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The external jugular vein returns the blood from the outside of the head, and runs along the neck, just under the skin.

The internal jugular is a very large vessel, lying deeper in the neck, and close to the carotid artery. It brings back the blood from the brain. The danger, in attempts at suicide, consists in dividing this vessel or the carotid artery, and not the external jugular vein. The axillary vein is made up of the vessels which bring the blood back from the arm. Besides the deep-seated veins, we have here a large superficial vessel, running along the outside of the fore-arm and arm, and called the cephalic vein; another on the inside, named the basilic. Between those in the fore-arm are found some veins called the median. At the bend of the elbow, these last make up two large trunks, of which one opens into the basilic, and the other into the cephalic vein. These are called vena mediana basilica and vena mediana cephalica. It is the latter veins, that surgeons generally bleed from, when that operation is performed in the arm; and as they run directly over the artery, the latter vessel is endangered by the lancet.

We have two large cutaneous veins to notice in the leg and thigh; viz. the saphena major, which runs up along the inner side both of the leg and thigh, and can be distinctly seen in the living person, when in the erect posture; the saphena minor, which runs over the calf of the leg. The former terminates in the femoral vein, near the abdomen; the latter, in the popliteal vein.

The disposition of the blood-vessels, as far as regards the supply of the body, is like that of the water-pipes in a city, viz. large and main trunks branching off by smaller pipes, and these again by still narrower tubes, in every direction, and towards every part in which the fluid which they convey can be wanted. So far the water-pipes which serve a town may represent the vessels which carry the blood from the heart. But there is another thing neces➡ sary to the blood, which is not wanted for the water; and that is, the carrying of it back again to its source. For this office, a reversed system of vessels is prepared, which, uniting at their extremities with the extremities of the first system, collects the divided and subdivided streamlets, first by capillary ramifications into

larger branches; secondly, by these branches into trunks; and thus returns the blood (almost exactly inverting the order in which it went out) to the fountain whence its motion proceeded.

The body, therefore, contains two systems of blood-vesselsarteries and veins. Between the constitution of the systems there are also two differences, suited to the functions which the systems have to execute. The blood, in going out, passing always from wider into narrower tubes; and, in coming back, from narrower into wider; it is evident, that the impulse and pressure upon the sides of the blood-vessels will be much greater in one case than the other. Accordingly, the arteries, which carry out the blood, are formed of much tougher and stronger coats than the veins, which bring it back. That is one difference: the other is still more artificial, or, if I may so speak, indicates still more clearly the care and anxiety of the artificer. Forasmuch as in the arteries, by

reason of the greater force with which the blood is urged along them, a wound or rupture would be more dangerous than in the veins, these vessels are defended from injury, not only by their texture, but by their situation; and by every advantage of situation which can be given to them. They are buried in sinuses, or they creep along grooves, made for them in the bones: for instance, the under edge of the ribs is sloped and furrowed solely for the passage of these vessels. Sometimes they proceed in channels, protected by stout parapets on each side; which last description is remarkable in the bones of the fingers, these being hollowed out, on the under side, like a scoop, and with such a concavity, that the finger' may be cut across to the bone, without hurting the artery which runs along it. At other times, the arteries pass in canals wrought in the substance, and in the very middle of the substance, of the bone: this takes place in the lower jaw, and is found where there would, otherwise, be danger of compression by sudden curvature. All this care is wonderful, yet not more than the importance of the case required. To those who venture their lives in a ship, it has been often said, that there is only an inch-board between them and death; but in the body itself, especially in the arterial system, there is, in many parts, only a membrane, a skin, a thread. For which

reason, this system lies deep under the integuments; whereas the veins, in which the mischief that ensues from injuring the coats is much less, lie in general above the arteries, come nearer to the sur face, and are more exposed.*

The Blood.

The veins and arteries are all filled with a fluid called Blood, usually considered the focus of life. It is of a rich and beautiful colour; vermillion coloured in the arteries, strong purple in the veins, and black, or almost so, at the right side of the heart; it feels thick and unctuous between the fingers, is of a slightly saline taste, and is various in various parts of the body.

The colour of the blood has been attributed to iron; and it is very true, that the colour appears to be entirely formed of it; for there exists no vestige of this metal in the washed and discoloured coagulum: but as, on the other hand, the blood does not become coloured without the concourse of air, and as oxygen alone is absorbed in respiration, it appears that the colour is owing to iron calcined by pure air, and reduced to the state of red oxide.

The blood, in passing through the lungs, is black, without receiving the influence of the air; and therefore supposed to want the stimulus, necessary to excite the left cavity of the heart. Hook and LOWER long since noticed the difference of colour in arterial and venal blood; and it has been since proved, by numerous experiments, that the fine vermillion colour of the former is produced solely by vital air, which it is capable of acquiring even through bladders, the coats of blood vessels, &c.

* How minute and multiplied the ramifications of the blood-vessels are, and how thickly they are spread over at least the superficies of the body, is proved by the single observation, that we cannot prick the point of a pin into the flesh, without drawing blood, i. e. without finding a blood-vessel. Nor is their diffusion less universal internally. Bloodvessels run along the surface of membranes, pervade the substance of muscles, and even penetrate the bones. Even into every tooth, we trace, through a small hole in the root, an artery to feed the bone, as well as a vein to bring back the spare blood from it: both which, with the addition of an accompanying nerve, form a thread only a little thicker than a horse-hair.

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The blood, the fountain whence the spirits flow,
The gen'rous stream that waters every part,
And motion, vigour, and warm life conveys
To every particle that moves or lives.

This vital fluid, through unnumber'd tubes,

Pour'd by the heart, and to the heart again
Refunded; scourg'd for ever round and round
Enrag'd with heat and toil, at last forgets
Its balmy nature; virulent and thin

It grows; and now, but that a thousand gates
Are open to its flight, it would destroy
The parts it cherish'd and repair'd before.
Besides, the flexible and tender tubes
Melt in the mildest, most nectarious tide

That ripening nature rolls; as in the stream
Its crumbling banks; but what the vital force
Of plastic fluids hourly batters down,

That very force those plastic particles
Rebuild-so mutable the state of Man.

ARMSTRONG.

This vital fluid never discontinues its interchangeable tide; but, night and day, whether we sleep or wake, still perseveres to sally briskly through the arteries, and return softly through the veins thus keeping the body in tenantable repair for the soul for the space of seventy or eighty years. But,

Learning did ne'er this secret truth impart
To the Greek masters of the healing art;
'Twas by the Coan's piercing eye unviewed,
And did attentive Galen's search elude.

Thou, wondrous HARVEY, whose immortal fame,
By thee instructed, grateful schools proclaim;
Thou, Albion's pride, didst first the winding way,
And circling life's dark labyrinth display.
Attentive from the heart thou didst pursue
The starting flood, and kept it still in view,
'Till thou with rapture saw'st the channels bring
The purple currents back, and form the vital ring.

;

The next thing to be considered, is the engine which works this admirable machinery, viz.

The Heart.

For our present purpose, it is unnecessary to ascertain the principle upon which the heart acts. Whether it be irritation, excited by the contact of the blood, by the influx of the nervous fluid, or whatever else may be the cause of its motion, it is something which is capable of producing, in a living muscular fibre, reciprocal contraction and relaxation. This is the power we have to work with; and the inquiry is, how this power is applied in the instance before us. There is provided, in the central part of the body, a hollow muscle, invested with spiral fibres, running in both directions, the layers intersecting each other; in some animals, however, appearing to be semicircular rather than spiral. By the contraction of these fibres, the sides of the muscular cavities are necessarily squeezed together, so as to force out from them any fluid which they may at that time contain: by the relaxation of the same fibres, the cavities are in their turn dilated, and, of course, prepared to admit every fluid which may be poured into them. Into these cavities are inserted the great trunks, both of the arteries which carry out the blood, and of the veins which bring it back. This is a general account of the apparatus; and the simplest idea of its action is, that, by each contraction, a portion of blood is forced by a syringe into the arteries; and, at each dilatation, an equal portion is received from the veins. This produces, at each pulse, a motion and change in the mass of blood, to the amount of what the cavity contains, which, in a full-grown human heart, I understand, is about an ounce, or two tablespoons full. How quickly these changes succeed one another, and, by this succession, how capable they are to support a stream or circulation throughout the system, may be understood by the following computation, abridged from Keill's Anatomy, p. 117, ed. 3: "Each ventricle will contain at least one ounce of blood. The heart contracts four thousand times in one hour; from which it follows, that there pass through the heart, every hour, four thousand ounces, or three hundred and fifty pounds of blood. Now the

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