fact, that even the aorta itself may be gradually obstructed at some distance from the heart, without the parts which it supplies being deprived of nourishment. From an attentive consideration of all these circumstances, it has been concluded, that the moderate increase of the area of the branches of large arteries; the acute angles at which they divide; their nearly rectilinear course; and the rare occurrence of inosculation between them, are designed to facilitate the rapid motion of the blood in them, so that it may arrive unchanged, and in the same state that it was in when projected from the heart, at that part of the body, for the nourishment of which it was intended: whilst, on the contrary, the great increase of the area of the smaller vessels, the variety of their angles, their tortuous course, and their frequent communications were designed to check the velocity of the blood's motion, when it has arrived at that part, where secretion is to be performed, and nutrition is to take place. Contrary opinions have indeed been maintained; and for the further discussion of this subject, we must refer the reader to the remarks on the circulation in the article PHYSIOLOGY.

Termination of the arteries.-When these vessels have become very minute, they terminate in two ways: they either turn back again, and become veins, and return the blood to the heart, or they send off fine vessels, which abstract something from the circulating blood, and are therefore called -secerning arteries. Though none but minute arteries are ever reflected to become veins, yet many of them are of sufficient magnitule to admit common waxen injection: and when this experiment succeeds, the continuity of the arteries and veins is very manifest. It seems therefore to follow from this facility of communication, that the mass of the blood is constantly and freely circulating, in order to undergo that change, which is effected in the lungs, whilst but a small part of it proceeds into the very minute arteries, for the purpose of having secretions made from it. For these arteries, however minute, must be considered large in comparison with the exility of others, which cannot be injected with wax, and even reject the red globules of the blood, or admit them in such small proportion, that they do not impart the red colour to the fluid, which moves in those vessels. Now, we may venture to affirin, that these globules do not much exceed in diameter the 150,000th part of an inch, which circumstance suffi

ciently shews the minuteness of the lesser arteries.

The secerning arteries are in general too minute to admit of demonstration; they are however evident in some glands; in the kidney, for instance, they may be seen continued into the excretory vessels. Subtile injections, when thrown into the larger arterial trunks, ooze out on the surfaces of membranes, and into the cellular substance, and they are generally supposed to be poured forth from the open orifices of secerning arteries. Analogy therefore, rather than actual demonstration, leads us to believe, that the secerning arteries abstract the particles of nutrition, or the materials which compose the fabric of the body, from the circulating fluids, and deposit them from their open mouths, so as by this means to build up and keep in repair the structure of the body.

Distribution of the arteries.-The great artery, whose branches supply the whole of the body, is named the aorta. It arises from the upper part of the left ventricle; and emerges from the heart, between the pulmonary artery and the right auricle. It first ascends in the chest; opposite the upper edge of the second rib it bends backwards till it reaches the left side of the spine, in which, situation it descends from the fourth or fifth dorsal to the last lumbar vertebra.

By the arch of the aorta is meant that part of the vessel, which ariges from the heart, and bends across the chest. It sends off the following branches: the two first arising at right angles close to the heart; the three following from the convexity of the arch:

1. Right coronary artery of the heart. 2. Left coronary artery of the heart. 3. Arteria innominata, a common trunk, dividing into

1. Right subclavian.

2. Right common carotid.

4. Left common carotid.

5. Left subclavian.

The common carotid artery is destined for the supply of the head. It emerges from the chest by the side of the trachea; mounts upwards in front of the vertebræ, and parallel with the trachea, till it reaches the upper margin of the thyroid cartilage, without sending off a single branch. At this part it divides into the external and internal carotid arteries, the former of which is distributed to the outside of the head; the latter to the brain.

1

Branches of the descending portion of the aorta in the chest.

1. Common bronchial artery.

2. Right and left bronchial arteries. 3. Esophageal arteries.

4. Lower intercostal arteries.

The aorta passes through the diaphragm at the lower part of the chest, and takes the name of abdominal aorta. It is still situated on the left side of the bodies of the vertebræ, and at the fourth lumbar vertebra it terminates by dividing into the two common iliac trunks.

Branches of the abdominal aorta. 1. Right and left phrenic arteries. 2. Cæliac artery.

a. Coronary artery of the stomach. b. Hepatic artery.

a. Duodeno-gastric, or gastro-
epiploic artery.

B. Superior pyloric artery.
7. Cystic artery.

c. Splenic artery.

.. Pancreatic arteries.

B. Short arteries to the stomach. 7. Left gastro-epiploic artery.

3. Superior mesenteric artery.

a. From 12 to 20 large branches to to the small intestine.

b. Middle colic artery.

c. Ileocolic artery.

4. Renal, or emulgent arteries.
5. Spermatic arteries.

6. Inferior mesenteric artery.

a. Left colic branch.

b. Internal hemorrhoidal branch.
7. Five pairs of lumbar arteries.
8. Two common iliac arteries.
9. Middle sacral artery.

The common iliac quickly divides into the external and internal iliac branches, of which the former goes to the thigh, the latter enters the cavity of the pelvis.

Branches of the internal iliac artery.
1. Ileo-lumbar artery.

2. Lateral sacral arteries.
S. Vesical arteries.

4. Middle hemorrhoidal.
5. Uterine branch.
6. Obturator artery.

7. Gluteal artery.

8. Ischiatic artery.

9. Pudendal artery.

a. External hemorrhoidal branches.

b. Artery of the perineum.

c. Dorsal artery of the penis.

d. Deep artery of the penis.

The external iliac artery, having changed its name for that of femoral, runs along the front of the thigh, and then bends inwards to the ham, where it takes the name of popliteal. It passes through the latter space to the leg, when it terminates by dividing into two, of which one runs along the front, and the other the back of the leg.

Branches of the external iliac artery. 1. Epigastric artery.

2. Circumflex artery of the ilium.

Branches of the femoral artery.

1. Branches to the lymphatic glands, and integuments.

2. External pudic arteries.

3. Deep-seated artery of the thigh.
a. External circumflex artery.

b. Internal circumflex artery.

c. First and second perforating

branches.

4. Branches to the neighbouring muscles. 4. Great anastomosing branch.

Branches of the propliteal artery.

1. Superior internal articular artery.
2. Superior external articular artery.
3. Middle articular artery.

4. Inferior internal articular artery.
5. Inferior external articular artery.
6. Anterior tibial artery.

7. Posterior tibial artery.

Branches of the anterior tibial artery. 1. Recurrent branch.

2. Various small muscular branches. 3. External and internal malleolar arteries.

4. Tarsal and metatarsal arteries. 5. Dorsalis hallicis.

Branches of the posterior tibial artery.

1. Large muscular branches to the soleus.

2. Medullary artery of the tibia.

3. Peroneal or fibular artery.
a. Anterior branch.

b. Posterior branch. 4. External plantar artery.

a. Four digital arteries. 5. Internal plantar artery. There is another large arterial trunk in the body, besides the aorta, called the pulmonary artery; this rises from the right ventricle, and conveys the venous blood to the lungs, for the purposes of respiration.

OF THE VEINS.

The blood is constantly moving in the arteries from the trunks into the branches; in the veins it follows a directly opposite course, and flows from the branches to the trunks.

There are seven large venous trunks in the body, to which all the blood is returned: three of these, viz. the superior and inferior vena cava, and the coronary vein of the heart, return the blood, which has circulated through the body into the right auricle of the heart: the other four are the pulmonary veins, and bring the blood back from the lungs to the left auricle.

The coats of the veins are thin when compared with those of the arteries; hence the blood can generally be plainly seen through them; and hence when divided they collapse, instead of presenting a circular section, as arteries do. It is difficult to separate them into coats, yet they are said to consist of two; viz. a smooth and highly polished internal one, which lines the canal; and a rough, cellular, external tunic, in which no muscular power resides. Hence

the circulation proceeds through these vessels merely by the impulse of the arterial blood, and is not aided by any action of the containing tubes.

The veins are much more numerous, and also larger than the arteries. In most parts of the body each artery has two veins lying by its side; and in many instances there is another numerous set of veins besides these. Hence the venous system is much more ca pacious than the arterial; and this difference is so great, that the veins are supposed to contain nine parts out of thirteen of the whole mass of blood. This great capacity of the venous system obviates the effects of any casual obstruction to the ready transmission of blood through the lungs; for the whole of the veins are not distended in a natural state, but serve as an occasional reservoir, in which the blood, constantly urged forwards by the heart, may be held till the cause of obstruction has ceased. But as such retardation in the course of the venous blood would tend to drive back the whole mass on the minute veins, which are the least able to bear it, such retrograde motion is prevented by valves, which exist in great numbers in the venous system. These are thin membranes, having a semilunar edge attached to the side of the vein, and a straight edge floating in the cavity of the vessel: they are placed in pairs. When the blood is going on in its natural direction, they lie close to the sides of the tube; but, when it attempts to return, the blood raises the loose edge, and that meets in the centre of the vessel with the corresponding part of the opposite valve, and thus closes the canal. Thus, when an obstruction takes place, each portion of a vein has to support that column of blood only which is contained between its own valves. Still, as these vessels possess no powers of their own, and are too far removed from the heart to feel its influence on the passage of blood through them, we find that the circulation is affected in them by external causes, as position, &c. Hence the legs swell after long standing; and hence also the veins of these parts are apt to become enlarged and varicous.

Distribution of the veins.-This is for the most part similar to that of the arteries, as each of the latter vessels have generally two accompanying veins, (which bear the same names as the concomitant arteries) named venæ sodales arteriarum. But in some situations there is a class of veins not corres ponding to the arteries, but running under the skin, and termed cutaneous or superficial

veins. These are found particularly in the extremities, and vary much in size at different times.

The vena cava superior is formed by three large trunks.

1. Vena azygos, which returns the blood from the sides of the chest, and runs along the middle of the spine.

2. Right subclavian, which is also made up by three venous trunks, viz. the internal jugular, the external jugular, and the axillary.

3. Left subclavian, formed in the same manner as the right.

The external jugular vein returns the blood from the outside of the head, and runs along the neck, just under the skin. We sometimes bleed from this in affections of the head.

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 these 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 in the latter veins that we generally bleed when that operation is performed in the arm; and as they run directly over the artery, the latter vessel is endangered by the lancet.

The inferior vena cava is a very large trunk, running along the spine at the right side of the aorta. It returns the blood from all the lower parts of the body. It is made up by the junction of the two common iliac veins; and as it ascends through the abdomen, it receives the following venous trunks: the lumbar, spermatic, renal, and the immense venæ cavæ hepaticæ.

The common iliac vein is formed by the junction of the external and internal iliacs. The latter brings back the blood from the cavity of the pelvis; the former returns it from the lower extremity.

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 saphrena 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 vena portarum is a large vessel formed by the union of those veins which belong to the stomach and intestines, the spleen and pancreas. It conveys the blood, which has circulated through those organs, to the liver, and it branches out in that gland as arteries do in other parts. Its blood is returned from the liver by the hepatic veins, which have been already noticed.

ORGANS OF ABSORPTION.

The absorbents are a minute kind of vessels found in animal bodies, which attract and imbibe any fluid that is brought near their mouths. They are so minute and transparent, as not to be discovered in ordinary dissection; but by great labour they have at length been detected in great numbers in every tribe of animals. As these vessels are transparent, their contents are visible, which circumstance occasioned them to receive the different denominations of lacteals and lymphatics. The former were so called, because they imbibed the chyle, a milky fluid, from the bowels; whilst the latter, containing much lymph, which they had taken up from all the interstices of the body, were therefore named lymphatics. The discovery of this system of vessels is referred to the seventeenth century. But at first their number did not appear sufficient to perform the whole function of absorption; neither had they been discovered in birds or fishes, whence anatomists still retained the idea that the veins participated in this important office. The merit of first demonstrating the absorbing vessels in those animals belongs to Mr. Hewson, who assisted in the labours of the first eminent anatomical school in London, where anatomy was most ably taught by Dr. Hunter. And it is to the immortal Hunter that we are indebted for fully proving the important doctrine, that the whole business of absorption is performed by the vessels which we are now considering. They have of late been injected in such great abundance, that they appear fully adequate to perform their office.

If, as we firmly believe, these vessels be the only ones which perform the office of absorption, they must exist in every part ^