 106 17. What can you state of transfusion as practised upon man? 106 18. What further can you say on the subject? 106, 107 19. What changes take place in the color of the blood in its journey through the system? 107 20. State all you can in relation to the circulation of the blood. 107 21. All, in relation to the size, shape, and location of the heart. 107, 109 22. How is the loss of power in the heart movements obviated? 109 23. Give a description of the formation of the heart. 109, 110, 111 24. What can you state of the ventricles and auricles of the heart? 110 25. Describe the action of the heart. 111 26. What special vitality does the tissue of the heart possess? 111 27. State all you can on the subject. 111 28. Describe the course of the blood through the cavities of the heart. 112 29. Describe the mechanism that regulates the heart-currents. 112 30. How do you account for the two heart-sounds at the front of the chest? 112 31. State what you can of the frequency of the heart's action. 112, 113 32. Of the activity of the heart. 113, 114 33. What do you understand by the arteries? 114, 115 34. State what you can of the arteries and the arterial system. 114, 115 35. What do you understand by the pulse? 115, 116 36. In what part of the body may the pulse be felt? 116 37. What further can you state of the pulse? 116, 117 38. What are the veins? 117 39. Where do they exist, and how are they formed? 117 40. Describe the valves of the veins and their uses. 117 41. Now give a full description of the construction of the veins. 117 42. What further can you state of the veins? 117, 118 43. What do you understand by the capillaries? 118, 119 44. What service do the capillaries perform? 118, 119, 121 45. Describe the circulation of the blood in the region of the heart. 118, 119 46. What can you state of the rapidity of the blood's circulation? 120 47. Of the process known as a.s.similation? 121 48. Of injuries to the blood-vessels? 121

{123}

CHAPTER VIII.

RESPIRATION.

_The Objects of Respiration--The Lungs--The Air-Pa.s.sages--The Movements of Respiration--Expiration and Inspiration--The Frequency of Respiration--Capacity of the Lungs--The Air we breathe--Changes in the Air from Respiration--Changes in the Blood--Interchange of Gases in the Lungs--Comparison between Arterial and Venous Blood--Respiratory Labor--Impurities of the Air--Dust--Carbonic Acid--Effects of Impure Air--Nature's Provision for Purifying the Air--Ventilation--Animal Heat--Spontaneous Combustion._

[Sidenote: 1. Difference between the two sets of capillaries? Change effected by respiration or breathing?]