Human Embryology and Morphology 26

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Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XXVI. Development and Differntiation of the Limb Buds

Evolution of the Limbs

The nature of the primitive structures from which limbs were evolved is still a much debated question. The manner of their development in vertebrate embryos makes it certain that they were not outgrowths from the vertebral system ; in every case they sprout out from the somatopleure, which encloses the body cavity, and are always supplied by the nerves of that lamina — the ventral branches of the spinal nerves. We are also certain that the limbs correspond to the pectoral and pelvic fins of fishes. It is clear that when land-living vertebrates were evolved, the slight structures which were equal to the balancing and finer movements of an animal suspended in water, had to undergo great modifications in order to become capable of moving and supporting the body on a solid medium. It was with the evolution of pulmoniferous land-living vertebrates that a very definite type of limb made its appearance. In all cases the limb of a primitive Tetrapod is built on the same plan ; it is made up of a basal segment or girdle, with a free part divided into proximal, middle and distal segments. The distal segment carried 5 digits.

Although man has departed greatly from the primitive mammalian type in the structure of his brain and trunk, yet in the elements which enter into the formation of his limbs he has retained more of the ancestral mammalian features than many other mammals. He retains the original number of digits ; the bones of his hand and foot are much less specialized than those of the horse. It is true that the skeleton of his lower extremity has been extensively modified for his plantigrade posture, yet under all the adaptational features one can see very clearly the outlines of a most primitive form. He comes of a stock which led an arboreal existence almost from the dawn of the mammalian type.

Embryonic Limbs

The limbs begin to appear at the end of the 4th week. A slight elevation or ridge is then seen to run along the dorsal border of the somatopleure, at some distance from the row of primitive segments formed in the paraxial mesoblast (Fig. 447). The limb buds spring from this ridge as flat processes with an upper, dorsal or extensor surface, and a lower, ventral or flexor surface. The two borders are anterior or cephalic and posterior or caudal. It is generally held that the lateral ridge, of which the limb buds are specialized parts, represents a continuous row of lateral fins. If this view is right, then the fore and hind limbs represent highly specialized fin-rays.

^ The following papers will give those interested a clue to the extensive literature on this subject : Osbum, Ainer. Journ. Anat. 1907, vol. 7, p. 171 ; Goodrich, Quart. Journ. Mic. Science, 1906, vol. 50, p. 333 ; E. Miiller, Anat. Hefte, 1909, vol. 39, p. 469 ; D. M. S. Watson, Journ. Anat. 1918, vol. 52, p. 1 ; W. K. Gregorv, Annals N. Y. Acad. Sc. 1915, vol. 26, p. 317 ; Prof. Wood Jones, Arboreal Man, 1915 ; Principles of Anatomy, 1920.

  • Development and Differentiation, see Bardeen's Monographs, A77ier. Journ. Anat. 1905, vol. 4, pp, 163, 265 ; vol, 6, p. 259 (muscles and nerves of lower extremity) ;

A section shows each bud to be composed of undifferentiated mesoderm, with a covering of ectoderm (Figs. 435, 454). It represents in structure a process of the undifferentiated mesoderm of the somatopleure or body wall ; hence the limbs are to be regarded, not as structures developed from the axis of the embryo, but as processes of the body wall. Extensions grow into each limb bud from the muscle-plate and skin-plate (dermatome) of the segments which are situated opposite the origin of the bud. Each corresponding segment of the spinal cord also sends to the limb bud a nerve process. At least seven body segments contribute to the formation of each limb (Fig. 439). Outgrowths from the myotomes into the limbs have been observed only in the embryos of lower vertebrates ; their occurrence in higher vertebrates is inferred. When the arm musculature becomes apparent as a mass in the 6th week, it shows no signs of separate segmental origin.

File:Keith1921 fig447.jpg

Fig. 447. Lateral View of a Human Embryo at the 28th day, showing the Limb Buds, Lateral Ridges, and Primitive Segments.

Changes in External Conformation

In the 5th week (Fig. 448) the limb buds are unsegmented ; in the 6th a constriction marks the hand off ; the position of the elbow being indicated in the same week. In the 7th week the fingers appear as thickenings in the webbed hand, the middle digit being indicated first. They become free at the end of the 8th week ; occasionally tlicy remain attached, the child being born with its fingers in a syndactylous condition. The shoulder remains buried in the body wall ; the skeletal structures of the arm and thigh are the first to be differentiated ; those of the forearm and leg precede the cartilaginous differentiation of the shoulder and pelvic girdle. In all the embryological changes the upper extremity is nearly a week ahead of the lower.

also Bardeen and Lewis, 1901, vol. 1, p. 1 ; Keibel and MalVs Manual of Embryology, vol. 1, 1910.

The Internal Differentiation of Tissues begins at the basal part of the limb and spreads towards the digits, the terminal phalanges being the last of the skeletal parts to become difierentiated (8th week). The mesoderm or mesenchyme becomes condensed in the axis of the bud and forms the cellular basis, or blastema, of the limb bones early in the 6th week. The skeletal basis is continuous, but where joints are to be formed there occur opener formations in the arrangement of the cells. Centres of chondrification appear in the skeletal blastema of the arm late in the 6th week (shaft of humerus) and the leg in the 7th week (shaft of femur).

File:Keith1921 fig448.jpg

Fig. 448. Four stages in the development of the Upper Limband 8th weeks. After Wilhelm His (1831-1904)}

Fig. 449. Four stages in the development of the Lower Limb — at the 5th, 6th, 7th and 8th weeks. After Wilhelm His (1831-1904)}

The condition of the skeletal blastema of the arm of a human foetus in the 7th week of development is shown in Fig. 450. The centres of chondrification have appeared for the humerus, radius, ulna and certain of the carpal bones ; the centres for the phalanges have not yet begun. The scapula, acromion and clavicle (outer part) are continuous ; a common centre appears for scapula and acromion, the outer clavicular blastema is chondrified separately. Before the end of the 2nd month the cartilaginous bases of all the arm bones have ajDpeared. Centres of ossification begin to appear in the latter part of the 2nd month, and correspond generally to the centres of chondrification. During the 3rd month the skeletal blastema between the chondrified bases of the bones, by a process of vacuolation within and between the cells, opens out into a cavity and forms the synovial membranes of the joints (Fig. 473). By the end of the 6th week the proximal muscles, vessels and nerves are appearing ; a week later, they are also apparent in the distal parts of the limbs. The tissue left over, not included in these structures, forms their sheaths, and the fasciae and connective tissue of the limb. The processes of the nerve cells to form the nerve-fibres, and of the muscle plates to form the musclefibres, grow in very early (see Fig. 454). The blood vessels appear first as a capillary plexus surrounding the ingrowing nerve buds ; in some mammals (the lemur, etc.) this embryonic plexus persists and forms the plexus mirabile. The limb vessels spread outwards from the segmental vessels.

^ ..„ rp, ci, , . iTj, . . .1, TT Skeletal Blastema of Lower Ex fig. 450. — The Skeletal Blastema of the Upper , .. ., , ,, i c ,i n,i Extremity of a Human Embryo in the 7th tiemity. — About the end 01 the 7 th drm^atLn^MiTn'dkated!'^*^(w!H':^Lewi?)°'^' week the blastema of the ilium becomes joined to the costal masses of the 1st, 2nd and 3rd sacral vertebrae (Fig. 451). The scapula, which at the beginning of the 2nd month lies opposite the 4th, 5th, 6th, 7th cervical vertebrae, retains its freedom (Fig. 450). By the end of the 7th week the cartilage centres have appeared for the majority of the bones of the lower extremity (Fig. 451). The centres for some of the tarsal and for the phalanges are formed before the end of the 2nd month, the terminal phalanges being the last. The acetabulum develops at the site of union of the iliac, ischial and pubic cartilages at the end of the 2nd month. At that time the femur has no neck — a condition seen in reptiles (Fig. 451). The neck begins to form early in the 3rd month. In the 3rd month the symphysis pubis is formed.

Fig. 451. The Skeletal Blastema of the Lower Extremity of a Human Embryo in the 7th week — 14 mm. long. (Bardeen.) Inset is the outline of the upper part of the lower extremity of a lizard. (Parsons.)

Torsion and Rotation of the Limbs

As the limbs are developed, the extensor surfaces of the knee and elbow are directed upwards. If the body of an adult were placed in the prone position, it would be necessary, in order to restore the limbs to their embryonic position, (1) to draw them out at right angles to the axis of the body, (2) to rotate the leg outwards so that the extensor surface of the knee is directed upwards, with the great toe in front and the little toe behind. (3) The arm, on the other hand, would require to be rotated inwards to bring the elbow (extensor surface) into the dorsal position. The rotation which brings the embryonic limbs into the adult position appears to occur at the junction of the limb girdle with the trunk.

Fig. 452. The Corresponding Points {A, B, C, and D) in the Ilium and Scapula.

Rotation at the Limb Girdle

To understand the extent of this rotation it is best to compare the scapula and ilium and pick out their corresponding points. The extensors of the thigh and arm may be taken as guides. The long head of the triceps and rectus femoris of the quadriceps manifestly correspond ; their points of origin — the anterior border of the ilium and axillary border of the scapula — may be regarded as homologous points. The other corresponding points are shown in Fig. 452. The sacral articular surface of the ilium corresponds to part of the supra-spinous fossa. To restore the limb girdles to their primitive and corresponding positions, the scapula has to be rotated so that its axillary or posterior border comes to occupy the position of its spine, while the ilium has to be placed at right angles to the spine and its anterior border rotated outwards until it occupies a position corresponding to the axillary border of the scapula. The free edge of the spine represents a former border of the scapula ; the supra-spinous blade of the scapula appears first in mammals.

There is a manifestly spiral twist in the humerus, but it is doubtful if this be in any way due to the torsion which the limb undergoes.

Professor Parsons ^ and Sir A. Geddes ^ have shown that although there is a direct correspondence in the elements of the upper and lower extremity, the correspondence is a reversed one — ^the right ilium representing a " mirror-image " of the right scapula which certainly is true. There is no evidence of a rotation of the elements of the limb-girdles during development. A reference to Fig. 453 will show that there is a correspondence between the structures on the distal border of the fore-limb and on the proximal border of the hind-limb. The subscapularis, teres major and latissimus dorsi {A), derivatives of a common flexor mass, correspond to the ilio-psoas — also the derivative of a common flexor mass {A^). The triceps and quadriceps (C, C^) also agree ; so do the olecranon and ulna with the patella and tibia. The specialization of the proximal digit of the hand to form a pollex, and of the first of the foot to form a hallux, occurs only in primates. The mirror-image theory particularly applies to the distribution of nerves. To explain this peculiar relationship, which exists between the fore and hind limbs of the same side in vertebrates, one is tempted to suppose that they represent anterior and posterior halves of a single primitive locomotory appendage ; the line of separation is represented by the adjacent borders of the limbs. On such a theory the adjacent borders should be constituted alike.

Fig. 453. Diagram of the Fore and Hind Limbs of the same side to show the " Mirrorimage " Relationship between their Constituent Parts. The vertical line passing through the umbilicus is regarded as the centre from which the two limbs have become differentiated. (After Parsons and Geddes.)

Segmental Nature of the Limbs

The nerves of the limbs, probably also the muscles, vessels and skin, are derived from a number of the primitive body segments. The 4th cervical to the 2n(i dorsal contribute to the formation of the upper extremity ; the 1st lumbar to the 3rd sacral to the lower, but even in man the extent to which the most anterior and most posterior of each of these contributes to the limb varies considerably. Since the processes of the skin and muscle plates of these segments retain in the limbs (so we infer from the study of limbs of lower vertebrates) their original nerve supply, it is evident that the muscles and skin of the human limbs may be assigned to their original body segments by a study of the distribution of the nerves. Such a study has been carried out by a great number of men during the two last decades.^ The primitive simple arrangement of muscle segments may be seen in the fins of certain fishes, but in man these segments have been divided and combined and special muscles formed from them ; yet the primitive arrangement can be recognized.

1 F. G. Parsons, Journ. Anat. and Physiol. 1908, vol. 42, p. 388.

^ Sir Auckland C. Geddes, Journ. Anat. and Physiol. 1912, vol. 46, p. 350.

Fig. 454. Section of the Arm Bud of a Human Embryo at the end of the 5th week. (Alex. Low.)

Nerve Supply of the Limbs. The Arm

It is important to note that the limb buds arise from the ventro-lateral aspect of the trunk (Fig. 454) near the junction of the somatopleure with the paraxial mesoderm. Therefore the nerves of the limbs are the nerves of the ventro-lateral zone — the lateral cutaneous branches of the typical segmental nerves (Fig. 455). The muscles are derived from the ventro-lateral sheet, which gives rise to all the muscles of the body wall. As soon as the limb buds appear, bundles of fibres from the anterior and posterior nerve roots of the corresponding body segments enter them and keep time with their growth. The limb nerves are at first so large in comparison with the size of the limb bud that they are crowded together and already form a plexus (Figs. 456, 457). As they enter the bud, the nerves encounter the condensed skeletal blastema at its base and divide into a dorsal or extensor set and a ventral or flexor set (Figs. 454, 455).

^ For references see Geddes, Journ. Anat. and Physiol. 1912, vol. 46, p. 350; also the researches of Bolk, Morph. Jahrb. from 1894 to 1898 ; A. T. Kerr, Amer. Journ. Anat. 1918, vol. 23, p. 285.

The relationship of the segmental nerves to the arm bud in the 6th week of development is shown in Fig. 456 — a drawing taken from Professor Streeter's research.^ The base of the arm is then situated in the cervical region ; the hypoglossal nerve issues almost at its anterior border. The arm descends tailwards during the 2nd month, the nerves consequently undergoing an elongation. The ventral divisions of the spinal nerves from the 5th cervical to the 1st dorsal have entered the bud, and already the chief nerves can be traced. The brachial plexus is formed ; the interlacing of fibres does not arise owing to a compression of the nerves due to a lack of room, but represents a physiological or functional adaptation. Professor Goodrich ^ found that in fishes only the posterior root or sensory fibres entered into the plexiform arrangement — the motor or ventral fibres proceed into the limb without exchanging fibres. By the beginning of the 3rd month all the muscles and nerves are differentiated. In Fig. 455 the distinction between the nerves of the extensor and flexor aspects of the limb is shown.

Fig. 455. Schematic Section to show the primitive grouping of the Nerves and Musculature of Limbs. (After Kollmann.)

In Fig. 457 the bud of the hind limb of the same embryo is represented. It will be seen that the stage of development is less advanced than in the arm. The crescentic base of the limb is in relationship with the spinal nerves from the 1st lumbar to the 3rd sacral. The crural and sciatic plexuses are continuous ; their separation occurs in the 7th week, when the ilium becomes attached to the costal processes of the sacrum.

^ Geo. L. Streeter, Amer. Journ. Anat. 1908, vol. 8, p. 285. ^ See reference on p. 425.

The nerve supply assists to indicate the body segments from which the arm is developed (Fig. 458). The 4th cervical is the most anterior, the 2nd dorsal, sometimes it is the 3rd, is the most posterior segment. Hence the arm is produced from seven, or more commonly eight, segments in all. Each segment contributes from its nerve, its muscle plate and probably also its artery (see p. 250). The typical distribution of a segmental nerve to the limb bud is shown diagrammatically in Fig. 455. Each segmental nerve, as is the case with the typical lateral cutaneous nerves, divides into a dorsal division for the extensor muscles, and ventral for the flexor muscles. The nerves to the extensor muscles form the dorsal divisions and cord of the brachial plexus ; the nerves to the flexor muscles form the ventral divisions and the outer and inner cords. The processes to the limbs from the skin plates and muscle plates are also divided into dorsal and ventral sets ; the one set making up the extensor aspect of the limb ; the other, the flexor aspect.

Fig. 456. The Arm Bud and its Nerves in a Human Embryo in the 6th week of development. (After Streeter.)

Fig. 457. The Bud of the Lower Extremity with its Relationship to Spinal Nerves in a Human Embryo in the 6th week of development. (After Streeter.)

Clinical and experimental research have shown that each of the seven or eight segments contributes to the cutaneous supply of the limb. The classical investigations of Sherrington ^ into the segmental distribution of the sensory nerves in the limbs of apes, showed that they are arranged in a definite and orderly manner (Fig. 459). The sensory distribution of the spinal nerves in the human arm is shown diagrammatically in Fig. 458. The distribution of the motor nerves of each segment is fully described in anatomical text-books.

Only three anomalous points in the arrangement of nerves in the upper limb require attention : (1) The segments which supply nerves for the arm are nearly constant. The extent, to which the 4th cervical and 3rd dorsal contribute, varies ; the degree of variation is markedly less than in the lower limb. (2) A part of the musculo-cutaneous nerve frequently joins the median below the insertion of the coraco-brachialis ; this communication is frequently seen in lower primates ; its meaning is not known. (3) A communication between median and ulnar in the forearm is also common and is seen constantly in some primates. The communicating branch passes with the deep branch of the ulnar nerve to the palm. It is also manifest that there is a correspondence between the musculo-spiral nerve on the proximal border of the arm and the sciatic on the distal border of the leg.

1 Sherrington, Journ. of Physiol. 1892, vol. 13, p. 621. 2 E

Fig. 458. The Distribution of the Posterior Roots of the Spinal Nerves on the Flexor Aspect of the Arm.

The Formation of Nerve Plexuses ^ depends on the following factors : (1) Each skin segment is supplied not only by its own nerve, but by the nerve of the segment in front of it and behind it. (2) A muscle segment, such as may be seen in the rectus abdominis, is supplied by its own and the two adjacent nerves, the fibres forming a plexus before entering the muscle. (3) Each muscle is formed by the combination of parts of two or more segments, and therefore its nerve rises from two or more spmal nerves. (4) The muscles of the limbs have migrated from their original positions and carried their nerves with them. (5) Most important of all, the afferent or sensory fibres from a muscle have to be linked to the centres of all the muscles which act as its antagonists or coadjutors. All these influences have led to the nerve fibres being assorted into definite cords at their first outgrowth.

1 H. Braus, Verhand. Anat. Gesellsch. 1910, p. 14 (Origin of Nerve-Plexuses).

Nerve Supply of the Lower Limb

Usually ten segments contribute to the nerve supply of the lower limb — the 12th dorsal to the 4th sacral (Fig. 460). The sensory nerves are derived from these segments ; the motor nerves begin at the 1st lumbar segment and end at the 3rd sacral. There is a considerable variation in the number of body segments or vertebrae to which the lower limb is attached ; usually it is the 25th vertebra which becomes the 1st sacral, but it may be the 26th or 24th (p. 55). Of these three forms, the first is the normal type (25th) ; the second the post-fixed type (26th) ; the third the prefixed type (24th). There is even a greater variation in the segments which contribute nerves to the limb ; the normal motor segments are the 1st lumbar to the 3rd sacral ; in the post-fixed type (more common than the next) the motor segments commence at the 2nd lumbar and cease at the 4th sacral ; in the pre-fixed type the motor segments commence at the 12th dorsal and end at the 2nd sacral. The spinal nerve which bifurcates and joins both lumbar and sacral plexuses is known as the nervus furcalis. In the normal type it is the 4th lumbar ; in the pre-fixed type it is the 3rd lumbar ; in the post-fixed type the 5th lumbar.

Fig. 459. — Diagram to show the typical manner in which the Posterior Nerve Roots are distributed in the Lower Limb (based on Sherrington's researches into the Sensory Distribution of the Limb Nerves of Apes).

The nervus bigeminus, normally the 4th sacral, may also vary in a corresponding manner.

The nerves to the extensor surface of the lower limb, the anterior crural (femoral), external popliteal (common peroneal), etc., represent the dorsal divisions of lateral cutaneous nerves (Fig. 455). The nerves to the adductor and flexor aspects, the obturator and internal popliteal (tibial), represent the ventral divisions. In a considerable number of individuals, the dorsal division (external popliteal) and ventral (internal popliteal) of the great sciatic separate in the pelvis, the external popliteal perforating the pyriformis.

The segmental distribution of the motor nerves in the lower extremities is given at length in text-books on anatomy. The muscular segments correspond approximately in their distribution with those of the skin.

Fig. 460. Flexor Aspect of the Lower Limb, showing the Sensory Distribution of the Segmental or Spinal Nerves.

It will be remembered that the perineal region is developed behind the limb buds of the lower extremities (Fig. 439) ; hence its nerve supply from the most posterior nerve segments (3rd and 4th sacral).

Sherrington found that the posterior roots of the limb nerves were distributed in a regular and simple manner in apes. His results are applied

to the lower limb of a human foetus in Fig. 459. The actual distribution in man, which has been partially worked out by clinicians, varies considerably from what might be expected from Sherrington's results (compare Figs. 459 and 460).

In the human leg and foot there is a tendency for the nerve fibres destined for the outer digits to proceed in the external saphenous (sural) nerve instead of by the musculo-cutaneous (superficial peroneal). The external saphenous nerve may supply the 4th and 5th digits (the ancestral form) in a manner similar to the ulnar nerve in the hand ; more frequently it is confined to the outer side of the 5th digit. The outgrowing fibres of the obturator nerve may be divided into ventral and dorsal parts by the blastema of the pubis. In such a case the more ventral fibres cross the ramus of the pubis and form the accessory obturator nerve.

Vessels of the Limbs

When the limb buds are being formed in the 5th week they are permeated by a capillary network, which in the case of the arm is chiefly fed by the artery of the 7th cervical segment, while in the case of the leg bud the chief axial artery arises from a pelvic arterial plexus — soon connected with the internal iliac (hypogastric) artery. During the 6th week the main arteries of the limbs are being evolved from pathways in the primary capillary plexuses ; by the end of the 8th week, all the important arterial channels have been laid down. Every student knows how frequently the arteries of the leg and arm depart from the arrangement which is regarded as normal. Comparative anatomy and embryology throw light on these arterial anomalies. ^

In Figs. 461, 462, the upper and lower limbs have been placed in corresponding positions — the extensor surfaces being directed upwards and a scheme of their arteries depicted in relationship to their skeletal elements. In each limb bud there is developed a main or axial artery, certain parts of which are suppressed in the 8th week while other accessory vessels are developed. The axial artery of the upper limb persists as the subclavian, axillary and brachial trunks, but in the lower limb the corresponding trunk (Fig. 362) is suppressed, as Professor Senior has shown, during the 8th week of development — save for the sciatic branch of the internal iliac artery and the anastomotic chain along the sciatic nerve which links together branches of the sciatic and popliteal arteries. In the flexor aspect of the elbow, as in the corresponding space — the popliteal — of the lower limb, the axial artery undergoes a degree of suppression. In the popliteal space, as we know from Professor Senior's investigations, the axial artery passes deep to the popliteus muscle ; the part which lies deep to the muscle becomes reduced during the 8th week and a new vessel develops superficial to the muscle. The part of the popliteal artery proximal to the popliteus muscle is derived from the axial vessel ; the part lying on the popliteus from the new trunk. In the anticubital space the corresponding axial vessel disappears, the terminal part of the brachial

^ For development of arteries see Prof. H. D. Senior, Journ. Anat. 1919, vol. 53, p. 1.31 ; Amer. Journ. Anat. 1919, vol. 25, p. 55 ; Erik Miiller, Anat. Hefte, 1903, vol. 22, p. 377. For comparative anatomy of vessels in limbs of primates see articles by Dr. Manners-Smith, Journ. Anat. and Physiol. 1910, 1911, 1912, vols. 44, 45, 46; E. Goeppert, Ergebnisse der Anat. 1904, vol. 14, p. 170.

In the fore-arm the axial vessel is represented by the anterior (volar) interosseus, continued into the hand to give ofi the palmar interosseus vessels — the primary blood supply of the hand. On the extensor or dorsal aspect of the interosseus membrane of the fore-arm develops the dorsal interosseus artery of the fore-arm fed by branches of the axial artery which perforate at the proximal and distal ends of the membrane (Fig. 461). In the leg the axial artery disappears, save its distal part, which is incorporated in the peroneal artery (Fig. 462). As in the fore-arm, perforating branches pass to the dorsal aspect of the interosseus membrane to form the anterior tibial artery.

Having thus traced the fate of the axial artery in each limb we now turn to the origin of the great secondary channels. The external iliac artery and its continuation, the femoral artery, open up a new channel to the lower limb along the course of the anterior crural or femoral nerve. The channel arises from the umbilical artery proximal to the origin of the internal iliac (the axial vessel) and by the end of the 7th week has effected a union with that part of the axial vessel which lies in the popliteal space (Fig. 462). In the upper limb there is no corresponding arterial trunk, although communications between the suprascapular (transverse scapular), circumflex and superior deep branch of the brachial artery may represent it. In both the leg and fore-arm more superficial secondary channels are formed — the ulnar and posterior tibial arteries and their branches which end in the superficial palmar and plantar arches (Figs. 461, 462). In all primates with the exception of man, the femoral artery, before piercing the adductor magnus, gives off a large branch — the saphenous artery — which accompames the long saphenous nerve and turns to the extensor aspect of the leg above the internal malleolus where it becomes the dorsal artery of the foot. At no stage of human development does the saphenous artery serve as a main channel, but the superficial branch of the anastomotica magna, which represents this vessel in man, is more highly developed at the 8th week than it is at any subsequent period. The saphenous artery corresponds to the radial of the upper limb.

Vas Aberrans

At a very early stage (7th week) there is developed along the superficial aspect of the median nerve an arterial anastomotic channel fed by a succession of branches which spring from the axial brachial vessel (Fig. 461). This channel frequently opens up in part, or even in its whole extent and gives rise to the greater number of arterial anomalies met with in the arm. The vas aberrans may replace the main artery, being known from the normal brachial artery by the fact that it lies superficial to the median nerve, whereas the usual vessel is deep to that nerve. The first ramus of the vas appears between the heads of the median nerve (Fig. 461). The ulnar or radial artery frequently arises from the brachial artery in the lower third of the arm ; in such cases the upper part of the radial or ulnar vessels will be found to be formed out of the anastomotic channel. In the fore-arm the median artery may be of large size, ending in the superficial palmar arch ; it, too, is formed out of the arterial anastomosis which is laid down in foetal life, superficial to the median nerve.

Superficial Veins

During the 6th week the terminal margin of the limb buds is fringed with a venous plexus which becomes broken up by the outgrowth and differentiation of the digits. The terminal plexus is drained by a vein which passes upwards on the fibular or peroneal margin of the limb, this marginal vessel becoming the superficial ulnar and basilic veins in the arm and the external saphenous vein in the leg. Later, radial and tibial marginal venous channels are formed, becoming the radial and cephalic veins in the upper limb and the long or internal saphenous in the lower. The cephalic vein originally crosses the clavicle and terminates in the external jugular vein as is the rule in apes, but later ends in the axillary vein, below the clavicle. In man only does the long saphenous vein terminate at the groin by piercing a hiatus in the fascia lata ; in all other primates it ends above the internal (mesial) condyle of the femur by joining the femoral veins in Hunter's canal.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures