Human Embryology and Morphology 25

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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 XXV. Body Wall and Pelvic Floor

Stages in the Evolution of the Body Wall

Behind the apparently simple arrangement of structures in the body wall of man lies a long history, only some of the later stages being known to us. Even in the lowest vertebrates the wall surrounding the pericardial and abdominal cavities is already muscular. We presume, however, there was a stage in which they were devoid of muscle, for in all vertebrates the musculature which enters the somatopleure, the lamina which forms the body wall of the embryo, arises from the muscle plates of the somites placed along each side of the dorsal median axis of the embryo (see p. 68). In fishes the musculature of each side of the body wall is arranged in two systems : (1) a vertebral, lateral or oblique system in which the ribs are embedded ; (2) a ventral or longitudinal system which extends from pharynx to tail. Both longitudinal and oblique systems are differentiated from one stratum. It is from a simple system of this nature that the musculature of the human body wall has been evolved (see Fig. 445).

Respiratory Stage

With the evolution of lungs the musculature of the body wall assumed a respiratory function.^ In fishes its chief use — if one excepts the part it plays in body movements — is to assist in the circulation of the blood within the body cavity — to drive it on towards the heart, and to expand or contract the cavity as the alimentary canal fills or empties. By means of ribs embedded in the septa of the lateral wall, the musculature of the body cavity became capable not only of compressing or diminishing the body cavity, but also of expanding it, and thus filling the lungs with air. In this manner the body musculature entered into the service of the lungs, and the nerve centres (respiratory centres) in the hind-bfain, which formerly regulated the movements of the gills and pharynx, came to have an automatic dominion over musculature of the body wall. The ribs, which served in the simple economy of the fish's body, became strengthened and firmly jointed to the vertebrae ; at the ventral ends of those encircling the lungs a supporting bar — the sternum — was evolved ; the primitive sheets of musculature became difierentiated to act on the ribs. In the latter part of the 2nd month when the lungs and pleural cavities are undergoing rapid development, respiratory transformations, similar in nature to those just mentioned, are taking place in the human embryo.

1 See R. H. Paramore's " Hunterian Lectures," Lancet, 1910, May 21st and 28th ; Prof. Wood Jones, Journ. Anat. 1913, vol. 47, p. 282.

2 F. Tourneux, Compt. Rend. Assoc. Anat. 1902 (Dev. of Walls of Thorax).

Mammalian Stage

We have already seen that the lungs of mammals develop within special cavities, which ultimately surround the heart ; as the pleural cavities expand they dislocate from the neck and depress within the body cavity a partition which completely divides it into thorax and abdomen. With the evolution of the diaphragm, and the disappearance of the lungs from the abdominal cavity, the body wall musculature became further modified, so that it can control the thoracic as well as the abdominal pressure. The evolution of pleural cavities effected a transformation in the thoracic part of the body wall. Their expansion and the differentiation of the thoracic wall are taking place during the latter part of the 2nd month of human development.

Orthograde Stage

It -is believed by many that the upright or orthograde posture is confined to man, and that it represents one of the more recently acquired human characters. This is certainly not the case ; man shares the orthograde posture with the group of primates with which he has so many structural affinities — namely, the anthropoid apes. Like man, they carry their bodies in an upright posture during progression. The smallest and most primitive of the anthropoid apes — the gibbon — is of ancient origin ; the orthograde posture is therefore an adaptation which has been long established in the higher group of primates. With a change of posture to the orthograde the action and fixation of the musculature of the body wall became greatly altered ; the mechanism of respiration was necessarily altered. The chest became wide or barrelshaped, the sternum broad ; the heart came to rest on the diaphragm. The muscles of the abdominal wall had not only to carry on their respiratory function ; they had also to support the abdominal viscera and to assist in emptying them. The mesenteric adhesions which take place during the early months of foetal life (see p. 286) are designed to give additional fixation to the viscera. The lower abdominal viscera came to rest on the pelvic floor ; the muscles of the tail, which rise within the pelvis of pronograde mammals, were modified to form a muscular hammock for the support of the viscera and the external tail disappeared. The caudal or coccygeal vertebrae are more reduced in the anthropoid apes than even in man. The spinal musculature and spinal column were altered to meet the new postural conditions.

Plantigrade Stage

If man shares , the orthograde posture with a group of higher primates, the power of plantigrade progression is peculiarly his own. Everyone recognizes that the foot, the leg, the thigh of man have undergone extensive structural alterations, but the fact is often overlooked that the process of adaptation has also led to marked structural changes in the body wall. The inguinal region especially has been modified. The great development and complete extension of the thigh have altered the musculature of the groin ; the inguinal (Poupart's) ligament has been evolved. These structural adaptations have weakened the human groin, and made it the commonest site of hernia. In the normal human upright posture the trunk is balanced on the pelvis ; the crest of the ilium and the external oblique have become modified for this purpose. The muscles of the abdominal wall not only support the abdominal viscera, and inaintain them during their respiratory excursions, but also take a part in producing and regulating the movements of the -body. Their functional value is often impaired in man, and hence he is the^subject of those forms of slipping _,or dropping of the viscera which are grouped under the name of visceroptosis, He is liable to many other varieties of static disablements.

Inguinal and femoral hernia occur so rarely amongst mammals generally that^they may be considered human peculiarities. Their frequency in man is due to certain structural changes in his pubo-femoral region, changes which have resulted mainly from his adaptation to upright progression. His susceptibility to hernia is due to : (1) The unique form of Poupart's ligament in man. It is scarcely developed in any other animal (Fig. 429). In the orang, for instance, also an upright primate, the external oblique has no attachment to the crest of the ilium, and takes no part in forming the outer part of Poupart's ligament (Fig. 429), the aponeurosis from the lower muscular idigitations terminating directly in the pillars of the external abdominal ring, thus strengthening the region of the inguinal canal. This is the usual termination in the mammalia. In man the anterior part of the iliac crest has grown into the lower digitations of the external obHque and severed them from their tendinous fibres, which now form the main constituent of Poupart's ligament. The digitations thus inserted to the iliac crest help in balancing the body.

File:Keith1921 fig428.jpg

Fig. 428, A. The Form of Pelvis and Inguinal Canal in Man., B. The corresponding forms in Pronograde Primates.

(2) The internal oblique and transversalis (conjoined parts) in the orang, and in all primates except man, arise from the firm tubular- sheath of the ilio-psoas, also froni the extensive anterior border of the ilium, and, arching over the spermatic cord, end in a long insertion on the iliopectineal line. They act as a powerful compressor or sphincter of the inguinal canal, and thus prevent 'hernia (Fig. 429, B).

(3) The human manner of walking and the great head of the human child at birth require a wide pelvis. All mammals adapted to the pronograde posture have a narrow pelvis, and hence a narrow anterior abdominal wall (Figs. 428 A and B) through which the inguinal canal passes very obliquely. The course of the canal is more direct in man. and therefore offers a greater facility to the escape of the abdominal contents.

File:Keith1921 fig429.jpg

Fig. 429, A. Poupart's Ligament and the Crural Passage of Man. B. Poupart's Ligament, Crural Passage, and Sphincter-like Conjojned Muscle of the Orang.

(4) Owing to the width of his anterior abdominal wall, the size of the space between the edge of the pelvis and Poupart's ligament (the crural passage) is very much greater in man than in any other animal (Figs. 429 A and B). In him, the most internal part of the passage is lef%-amfilleH,. and this unfilled space forms the femoral or crural canal through which femoral hernia may escape. The formation of the femoral canal has, ^ therefore, no embryological basis ; it is not like the inguinal canal the site of an embryological outgrowth of peritoneum. The crural passage is relatively larger in women than in men, owing to the greater size of the female pelvis, and hence femoral hernia is much more common in women than in men. Some hint as to the method of treatment of hernia in- man may be obtained from a consideration of the arrangement of structures ' which prevent them in other animals.

(5) Perhaps the most important factor in the causation of hermajn man is the compression to which the abdominal contents are subjecteti by the contraction of the musculature of the abdominal parietis during strenuous efforts, such as the lifting of heavy weights or the carrying of excessive burdens.

The Pelvic Floor

Coccyx. — The retrograde changes undergone by the coccyx in the evolution of the human body are intimately connected with the formation of the pelvic floor. The. coccyx in man is commonly comjDOsed of four vertebrae, more or less vestigial in nature, which represent the basal caudal vertebrae of tailed mammals. Evidence of their vestigial or retrograde nature is to be found in :

(1) Only their centra are developed — with the exception of the first, which shows partial formation of transverse processes and neural arches (superior cornua) ;

(2) Delay in the appearance of the centres of ossification. These, instead of beginning in the 8th week as in a typical vertebra, commence after birth. The centre for the 1st coccygeal vertebra appears in the 1st year, that for the 4th vertebra about the 25th year ; the 2nd and 3rd at intermediate periods. All four are fused into one piece about the 30th year.

(3) Late in life, between the 40th and 60th year, the coccyx unites with the sacrum.

The number of coccygeal vertebrae varies ; four is the normal number, but there may be three or five. In the 7th week embryo as many as eleven coccygeal vertebrae have been counted. The first coccygeal vertebra may join the sacrum, making six sacral vertebrae. The coccygeal vertebrae in anthropoids are more reduced as regards the development of their parts than in man.

The evidence of the former existence of a true tail in the ancestral human stock consists of :

  1. From the 5th to the 8th week the coccygeal region of the spine protrudes (Fig. 430), and the vertebrae number from 8 to 11 ; the notochord is traceable beyond the vertebral segments.
  2. Vestiges of the extensor and flexor muscles of the tail are frequently found (10 % of bodies) on the dorsal and ventral aspects of the sacrum and coccyx. Occasionally small nodules of bone are found in front of the human coccyx, spanning the continuation of the middle sacral (caudal) artery ; these nodules represent the chevron bones or haemal arches of tailed mammals. The depressors of the tail are attached to the chevron bones (see Fig. 431).
  3. True tails, consisting of external prolongations of the coccygeal region, commonly fibrous, rarely containing vertebrae, occasionally occur.
  4. The post-anal pit, always to be seen in the newly born child, marks the point at which the coccyx disappears below the surface early in the 3rd month. In man the coccyx forms part of the perineal floor. Instead of projecting far beyond the gut, as in tailed mammals, it terminates

Pelvic Floor is peculiarly extensive in man, an adaptation to his upright posture. The floor is formed by the following structures : (1) The levator ani and its sheath (recto-vesical and anal fasciae) on each side ; (2) The coccyx and coccygeus muscles ; (3) The constrictor urethrae and triangular ligament ; (4) The pyriformis and its sheath may also be included.

Development of the Pelvic Floor

The pelvic floor has been evolved in man by a transformation of the tail and the caudal muscles. The arrangement of tail muscles in a four-footed mammal, such as the monkey or dog, is shown in Fig. 431, A, and the modification of this form in anthropoids and man in Fig. 431, B. In mammals, two muscles, the pubococcygeus and ilio-coccygeus act as depressors of the tail, which in four-footed animals plays the part of a perineal shutter ; in orthograde primates the tail no longer helps to close the perineum, its muscles being required for the support of the pelvic viscera. In pronograde apes these muscles are attached to the small V-shaped chevron bones on the under surface of the basal caudal vertebrae (Fig. 432). Another muscle, the ischio- or spino-coccygeus, acts as a lateral flexor of the tail. It is attached to the transverse processes of the caudal vertebrae, and rises from the dorsal border of the ischium. In man the pubo-coccygeus and ilio-coccygeus are blended into one sheet and form the levator ani. The shrinkage of the tail leaves the muscle partly stranded on the ano-coccygeal ligament (Fig. 431, B). Other fibres of the pubo-coccygeus lose their primary insertion to the coccyx, and become attached to the prostate, central point of the perineum, and to the anal canal. Both muscles, especially the ilio-coccygeus, retain in part their primitive attachment to the coccyx (cauda). The spino-coccygeus, or coccygeus muscle, is partly fibrous in man, its outer laminae forming the small sacro-sciatic ligament ; its inner laminae remain muscular and form the coccygeus. In man too, the origin of the ilio-coccygeus has sunk from the pelvic brim of the ilium on to the obturator fascia (P. Thompson) ; traces of the primitive origin from the pelvic brim can often be detected (Fig. 433). The white line, a structure peculiar to man, marks the new point of origin of the levator ani from the obturator fascia.

File:Keith1921 fig430.jpg

Fig. 430. The rise and retrogression of the caudal vertebrae during the 2nd month of development. (After Kunitomo.)

1 The following are some of the British papers dealing with this subject : P. Thompson, Myology of the Pelvic Floor, Manchester, 1899 ; R. H. Paramore, Lancet, 1910, May 21st and 28th. In the Journal of Anatomy and Physiology the following papers have appeared : P. Thompson, 1901, vol. 35, p. 127 ; A. M. Paterson, 1907, vol. 41, p. 93 ; D. Derry, 1908, vol. 42, p. 97 ; G. Elliot Smith, 1908, vol. 42, p. 198 et seq. ; J. Cameron, 1908, vol. 42, p. 438.

Fig. 431. — Diagram to show the Pelvic Muscles of a Pronograde Ape (A) and of an Orthograde Ape (B).

In fishes {selachians) the levator ani is represented by a backward continuation of the rectus abdominis (Paramore). The pelvic part of the rectus is attached behind to the tail ; anteriorly it is attached to the movable pelvic girdle. The cloaca of the dog-fish passes out between the right and left primitive representatives of the levator ani, which can compress the cloaca, not by depressing the tail as in mammals, but by pulling the pelvis backwards.

Pelvic Fascia and Fasciae in General

It has been customary to regard fasciae as sejDarate structures forming distinct sheets with devious and complex courses. It is possible by dissection to prepare and display them according to accepted descriptions, but the structures so displayed are artificial and not the true structures which the surgeon or physician has to deal with in actual practice. Embryology is the best guide to their nature. Take, for example, the development of the fasciae seen on making a section of the upper arm (Fig. 434). When the limb bud has appeared, which it begins to do about the end of the 4th week of development, a section through it reveals a syncytium of mesodermal cells, the blastema of bones, muscles, etc., surrounded by a covering of ectoderm (Fig. 435). Very soon the central cells near the axis of the bud are densely grouped and form the basis of the skeletal axis. Others, derived from extensions of the primary muscle plates (Fig. 435), arrange themselves to form the biceps, triceps and muscles of the arm ; others become the walls of vessels and the sheaths of nerves. After these various groups of cells have become differentiated, there is left over a cellular residue in which the highly differentiated cell-groups are enmeshed. The undifferentiated mesoderm forms the connective tissue or fascial system of the part. From the manner of its origin it is evident that the connective tissue system — ^the fasciae and septa — must form a continuous sponge-work of sheaths, each being in continuity with that of every surrounding structure. The sheaths of the biceps, triceps and brachialis anticus, the periosteum of the humerus, the deep fascia, internal and external intermuscular septa, the sheaths of the vessels and nerves of the arm, represent the mesodermal tissue which was left over after the individual structure of the brachium were differentiated, and are, from the manner of their origin, necessarily in contmuity (Fig. 434). They can only be artificially separated from each other. It is more accurate and easier to describe fasciae, then, not as separate structures, but as adjuncts of the structures which they surround or ensheath. As to the manner in which connective tissue is developed, there are two opinions : (1) that the substance of the cell body elongates and forms a fibre ; (2) the more probable, that fibres are formed in a substance which lies outside the cell body, but is under the influence of the cell.

Fig. 432. — The Pelvic-caudal Muscles of a Monkey.

Fig. 433. — The Pelvic Muscles of Man — corresponding to those shown in Fig. 432.

Fig. 434.— Section across the Upper Arm to show the continuity of its Fascial System.

Fig. 435. — Section of a Limb-bud to show the manner in which its tissues become differentiated. (After Kollmann.)

The Pelvic Fascia, which strengthens the pelvic floor, is composed of the sheaths of four muscles : (1) Levator Ani ; (2) Obturator Internus ; (3) Pyriformis ; (4) Constrictor Urethrae and deep Transversus Perinei.

The fibrous capsules of the following viscera also form part of it :

(1) Prostate and Vesiculae Seminales in the male ; (2) Vagina and Uterus in the female ; (3) Bladder ; (4) Rectum. Under the title of pelvic fascia these eight elements are combined. To these must be added the important sheaths of the vessels — especially of the vesical, uterine and perineal arteries.^ I. The Obturator Fascia is the sheath on the inner or pelvic aspect of the obturator internus ; the sheath on the outer side of the muscle is formed by the periosteum and obturator membrane. The obturator fascia is attached at the circumference of the muscle. There it becomes continuous with the periosteum of the os innominatum. The part above the white line (supra-linear) is intra-pelvic ; the part below (infra-linear) is perineal and situated on the outer wall of the ischio-rectal fossa.

II. Recto- vesical and Anal Fasciae. — The levatores ani form a muscular floor for the pelvis, stretching from the white line of one side to the white line of the other. The sheath on their under surface — on the inner wall of the ischio-rectal fossa^ — forms the anal fascia. On the upper surface, their sheath forms the greater part of the recto-vesical fascia. The pelvic viscera rest on the upper surface of the levatores ani and the capsviles of the viscera are continuous with the sheath on the upper surface of the muscles. The combined visceral capsules and upper sheath of the levatores ani form the recto-vesical fascia.

III. The Triangular Ligament is situated in the neighbourhood of the constrictor urethrae muscle (Fig. 436), but it can scarcely be regarded as its sheath. It is rather a fibrous septum for giving attachment to the prostate on its deep or pelvic surface and to the bulb and root of the penis on its lower or perineal aspect (Delbet, EUiot Smith). The inferior transverse fibres of the constrictor form really a separate muscle — the deep transverse perineal. The apex of the prostate rests on the muscle, its fibrous capsule being continuous with the posterior layer of the muscle sheath — the deep layer of the triangular ligament.

^ For literature see F. P. Mall, Amer. Journ. Anat. 1901, vol. 1, p. 329 ; A. von Szily, Anat. Hefte, 1907, vol. 33, p. 225 ; J. S. Ferguson, Ainer. Journ. Anat. 1912, vol. 13, p. 129 ; Korff, Ergebnisse der Anat. 1907, vol. 17, p. 247.

- See references on p. 409 under the names of Prof. A. M. Paterson and Prof. Elliot Smith.

Fig. 436.— The Constrictor Urethrae Muscle.

IV. The inner sheath of the pyriformis forms the pyriform fascia. The coccygeus is continuous with the levator ani and its sheath forms part of the recto-vesical fascia. The loose perirectal sheath is also continuous with the tissue of the fascia pyriformis.

The pubo-prostatic ligaments and the lateral vesical ligaments are. strengthened parts of the fibrous capsule of the prostate, which provide the bladder with a pubic fixation. The vesical musculature, in emptying the bladder, acts from the pubic fixation thus obtained. The great strains to which the pelvic vessels are exposed when the pelvic floor and viscera are depressed in forced muscular efforts renders a strong fibrous protective sheath necessary. Hence the tough fibrous coating round the uterine and vesical vessels. Alcock's canal is formed from the fibrous sheath round the pudic artery and nerve (Elliot Smith).

Cervical Fascia

From what has been said of the pelvic fascia, the nature and arrangement of the cervical fascia will be readily understood. It is composed of (1) the sheaths of the cervical muscles (sterno-mastoid, etc.) ; (2) of the sheaths of vessels (carotid sheath, etc.) ; (3) the sheaths of nerves (axillary sheath, etc.) ; (4) the fascial capsules of viscera, such as the thyroid body, salivary glands, and pharynx. The carotid sheath and sheaths of the great vessels from the base of the skull to the pericardium within the thorax are formed to a great extent from mesodermal tissue which was developed within the visceral arches of the pharynx. At first the pericardium lies beneath the mouth and pharynx. With the development of the neck at the end of the 2nd month of foetal life, the cervical structures and their sheaths become stretched, but they maintain the ancient connection between skull base and pericardium.

The muscular sheaths on the inner aspect of the transversalis, iliacus and psoas also have been regarded as forming distinct fasciae.

On the other hand, some fasciae are quite discrete structures. The palmar fascia is part of the palmaris longus muscle ; the plantar, part of the plantaris muscle ; the vertebral aponeurosis or fascia, part of the layer of muscle which is represented by the serratus posticus superior and inferior ; the epicranial aponeurosis is part of the platysma sheet. The middle layer of the lumbar fascia represents a primary septum developed between the dorsal and ventro-lateral groups of musculature (see p. 68).

Fascial structures have also a distinct relationship to the lymphatic system. Lymphatics follow the septa and capsules of glands and muscles ; the lymphatics of the lung collect in the connective tissue separating its lobules. The most remarkable of all the capsular tissues of the body are those represented by the membranes of the central nervous system ; there the cerebro-spinal spaces, or clefts, have separated the cerebral capsule into three layers — the pia mater, arachnoid and dura mater.

Leonard Hill has also drawn attention to the part which ensheathing fasciae play in assisting the circulation of the blood. Every contraction of the muscles of the thigh tends to force the venous blood within the sleeve formed by the fascia lata on towards the heart.

^ See Prof. F. G. Parsons, Journ. Anat. and Physiol. 1910, vol, 44, p. 153.

Body WaU. — Having thus traced the evolution of the pelvic jfloor and discussed the nature of fasciae generally in connection with the pelvic fascia, we pass on to consider the development and nature of the abdominal and thoracic walls.

Bilateral Symmetry of the Body. — From a developmental point of view the body is made up of two symmetrical halves ; each half of the embryonic plate, taking the medullary groove as the line of division, contributes equally to the formation of the body. Each produces a half of the nervous system, each a half of the vascular, muscular and alimentary systems, so that each individual is in reality made up of two identical halves, right and left. Although each side of the body rises from the same blastocyst, yet each becomes specialized structurally and functionally so that, as development goes on, there appears a very remarkable, asymmetry.

Fig. 437. FlG. 438. -Diagram of the Structures formed in the Median Ventral Line of the Body. -the Median Ventral Line in an Embryo of 4 weeks, to contrast with the Corresponding Line in the Adult.

Ventral Line of the Body

The structures within the right and left body walls become united along the ventral line from the mouth to the anus (see Fig. 437). The mesoderm, muscle plates, dermatomes, nerves and cartilaginous outgrowths, which are produced on each side of the median dorsal line of the body, meet on each side of the median ventral line. In this line are developed the symphysis of the lower jaw, the body of the hyoid bone (copula), the white line of the neck and angle of the thyroid cartilage, the sternum, the supra-umbilical part of the linea alba, umbilicus, infra-umbilical part of the linea alba, symphysis pubis, the septum of the penis, and of the scrotum and perineal raphe. The ventral line is continued forwards on the face between the parts derived from the mesial nasal processes.

The idea was at one time prevalent that the whole of this line was formed by the fusion of one somatopleure with the other ; the median ventral line was the suture formed by the union. Such is not the case. The blastoderm, which lies at first like a cap on the yolk sac (Fig. 18), is produced or folded anteriorly to form the fore-gut and the part of the body above the umbilicus ; it is produced posteriorly to form the hind-gut and the part of the body below the umbilicus. The blastoderm grows out from the umbilicus to form the embryo in much the same way as a soap-bubble is blown from the bowl of a pipe. In an embryo, at the commencement of the 4th week, the greater part of the ventral line is occupied by the umbiHcus (Fig. 438). At that time the umbilicus is 3 mm. long, the entire ventral line being about 4 mm. At the end of the 7th week the ventral line measures 15 mm., the umbilicus retains its former size, about 3 mm.

Fig. 439. — Diagram of a Human Embryo (6th week) showing the Arrangement and Extension of the Mesoblastic Segments. (After A. M. Paterson.) The first and last of each segment entering into the formation of the limbs is stippled. The position is indicated in which the sternum is formed.

At first the somatopleure shows no trace of segmentation. The paraxial masses of mesoderm become segmented early and form the muscle plates (Fig. 65). From each muscle plate of the primitive segments a process grows down into the somatopleure (Fig. 439). The somatopleure thus becomes segmented secondarily, the process of segmentation spreading from the dorsal to the ventral side of the plate, but along the median ventral line of the body wall, a band of the primitive mesodermal tissue remains unchanged and undifferentiated. In the ventral band between the left somatopleure and the right are formed the sternum and the linea alba (Fig. 437). In lower vertebrates, in fishes, and to a less marked extent in amphibians and reptiles, the myotomic segments remain distinct from end to end of the trunk.

Formation of Ribs

Ribs, like all true skeletal bones, pass through three stages : (1) They are represented by a mesenchymatous or membranous basis in the fibrous tissue (septa) between the muscular segments of the somatopleure (Fig. 439). The condensation of the costal mesenchyme appears at the beginning of the 5th week as a separate vertebral element.

(2) The mesenchymatous basis or blastema of the rib becomes cartilaginous.

(3) Ossification of the cartilage begins in the 8th week, but the process of ossification leaves the ventral parts of the costal segments untouched ; they form the costal cartilages ; in lower forms they become ossified and form sternal ribs. The process of chondrification begins at the dorsal end of the ribs in the 6th week, and spreads ventrally, thus repeating the order in which the blastema was laid down. The extension ventralwards of the ribs corresponds with the growth and expansion of the lungs ; at the beginning of the 7th week they scarcely reach the lateral or axillary line of the body, but by the end of this week they have effected a junction with the sternal bars (Fig. 443). The ribs from the 1st to the 7th are developed in the somatopleure over the pericardium. In lower vertebrates, such as reptiles, each rib articulates with the neural arch of a vertebra by two heads, dorsal and ventral (Fig. 60). The tuberosity of a rib represents its dorsal head. In man, with the exception of the first and last rib, or in some cases, the two last ribs, the costal head is placed opposite an intervertebral disc, for in position the disc represents the ventral or chordal part of a primitive vertebra. In the case of the first rib the head has shifted backwards to the body of the first vertebra, while in the 12th and sometimes the 11th, the head and tuberosity are fused, and both articulate with the part of the vertebra which represents a transverse process.

The Sternum

In man and anthropoids the sternum has become flat and highly modified with the alterations in the shape of the thorax (Fig. 372). With the adaptation to the upright posture the thorax becomes flattened from back to front ; its transverse diameter is as great, or greater, than the autero-posterior. The type of respiration is greatly altered. The sternum also becomes wider and shorter. To understand the nature of this change, it is necessary to note the characters of the sternum of a pronograde mammal, such as the dog or ape (Fig. 440). In such, the sternum is typically made up of seven segments : 1. A modified anterior segment, the pre-sternum ; 2. Five narrow, cylindrical segments or sternabrae, forming the body of the sternum ; 3. The ensiform process, a hind segment, complex in nature and ending in the middle ventral line. the ensiform process frequently bifurcates and is never segmented.

1 For development and differentiation of ribs see Charles R. Bardeen, Amer. Journ. Anat. 1905, vol. 4, p. 163 ; also p. 265 ; Geddes, Journ. Anat. and Physiol. 1913, vol. 47, p. 18. For ossification of ribs : Franklin P. Mall, Amer. Journ. Anat. 1906, vol. 5, p. 433.

The chief changes in the human sternum are : 1. Each segment has become flat and wide ; 2. The segments of the body fuse together during the years of adolescence, the fusion beginning behind and passing forwards ; 3. The 4th sternabra of the body is usually vestigial and is probably made up of two or more fused segments.

In low primates 8 or 9 pairs of ribs may reach the sternum, six or more sternabrae being then present. In man the number has been reduced to 7 pairs, the sternal ends of the 7th pair lying in front of the fourth sternabra. It is not uncomimon to find the 8th rib reaching the sternum, especially on the right side ; it is rare to find the 7th pair fail to reach the sternum. The more frequent presence of an 8th sternal rib on the right side is due to right-handedness (Cunningham), or, as seems more probable, to give a more secure origin to the right costal fibres of the diaphragm, which have a greater resistance to overcome during inspiration, than those of the left side. In man and the anthropoid apes a new feature appears in the lower costal cartilages. The 5th, 6th and sometimes the 7th throw out processes which articulate with the cartilage below. When, during inspiration, the diaphragm raises the chest, these articulations permit it to elevate the 5th and 6th pairs of ribs as well as the 7th pair.

Fig. 440

Fig. 441. -The Form of Sternum in a Pronograde (quadrupedal) Mammal. -The Form of Sternum m a Mammal adapted to the Orthograde (upright) Posture. The Points of Ossification are also shown.

Morphology of the Sternum

In amphibia the ventral parts of the shoulder and pelvic girdles develop towards the ventral median line. In ^ The account given by Paterson (Hunterian Lectures, 1903) has been followed with some modifications. For an introduction to the more recent literature see the median line a rod of cartilage is formed between them (Fig. 442). The median rod is differentiated as right and left bars from the ventral parts of the limb girdles. The right and left bars fuse to form the median cartilage. The median rod between the shoulder girdles becomes the sternum ; it is divided into three parts — anterior, which projects in front of the girdle (omo-sternum or supra-sternum) ; posterior, behind the girdle ; and the middle, with which the shoulder girdle articulates (Fig. 442, A). The sternum affords a basis from which muscles act on the shoulder girdle, and also a ventral basis for the articulation of the shoulder girdle. In all classes of vertebrates, the sternum is developed over and shields the heart. The median cartilage of the pelvic girdle is similarly divided into anterior, middle and posterior parts (Fig. 442, B).

Fig. 442. — The Cartilages developed on each side of the Median Line between the Shoulder and Pelvic Girdles. A, the shoulder girdle of the frog ; B, the pelvic girdle of sphenodon. (The term " epi-sternum " is wrongly applied in Fig. A ; it should be omo-sternum or supra-sternum. There is now a general agreement that the term epi-sternum should be applied to the membrane bone formed between the clavicles.) The evolution of a costal type of respiration in reptiles leads to a further stage of development. Some of the costal processes of the vertebrae grow towards the median ventral line, some of them reaching and articulating with the middle part of the bar between the shoulder girdles ; this part now serves as a fulcrum or sternum for both ribs and girdle. Such a condition is also seen in birds and monotremes (Fig. 466). In the higher mammals, the ventral part of the shoulder girdle retains only its ventral connection with the sternum through the clavicle ; it still serves as the basis of origin for muscles which act on the shoulder girdle and on the arm. Its chief purpose has become respiratory. In the human sternum the three parts of the primitive sternum can be recognized : the supra-sternal bones (Fig. 441), which are only rarely separated from the presternum, represent the anterior part (omo-sternum) ; the manubrium and body, the middle part of the shoulder girdle sternum ; and the ensiform process, the posterior part.

Whitehead and Waddell, Amer. Jovrn. Anat. 1912, vol. 12, p. 89 ; F. B. Hanson, Anat. Rec. 1920, vol. 17, p. 1 ; Amer. Journ. Anat. 1919, vol. 26, p. 41.

Development of the Sternum

In Fig. 443 four stages in the development of the human sternum are represented. Stage A shows the extent to which the ribs have become chondrified at the end of the 6th week ; the cellular costal blastema, into which the process of chondrification is spreading, is not shown. In the following week (Stage B) the process of chondrification has reached the middle line in the region of the presternum .

Fig. 443. — Four Stages in the Chondrification of the Human Ribs and Sternum and showing the Fusion of the Sternal Bars. (After Charlotte Miiller.) A, end of 6th week ; B, end of 7th week ; C, end of 8th week ; D, end of 10th week.

The ventral ends of the ribs are now joined together by a ventral or lateral sternal bar. The sternal bars in the region of the presternum have begun to fuse together across the middle line. At their anterior extremities they are joined by the ventral cartilaginous end of the clavicle. In the presternum there is thus an element apparently derived from the ventral end of the clavicle. In Stage C, about the end of the 8th week, the process of fusion is advanced, but the projection of the foetal heart and liver at this time (see Fig. 45), tends to keep them apart. Each sternal bar has now 7 ribs continuous with it, and its posterior end is free. Early in the 3rd month (Stage D) the process of fusion is complete, the cartilaginous basis of the sternum has been formed by the fusion of right and left bars.

At the end of the 2nd month the diaphragm is descending to its final position, the pleural cavities are rapidly forming, and the liver is assuming a more abdominal position. Charlotte Miiller,^ whose illustrations are represented here, found that the mesenchymal sternal bars were chondrified as direct extensions from the ribs.

The sternum is thus developed in the median ventral line over the pericardium and between the mandible in front and the umbilicus behind (Figs. 437, 439). The mesoderm condenses during the 5th week on each side of this part of the median line to form the right and left mesenchymal halves of the sternum, which anteriorly are continuous with the bases of the ventral part of the shoulder girdle (Fig. 444). These two halves, the right and left mesenchymal sternal bars, fuse gradually in the middle line, the process of fusion commencing at the presternum and spreading backwards.

Fig. 444. — The Sternal Bars in an Embryo of 7 weeks. (After Paterson.)

The sternum is regarded by Paterson as a structure rising independently of the ribs on each side of the median ventral line. This, however, is not the commonly accepted view. Ruge's researches led him to the conclusion that the segments of the sternal bars were produced as buds from the ventral ends of the ribs. The evidence of comparative anatomy and the difference in the type of the cartilage cells in the costal and sternal elements negative Ruge's interpretation.

In its development the sternum passes through three stages — fibrous, cartilaginous and bony.

1. Fibrous or mesenchymal Stage. — In the 7th week (Fig. 444) the costal cartilages are already chondrified. The mesoderm on each side of the median line, in which they end, has become condensed, and forms the membranous basis of the two sternal bars (Paterson). The bars begin to fuse together anteriorly.

^MorpTi. Jahrb. 1906, vol. 35, p. 591.

2. Cartilaginous Stage. — The blastema of each sternal bar begins to chondrify in the intervals between the ends of the costal cartilages. The process of chondrificatidn and fusion proceed apace, and by the commencement of the third month the segments of each side have united to form the cartilaginous sternal bars (Paterson). Fibrous joints are subsequently formed between the presternum and mesosternum and between the mesosternum and ensiform process. A fibrous, and then synovial joint, is also developed at the union of the costal cartilages with the sternum, except in the case of the first pair, where a synovial joint is only occasionally present.

3. Ossification. — A centre appears for each sternabra ; those for the third and fourth of the mesosternum are frequently double, one being placed on each side. The centres for the 4th mesosternal segment are frequently absent. The centre for the presternum (there may be two or even more) appears about the 4th month ; the centres behind appear in the 6th and 7th month ; that for the 4th sternabra of the mesosternum appearing about the time of birth ; that for the ensiform four or five years after birth. The process of fusion of segments begins behind about puberty ; the segments of the mesosternum are united together by the 25th year. Occasionally a median foramen may be seen in the sternum ; it is due to imperfect union of the sternal bars.

Sterno-Manubrial Joint becomes of great functional importance in man and those primates adapted to the upright posture. Even in old age this joint is rarely ossified (8 per cent., Paterson). In man a considerable respiratory movement occurs between the manubrium and body of the sternum. The manubrium moves in continuity with the ventral ends of the first pair of ribs ; the body of the sternum follows the excursion of the 3rd to the 7th pairs of sternal ribs. As a rare abnormality (commoner in black than in white races) this joint is formed between the first and second segments of the mesosternum.


Clear evidence of the origin of the sternum from the shoulder girdle is to be seen in the presternum. In the earlier developmental phases, it is continuous with the precoracoid element in the ventral end of the clavicle (Figs. 442, 443, 444). It is separated from this element by the development of the sterno-clavicular joints and meniscus. In over 80 per cent, of bodies the upper border of the human manubrium sterni shows traces of the supra-sternal bones which represent the anterior parts (omosternum, epicoracoids) of the primitive sternum. Very rarely these bones are separate (Fig. 441) ; commonly they are present as elevations or nodules on each side of the suprasternal notch (Paterson). The interclavicular ligament, which represents the interclavicle of birds (episternum), reptiles and monotremes (Fig. 466), is attached to the presternum.

^ Keith, Further Advances in Physiology, edited by Leonard Hill, 1909 (Arnold) ; Journ. Anat. and Physiol. 1896, vol. 30, p. 275.

Linea Alba

The separation of the sternal bars is not a reproduction of an ancestral phase, but is simply due to an embryological convenience to accommodate, first the yolk sac and later the large heart and liver of the embryo. In Fig. 445 is shown the early condition of the linea alba — from the classical research by Bardeen and Lewis. ^ The umbilical cord is still distended by a loop of intestine, and the two recti are wide apart, separated by the mesial ventral membrane — the primitive linea alba. The two sternal bars are also held apart by the condition of the umbilical structures ; indeed, the primitive linea alba is not only wide, but also extends from the neck to the perineum. In the 10th week the intestines return from the umbilical cord to the abdomen, the chest wall expands before the growing lungs and the mesial ventral line becomes gradually closed.

Fig. 445. — The Primitive Linea Alba in a Human Foetus in the 8th week — 20 mm. long. (After Bardeen and Lewis.) Only the right half of the body is shown ; the rectus abdominus is lateral in position, it and the sternal bars being kept from the mesial ventral line by the structures in the neighbourhood of the umbilicus.

Fig. 446. — Transverse Section of the Thoracic Wall of a Lizard to show the Primitive Arrangement of the Muscular Strata of the Body Wall.

In Fig. 446 a transverse section is shown of the muscular layers in the anterior or thoracic body cavity of a lizard, and which also represents a stage in the evolution of the musculature of man's body wall.^ It will be seen that there are three layers : an outer represented by the rectus and external oblique ; an inner by the transversalis, and a middle double layer — the internal and external intercostals. In the abdomen they are combined in one layer — the internal oblique. The three layers are functionally different ; the transversalis is a constrictor of the body cavity ; the middle layer is mainly respiratory in its action ; the outer is also respiratory, but chiefly concerned in body movements.

^ C. R. Bardeen and W. H. Lewis, Aryier. Journ. Anat. 1901, vol. 1, p. 1 ; Amer. Journ. Anat. 1901, vol. 1, p. 145 (Nerves of Abdominal Wall).

^ Keith, Journ. Anat. and Physiol. 1905, vol. 39, p. 243 ; Kazzander, Anat. Hefte, 1904, vol. 23, p. 541 (Dev. of Rectus Abdominis).

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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