Difference between revisions of "Talk:Paper - Development of the internal mammary and deep epigastric arteries in man (1898)"

From Embryology
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A. A. VI
S. 601.
2. Fick.
3. Fick.
4. Bardeleben,
A. A. IIIS. 324.
5. Bardeleben.
6. Bardeleben.
7. Bardeleben.
8. Bardeleben.
9. Bardeleben.
10. Bardeleben.
11. Bardeleben.
12. Shepherd,
Jour. Anat. and Phys. XIX-p.311.
13. Shepherd.
14. Shepherd.
Wallace, Jour. Anat. and Phy. XXI-p. 153.
Christian, Specimen I.
Christian. Sj)ecimen 11.
\ R.
\ R. ( L.
(?) L.
(L. I ! R.
15. Shepherd.
16. Shepherd.
i R.
17. Shepherd.
(R. 1 L.
Origin. Tendons of steruo-mastoid on both sides and upper digitations of pectoralis major on both sides.
Digitations of pectoralis major by means of two horizontal tendons to right and left at upper edge III rib.
Tendon left sterno-mastoid, few fibres from tendon right sternomastoid, manubrium sterni and upper digitations of pectoralis major of both sides.
Manubrium sterni and upper digitations of pectoralis major of both sides.
Tendon sterno-mastoid.
Right tendon sterno-mastoid. Both tendons of sterno-mastoid.
Right tendon sterno mastoid.
Manubrium sterni. Manubrium sterni.
Fascia of pectoralis major at II rib and left sterno-mastoid.
Right sterno-mastoid and some fibres from left pect. major.
Manubrium sterni.
Manubrium sterni.
Both sterno-mastoids and right pectoralis major.
Sterno-mastoid and sternum opposite II and III rib.
Sternum opp. II rib.
Sterno-mastoid and right pector.
Pect. major over manubrium
sterni. Pect. major over manubrium
Manubrium sterni. Manubrium sterni.
Right tendon sterno-mastoid and few fibres from left pectoralis •major.
Manubrium sterni and both tendons of sterno-mastoid.
Manubrium sterni and both tendons of sterno-mastoid.
Tendon of sterno-mastoid, fascia over manubrium sterni and a small fasciculus from II rib.
Fascia over I interspace.
iQsertlon. Rectus sheath at VI and VII ribs, digitations of ext. oblique arising from V rib and from fascia over sternum at level V rib.
Rectus sheath at VI and VII ribs and fascia of lowest digitations of pectoralis major.
Rectus sheath at V and VI ribs and fascia of pectoralis major and fasciculi to III, IV and V
Rectus sheath at VI rib, and a deeper layer to V rib and intercostal membrane between V and VI ribs.
Rectus sheath at VI rib and two thin fasciculi to left pectoralis major.
Rectus sheath at VI rib. Rectus sheath at VI rib.
Rectus sheath at VI rib.
Rectus sheath. Rectus sheath.
Rectus sheath at V rib.
Rectus sheath.
Rectus sheath.
Rectus sheath.
IV costal cartilage, border of sternum opposite V and VI ribs and aponeurosis of ext. oblique.
Ill costal cartilage and border of sternum.
Fasciculus into lower segment pectoralis fasciacontinuing with fibres of pector., lovcer end of sternum and ensiform cart.
Aponeurosis of ext. oblique.
III costal cartilage.
IV costal cartilage and fasciculus into fascia over pectoral is major.
A slip to fasc. over pect. maj. and a second to same lower down.
Pectoralis major and sternum opposite IV costal cartilage.
One fasc. to III cost, cart., a second to lower sternum and third to fasc. over pect. major.
Ill, IV, V and VI costal cartilages and border of sternum.
Rectus sheath at V rib. Rectus sheath at V rib.
V costal cartilage.
V' costal cartilage.
Remarks. Intra vitam contracted with sterno-mastoid inraisinghead.
Motion with sterno mastoid observed.
Nerve Supply. Ill and IV intercos.
IV intercos.
II, III and IV intercos.
Ill and IV intercos.
II and III intercos.
II and III intercosII and III intercos.
II intercos.
II intercos.
II intercos.
III intercos.
Ill intercos.
Ill and IV intercos.
Ill (?) intercos.
Int ant. thoracic. Anenceph.
Ant. thor. (?) Ant. thor. (?)
Int. ant. thor. and III intercos.
Int. ant. thor.
Int. ant. thor.
Int. ant. thor.
Int. ant. thor. Int. ant. thor.
Ext. ant. thor.
Ill intercos. Ill intercos.
Branch from Brachial plexus and a small twig from intercos.
Ant. thoracic.
Between the two remaining hypotheses there has been much discussion. Bardeleben advanced the theory that it belonged to the same plane of muscles as the M. rectus abdominis and M. sterno-cleido-mastoideus, and Testut the theory that it represents a connecting link between the M. obliquus abdominis externus and M. sterno-cleido-mastoideus, a condition normally found in the snake. However, Le Double claims that this latter condition does not hold and that the muscle found in the snake is represented in man by a deeper layer. It is probable that there is no exact analogue of this muscle in the lower animals. In some animals, as the Armadillo, beaver and Echidna the M. sterno-cleido-mastoideus extends down on the sternum (Turner) bxit not so far as the VI rib, and there is no connection with the M. rectus abdominis, which lies in a deeper plane. In all cyano-morphous primates (Keith) a M. supracostalis anterior occurs which is a digitation of the M. rectus abdominis arising from the I rib, but this lies beneath the M. pectoralis major. However there can be no doubt that Bardeleben was justified in his conclusion that a close relationship exists between the M. sternalis and the M. rectus abdominis since he and others have reported undoubted cases where the M. sternalis received its nerve supply from the intercostal nerves in a manner similar to the M. rectus abdominis.
This view of Bardeleben was generally accepted until P. S. Abraham reported some cases in anencephalic monsters, receiving nerves from the brachial plexus through the anterior thoracic branches. Soon after this Cunningham reported similar cases from adult cadavers. Such a nerve supply suggested a close relationship with the M. pectoralis major and from the fact of the long course of the nerve through and over the M. pectoralis major and from the recurrent course of many of the lower anterior cutaneous branches of the intercostal nerves perforating the fascia beneath the M. sternalis to run around the inner border of this muscle, it was concluded that it is an aberrant portion of the M. pectoralis major which had rotated inward and downward to this present position. This conclusion seems to be fully in accord with the facts and to be justided by the nerve supply which we regard as the link between present position and the myotome from which it arose. That this muscle is really often innervated from the brachial plexus is further shown by the contraction of the M. sternalis when the brachial plexus is electrically stimulated.
Here then we have two views as to the origin of this muscle, conflicting but both apparently well justified. Can these views be in any way harmonized ? From the twenty cases tabulated above it is readily seen that these muscles divide naturally into two classes, one with a fixed insertion into the middle ribs or margin of the sternum and supplied by nerves from the brachial plexus, the other with a less fixed insertion into the sheath of the rectus and a nerve supply from the perforating branches of the intercostal nerves, the origin of the two classes being very similar. From this we conclude that in the musculus Sternalis we have a muscle functionally always the same — probably antagonistic to the triangularis sterni— but that we have included under one term two distinct muscles, one closely related to the M. rectus abdominis, and the other an aberrant portion of the M. pectoralis major, the former supplied from the intercostal nerves and arising in the embryo from thoracic myotomes, the latter supplied from the brachial plexus and arising from cervical myotomes. Therefore, from the standpoint of innervation and development we agree with the results obtained by Bardeleben that the name M. sternalis has been used as a general term and that it should be reserved for those presternal muscles associated with the M. pectoralis major, while the name M. rectus thoracis should be applied to those related to the M. rectus abdominis.
The cadaver from which specimen No. 1 was taken showed a number of additional muscle anomalies in the region of the course of travel which must have been followed by M. sternalis in its development. When one sees a marked anomaly he should look for and will frequently find associated variations. These may indicate the forces at work to produce variation, and for this reason I enumerate those which accompanied Specimen I.
Mnsadus deido-hrjoideus. — A M. cleido-hyoideus (Fig. 3) is found on the left side, occurring with no variations in the M. sterno-hyoideus, M. sterno-thyroideus or M. omo-hyoideus. It occurs as a thin narrow ribbon of muscle fibres more delicate than the M. omo-hyoideus. It arises from the body of the hyoid bone just above and overlapping the external third of the M. sterno-hyoideus and internal half of the M. omo-hyoideus. From this origin it passes downward and slightly outward to be inserted into the clavicle just posterior to the clavicular insertion of the M. sterno-cleido-mastoideus. Just before reaching the clavicle the muscle spreads out fan-like to be inserted by an aponeurosis about twice the breadth of the muscle and extending along the clavicle from a little internal to the middle point of the insertion of the M. sterno-cleidomastoideus to a point about one cm. external to its insertion.
Quite frequently the M. omo-hyoideus gives off a slip to be inserted into the clavicle and consequently this muscle may be regarded as an aberrant portion of the M. omo-hyoideus here entirely split off except at its very origin. Its nerve supply is apparently the same as that of the M. omo-hyoideus, a fact supporting this view of its histogenesis.
M. sterno-thyroideus. — On the left side the M. sterno-thyroideus is normal as to its size and attachment. The muscle of the right side is fully twice as broad as that of the left and somewhat thicker. Its origin from the sternum is normal. Its insertion is by three heads, each more or less distinct. Of these the inner is inserted as usual on the inferior surface of the oblique ridge of the thyroid cartilage and represents in size and insertion almost the normal muscle. The middle head continues up closely associated with the outer border of the M. thyro-hyoideus, which latter is somewhat narrower than the one of the opposite side. Most of the fibres of this head are inserted into the anterior inferior and middle border of the hyoid bone, while a few are inserted into the middle jiart of the superior border of the thyroid cartilage. The outer head consists of a distinct ribbon of fibres running up to be inserted into the deep cervical fascia and sheath of the carotid artery intimately blending with these structures. The main part of the muscle is supplied by a branch from the hypoglossal loop, while the middle and outer heads receive fibrils from a branch of the hypoglossal nerve given off above that to the M. thyro-hyoideus.
Fig. 1.— Sketch sliowiu!;- the nttiiclinient of tlie M. steniali: in Spefimei] No. I.
Fig. 2. — The attachment of the M. sternalis in- Specimen No. II.
Fig. 3.— The neck of Specimcu I, to tlie .M. cleido-hyoiilens to the clavicle.
Fig. 4. — The axilla of Specimen No. I, to show the slip from the M. latissimus di>rsi to thi' pcctoralis nn)jor.
Arising on the right side from the tendinous portion of the M. digastricus, where it is held down to the hyoid bone by a strong loop of fiiscia, is found a small muscle about one cm. wide, nearly cylindrical in shape and running downward and outward around the larynx for about 6 cm. to be inserted into the fascia about the cornicula laryngis (cartilage of Santorini). Its nerve supply consists of a fine fibril arising from the hypoglossal nerve.
M. trapezius. — The M. trapezius of the right side gives off a fasciculus about 5 cm. wide and 1.5 cm. thick which is inserted into the clavicle at the inner part of its middle third just external to the origin of the clavicular portion of the M. sternocleido-mastoideiis. This fasciculus extends upward to join the anterior border of the main portion of the M. trapezius about 10 cm. from its insertion into the occipital bone. No special distinct nerve supply could be found for this fasciculus.
M. latissimus dorsi. — On eacli side of the subject the M. latissimus dorsi (Fig. 4) possesses an accessory tendon of insertion. The larger normal tendon passes as usual beneath the axillary artery and brachial nerves while the accessory tendon passes over, thus forming an arch for the nerves and vessels. The regular head is inserted into the bicipital groove of the humerus while the accessory one is inserted along with the tendon of insertion of the sternal portion of the M. pectoralis major into the anterior bicipital ridge. The tendinous fibres of this latter are intimately associated with the pectoral tendon for about 2 cm. and then separating pass down as a rounded tendon to join the muscle fasciculi which come off from the main muscle about 6 cm. from its humeral insertion. The accessory head thus formed is about one-fifth the size of the other tendon, which is normal and 7.5 cm. in length.
The two heads form a triangle with the long and short heads of the M. biceps and M. coraco-brachialis as a base. Through this pass the axillary artery and vein, the median, musculo-spiral, ulnar, internal cutaneous and lesser internal cutaneous nerves. So far there is almost perfect symmetry in these structures on the two sides of the cadaver, but the nerve supply to each head is apparently different, though the ultimate origin of the individual nerve fibres from the cord may possibly be quite the same.
The accessory head of the M. latissimus dorsi of the left side is supplied and apparently solely supplied by a rather small nerve entering the muscle substance. This can be traced up to the lower border of the M. pectoralis minor, there to join a nerve which is unmistakably the internal anterior thoracic, since it supplies the M. pectoralis minor sending some of its fibres through this muscle to supply the M. pectoralis major. On this side of the body the internal anterior thoracic nerve arises by two divisions and probably receives fibres from the VII and VIII cervical nerves.
On the right side, at first sight, the accessory head would seem to receive its nervous supply from the II and III intercostal nerves, since two stout branches from these sources enter the muscle substance. However, on closer examination, these appear to pass through the muscle substance without giving off any fibres to the muscle and to end further on as cutaneous nerves. As no other definite nerve fibre could be traced to this head, it must be supplied by a fibre from the middle or long subscapular nerve, running up in the muscle substance from the main muscle body. If this be the case these fibres would ultimately have about the same source in the cord as those supplying the accessory head on the left side as may be seen by comparing the diagrams of the two plexuses.* On the right side the long subscapular nerve arises from the posterior cord of the plexus just after this cord receives a branch from the inner cord. Thus this long subscapular nerve probably consists of fibres from the VII and VIII cervical nerves, the same nerves as those supplying fibres to the accessory head of the M. latissimus dorsi of the left side through the branch from the internal anterior thoracic nerve.
On neither side does the M. latissimus dorsi receive a slip from the tip of the angle of the scapula.
M. extensor carjii radialis accessorms. — This muscle occurs only in the right arm. Its origin is from the radial side of the M. extensor carpi radialis longior by a slip separating about 4 cm. from the origin of this muscle. About 5 cm. above the annular ligament this slip becomes muscular. Just above the ligament it passes under the tendon of the M. extensor carpi radialis longior to be inserted with the M. extensor carpi radialis brevior into the base of the metacarpal bone of the middle finger. It is innervated by a branch of the musculocutaneous nerve.
M. extensor digiti tcrtii. — Occurs only in left hand. Its origin is from the posterior shaft of the ulna below the origin of M. extensor indicis. Its tendon passes down with that of the M. extensor indicis through the same compartment in the annular ligament, and is inserted finally into the tendon of the M. extensor communis digitorum belonging to the middle finger and on its ulnar side. Its nerve supply is from the posterior interosseous branch of the musculo-spiral nerve.
Scapula. — On either side of the body, the tip of the acromial process is separate from the scapula, being connected to the spine of the scapula by a firm ligamentous band of connective tissue surrounding the proximal ends of bone lying in juxtaposition. The ends of the bones are covered with cartilage, and all are so firmly bound together that very little motion is possible.
All of these eight variations, in addition to the M. sternalis, are along the course the diaphragm takes in its excursion, during development, from the neck to its permanent location. It is to be noted that all this takes place before the embryo is four weeks old, and that at this time the muscles are just beginning to be formed from the myotomes.
Aug. 22, 1898.

Latest revision as of 15:52, 16 February 2020


Mall FP. Development of the internal mammary and deep epigastric arteries in man. (1898) Johns Hopkins Hospital Bulletin 9: 232.

By Franklin P. Mall.

(From the Anatomical Laboratory of the Johns Hopkins University .)

The great importance of the fact that the main arteries of the vertebrates arise directly from the aortic arches has been shown repeatedly in ontogenetic and phylogenetic studies. Furthermore, it gives us a scientific basis for the explanation of the numerous variations which may occur while this simple system of arteries is being transformed into the complex system as found, for instance, in man.

While the modification of the aortic arches is taking place, certain arteries become larger, others disappear, while in others tlie direction of the circulation is reversed. The laws governing these changes have been discussed extensively by Thoma in his numerous communications. It is the law of use and disuse expressed in this instance by rapidity. For a given vessel at a given time in its development there is a certain rapidity of circulation through it at which it ceases to grow. If the rapidity of the circulation is increased the vessel becomes larger ; if it is diminished the vessel becomes smaller ; if the circulation comes to a standstill the vessel disappears altogether. This law is constantly at work throughout development in the grown animal and in all pathological changes. It does not appear to govern the firstgrowth of the capillaries but itgoverns the growth of the vessels after the circulation in them is once started.

Any one who has studied the growth of the blood-vessels extensively in living animals as well as in fixed preparations is well aware of the fact that blood-vessels are constantly being formed and destroyed. This process is so extensive that at almost any stage of the development of an animal we may safely say that more blood-vessels have disappeared than are present at that time.

With the above ideas clearly in view, I have studied the extension of the blood-vessels into the body- walls of the embryo to see whether or not some secondary system, regular like the aortic arches, appears to be subsequently converted into the

main arteries of the body- walls and the extremities. That the vertebral artery is formed by the union of a number of branches from the descending aorta has been shown by His,* by Froriepf and by Hochstetter.J These branches are the segmental arteries from the aorta while it is still in the branchial region. The aorta soon shifts away from the branchial region with the bending of the head and the development of the neck. While this is taking place the segmental arteries unite at their distal ends to form the vertebral artery while the communicating branches are gradually broken off as shown in Fig. 1, V, from a human embryo of the fourth week. This process continues down to the seventh cervical segmental artery, where it ceases. At this point the subclavian artery arises, Fig. 1, S, as shown by Hochstetter. From now on for a few segments the process is very diflBcult to follow, but further on throughout the thoracic region of the embryo the process is simple. This is easily accounted for by the perfect segmental arrangement of this portion of the body throughout its development.

There are, however, two other sets of segmental arteries in the thoracic region of the body which appear before this set is formed and gives rise to the arteries of the body-walls. The first set is already well developed in embryos at the end of the second week.§ They arise from the aorta and pass immediately to the umbilical vesicle, there to break up into a plexus which is collected by the omphalo-niesenteric veins. Soon these arteries disappear and the omphalo-mesenteric artery takes their place. These vessels are of course on the mesial side of the ccelom.

Hi8(W.) Anat. mensch. Embryonen. III. Zuf Geschiehte der Organe, Leipz., 1885.

fFroriep (A.) Arch. f. Anat. u. Physiol., Anat. Ablh., Leipz., 1SS6, S. 69-1.50.

t Hochstetter (F.) Morphol. Jahrb., Leipz., Bd. xvi, 1890.

? Mall (F. P.) J. Morphol., Bost., Vol. xii, 1896-7, Fig. 16.

September-October, 1898.]



Before this first system has disappeared, and before the permanent intercostal vessels have appeared the second set of segmental arteries arises from the descending aorta, passes on the lateral side of the ccelom into the menibrana reuniens, as is indicated to us by a figure given by von Kiilliker,* as well as by the description of His.f Von Kiilliker pictures a cow's embryo with the whole membrana reuniens filled with a minute plexus of veins which radiate from the myotomes towards the umbilical cord, while His describes this same region in the human embryo as filled with branches of the umbilical vein which empty into the sinus reuniens above, and into the umbilical vein below. According to His's description they arise when the communication between the umbilical veins and the sinus reuniens is severed. Although the picture given by von Kiilliker does not correspond with His's description it does not contradict it, nor is it peculiar to the cow's embryo. I have in my collection a well-preserved human embryo (No. LXXVI), in which the membrana reuniens is filled with a plexus of veins much like

Fig. 1. — Arterial system of a humau embryo four weeks old (No. II). Enlarged 10 times. F, vertebral artery; P, pulmonary artery ; C, creliac axis; S, subclavian artery ; 4, fourth dorsal seijmeutal artery.

that in the cow's embryo. The specimen was taken from the uterus seven hours after the death of the woman, and without opening, the ovum was hardened in absolute alcohol. All the vessels down to the capillaries are filled with blood, thus making it an excellent specimen for the study of the blood-Tessels. It represents a stage somewhat more advanced than the one pictured by von Kiilliker, as the plexus of veins does not cover the whole membrana reuniens. The ventral wall over the heart near the liver contains no vessels, while the membrana reuniens covering the upper end of the heart is filled with a plexus of vessels which communicate with the capillaries of the mandibular arch. ., There is an extensive plexus through the arm and lateral body walls which extends through the menibrana

von Kolliker (A.) Grundrisa der Eutwicklungsgeschichte des MenschenundderhiJherenThiere. 8°. Leipzig, 1880. S.103, Fig. 85.

fHisCW.) Anatomie menschlicher Embryonen. Ill, Zur Geschichte der Organe. Leipzig, 1885, 8°. S. 206, also Fig. 130,

reuniens covering the liver, and finally encircles the cord and communicates with the umbilical veins.

The specimen just described is about 22 days old and, although I have six other good embryos between 14 and 28 days old, I find no such plexus in the membrana reuniens, although in all but one of them (14 days) the arm shows a rich plexus of capillaries filled with blood. In stages older than four weeks I find no blood-vessels in the membrana reuniens with the exception of that portion encircling the cord, where there is a rich network of veins. Although I have a number of excellent specimens of five or six weeks, the membrana reuniens over the heart and liver contains no blood-vessels until it is invaded by the ventral plate, which is accompanied by the development of the intercostal vessels. In pigs' embryos this extensive membrane is also free from veins, with the exception of the zone encircling the cord, which again has a venous plexus more marked, however, than in the human embryo.

It appears, then, that during the third week of development, while the umbilical veins still empty into the sinus reuniens, an extensive plexus is formed throughout the greater extent of the membrana reuniens, which receives blood from the aorta on its dorsal side, and empties it into the umbilical vein on its ventral side. As the umbilical vein changes its position to enter the liver, this circulation through the membrana reuniens is broken up as the much earlier circulation through the umbilical vesicle was broken up.

The earliest collecting vein for the descending aorta is the omphalo-mesenteric vein ; next, it is the umbilical vein, and finally, when the abdominal walls are comjileted, it is the cardinal. This in turn is partly converted into the vena cava inferior.

The permanent arterial system is already well outlined in embryo II (Fig. 1). The aortic arches and segmental arteries are sufficiently well marked to permit one to number them. The vertebral artery is in process of development, it being formed by a union of a number of segmental arteries, as shown by His, by Froriep, and by Hochstetter. The seventh cervical segmental artery gives rise to the subclavian artery. The lower cervical, all the dorsal and lumbar segmental arteries, are concerned in the development of the thoracic and abdominal walls. Fig. 1 illustrates the extent of the arteries. It shows a simple arrangement from the vertebral to the hypogastric artery. The lower lumbar arteries are not shown. A section of this embryo is given in Fig. 2. It shows the relation of a segmental artery to the myotome. The segmental arteries supply primarily the spinal cord and ganglia by two groups of branches, one near the middle line and one more lateral. A more ventral group of segmental arteries supplies the Wolffian body. The blood from all these groups of arteries is collected by the cardinal veins.

The lateral group gives rise to the intercostal arteries by first supplying the myotome, and, as this grows into the membrana reuniens by a process of budding, the vascular loop follows it. In so doing the loop is first on the dorsal side of the sympathetic and finally on its lateral side, thus making the sympathetic cord cross the intercostal arteries and veins on their ventral side, as is the case in the adult.

No sooner has the vascular loop extended to the lateral side of the sympathetic cord than it begins to anastomose with



fNos. 90-91.

neighboring segmental loops, as single vessels near the subclavian and hypogastric arteries, and as a plexus midway between these two. Tliis gives us at tliis early period a complete lateral anastomosis from the subclavian artery to the femoral, as Fig. 3 shows. It remains only for this system to shift around towards the median line with the muscle, nerves and ribs to form the condition of things as found in the adult.

Fig. 2. — Section through a human embryo lour weeks old (No. II). Enlarged 55 times. 4, aorta ; FW, foramen of Winslow ; MB, membrana reuniens; L, liver; R, heart.

Fig. 3 shows that the upper and lower segmental arteries of the series do not correspond at this early time with the same in the adult. Above, the superior intercostal is missing, while below, there is only a small fourth lumbar artery present, and it arises from the middle sacral. Iliolumbar and circumflexiliac arteries are altogether wanting, and I should judge from the relation of the arteries in this embryo, that the arch formed by the iliolumbar and circumflex-iliac is of secondary origin and has nothing to do with the segmental arteries. That they form anastomoses with the lower lumbar arteries in the adult can be explained in other ways.

The hypogastric artery is present long before the segmental arteries are formed near its junction with the aorta, and on this account we can no more call the trunk of the common iliac artery segmental than we can apply the same term to the

descending aorta. We can only locate its origin in the neighborhood of the fourth lumbar artery.

Hochstetter has settled definitely that the subclavian is a branch from the seventh cervical segmental artery.* Between the seventh cervical and the third dorsal we have three segmental arteries. In Fig. 1 the segmental arteries in this region are all simple with the exception of the seventh, which sends a large branch into the arm. From this stage to the one pictured in Fig. 3 there is a jiimp, but in it we see the intermediate stage between Fig. 1 and the adult.

Fig. 3. — Arterial system of a human embryo six weeks old (No XLIII). Enlarged 5 times.

All of the arteries below the vertebral are destined to pass behind the sympathetic, and it is excluded only on account of its direction. In Fig. 3 the eighth cervical segmental passes on the ventral side of the nerve, which shows conclusively that it must either be a new artery or a secondary connection between the eighth segmental and the subclavian. Since the first and second intercostal arteries pass behind the sympathetic in this embryo and in front of it in the adult, we must accept Hochstetter's opinion that the superior intercostal is formed by secondary connections between the upper intercostals and the subclavian. If the old connection remains, it forms the arteria aberrans.

I stated above that the sympathetic lies in front of the subclavian, while in the adult it lies in a great part behind it. Hochstetter explains this change of position by a wandering of the trunk of the artery through the group of embryonic nerve cells. In earlyembryos the sympathetic system resembles a group of sprouts from the segmental nerves which cross the segmental arteries, and the sympathetic cord is of secondary formation. This cord grows very rapidly during the fifth and sixth weeks, to form a great mass of cells extending from the vagus gang

Hochstetter numbers the segmental arteries to correspond with the vertebrK above them. Throughout I number the arteries with their .accompanying nerves. He states that the subclavian arises from the sixth segmental, this being the same artery that I call the seventh cervical segmental.

September-October, 1898.]



lion to the adrenals, connecting all of the branches with the segmental nerves to make of them rami conmiunicantes. This all goes on hand in hand with the descent of the heart into the thorax. At the same time the arm is rotating towards the ventral median line, and drags with it the subclavian artery. In so doing the subclavian is dragged into the sympathetic cord in its earliest stage, thus allowing the greater portion of the cord to be developed on its dorsal side. The portion of the sympathetic which from the first lies on the ventral side of the subclavian becomes the ansa subclavia.

The descent of the heart into the thorax on the inside with the descent of the arm over the clavicle on the outside of the body causes great tension on the upper intercostal arteries, and favors the new formation of blood-vessels in a more direct line. This is the reason why the main branch of the superior intercostal is a secondary and direct artery from the subclavian.

The simple diagram, Fig. 4, shows the origin of the main ai'teries of the trunk from the aortic arches and segmental arteries. This compared with Figs. 1 and 3 will explain itself. In the diagram the vessels which remain are black ; those which disappear, outlined; and those new formed, striated.

Resume : — While the aortic arches are forming, the arteries arising from the descending aorta pass on the mesial side of the coelom to the umbilical vesicle, and the blood passing through them is collected by the omphalo-mesenteric veins. Soon the segmental arteries arise, unite and shift backwards in the head region to form the vertebral artery. In the trunk they also unite within the lateral body-wall to form the internal mammary and deep epigastric arteries. Thisanastomosing arch, lying immediately below the tips of the ribs and the rectus abdominis muscle, wanders with them to the ventral middle line, the commrinicating branches forming the intercostal arteries. The whole course of the anastomoses between the tips of the segmental arteries takes on in its wandering the shape of the letter

Z (Fig. 4), the upper angle marking the origin of the subclavian. With the rotation and descent of the arm the subclavian is dragged partly through the sympathetic cord ; the origin of the superior intercostal is shifted, by the formation of a new anastomosis, from the dorsal to the ventral side of the sympathetic cord.

Fifi. 4. — Diagram to show the development of the arteries of the trunk from the aortic arches and segmental arteries. The arteries which remain are black; those which degenerate are outlined; those newly formed are striated. /. C, internal carotid; E. C, external carotid; J?. A., bulbus aortae ; P., pulmonary artery; A. D., descending aorta; v., vertebral; S., subclavian; F., femoral; C, umbilical; 7, seventh cervical; 3 and 12, third and twelfth dorsal.

The first vein collecting the blood from the abdominal aorta is the omphalo-mesenteric ; next the umbilical, and finally, the cardinal assumes this function.


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