Book - Buchanan's Manual of Anatomy including Embryology 4

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Chapter IV The Bones Of The Trunk

A. The Vertebral Column

The vertebral column is composed of thirty-three vertebrae in the young subject, and these in the adult are divided into two classes—namely, movable and immovable, or fixed. The vertebrae are subdivided into five groups—cervical, thoracic, lumbar, sacral, and coccygeal.

Component Parts of a Complete Vertebra. —A complete vertebra is composed of a body or centrum; a vertebral arch {neural arch) ; a spinous process, or neural spine {neurapophysis) ; two transverse processes; four articular processes (. zygapophyses ), two superior and two inferior; and a vertebral foramen. The centrum forms the anterior or ventral part of the bone, and is somewhat disc-shaped. The neural arch consists of two halves, the anterior portion of each being the pedicle, and the posterior portion the lamina. The pedicles present, above and below, the superior and inferior vertebral notches. The spinous process is formed by the fusion of the two laminae in the median line posteriorly. The transverse processes project outwards, one at either side, from the neural arch at the junction of the pedicle and lamina. The articular processes, two superior and two inferior, project upwards and downwards from the junction of the pedicle and lamina at either side, and they are covered by cartilage. The vertebral foramen is enclosed by the body and neural arch. It is bounded in front by the posterior surface of the body, on either side by a pedicle and lamina, and behind by the fusion of the laminae to form the spinous process.

Structure of a Vertebra. —The body is composed of cancellous tissue, covered by a thin layer of compact bone. The chief lamellae are disposed in almost vertical curves, the convexities of which are directed towards the periphery. Crossing these there are horizontal lamellae which are nearly parallel with the superior and inferior surfaces. The cancellous tissue is permeated by venous channels which converge to the two large foramina on the posterior surface of the body. The neural arch and its processes are chiefly composed of compact bone, the amount of cancellous tissue being for the most part small.

In the above description the terms ' body ’ and ‘ centrum ’ have been used as synonymous, but this is not strictly true. By ‘ body ’ is meant all that part of the vertebra which does not in the adult appear to belong to the neural arch. By ‘ centrum * only that part of the body is indicated which develops from its own centre of ossification. Later on it will be seen that a small part of the side of the body belongs morphologically to the neural arch.

The Cervical Vertebrae

The cervical vertebrae are seven in number, and they occupy the region of the cervix or neck. The distinctive character of all cervical vertebras is the presence of an aperture at either side of the body, called the foramen transversarium. The first or atlas, the second or axis, and the seventh or vertebra prominens have such pronounced characters that they require a special description.

Fig. 80. The Sixth Thoracic Vertebra (Superior View).

A Typical Cervical Vertebra. The body is small and box-shaped; wider sideways than from before backward. On its upper surface there is a nemo central lip on either side, which embraces the bevelled lateral border of the lower surface of the body above. The whole surface thus concave from side to side, and the posterior lip is on a

? ? iT ^ ? -f ve ^ ^ an the anterior. The inferior surface is bevelled laterally, and its anterior lip is on a lower level than the posterior, e m enor sur ace is convex from side to side, and concave from before ac wards. The superior and inferior surfaces give attachment to the intervertebral discs The anterior surface is convex from side to side, iS, from above downwards. It is covered by the anterior Thp^ a / • gament, and it presents a number of nutrient foramina, tiO Of tvllrh surface , 18 and presents several nutrient foramina, escaoe of the* & , ei J dier Slde °f the middle line, are large, for the the Dosterior 1nrioIf r ^k r t\^ einS * P oster ^ or surface is related to

inner bound a rv off] 11 ^ amen ^- Each lateral surface forms the inner boundary of the foramen transversarium.

The pedicles spring at either side from the posterior part of the lateral surface of the body, where each encroaches rather nearer the upper than the lower surface. They are smooth and almost cylindrical, and their direction is outwards and backwards. Above and below each pedicle there is a well-marked vertebral notch , the superior being narrower and slightly shallower than the inferior. The superior notch lodges a spinal nerve. When two vertebrae are in position the contiguous vertebral notches, at either side, complete an intervertebral foramen through which emerges the cervical nerve, and it will be noticed that, in its exit, the nerve must pass behind the foramen transversarium.

Fig. 81.—The Fifth Cervical Vertebra. A, superior view; B, lateral view.

The laminae spring each from a pedicle. They are compressed from before backwards, and their direction is backwards and inwards to the middle line, where they fuse, and so give rise to the spinous process. The upper border and adjacent portion of the posterior surface of each lamina give attachment to the ligamentum flavum connecting it to the lamina above, whilst the lower part of the anterior surface near the lower border gives attachment to the ligamentum flavum connecting it to the lamina below.

The spinous process is formed by the fusion of the two laminae. It is triangular, and its direction is backwards and slightly downwards. Superiorly it presents an antero-posterior ridge for an interspinous ligament, and interiorly a grooved surface, also for an interspinous ligament. It terminates behind in a bifid extremity, which, in the case of the third, fourth, and fifth vertebrae, presents a distinct triangular notch. The bifurcated extremity gives attachment to the deep fibres of the ligamentum nuchae. The cervical spinous processes are very short, except those of the sixth and seventh, especially the latter, and in this way backward flexion or over-extension of the neck is not interfered with.

The articular processes spring from the junction of the pedicle and lamina at either side. Each is nearly circular, the plane being oblique, and the surface almost flat. The superior pair look backwards and upwards, and the inferior pair forwards and downwards.

The transverse processes ( ’diapophyses) spring at either side from the junction between the pedicle and lamina, and are serially homologous with the transverse processes of a thoracic vertebra. Each terminates in a projection, known as the posterior tubercle.

The costal processes {parapophyses) project outwards from either side of the body anteriorly, and are serially homologous with the vertebral part of a rib. Each terminates in a projection, known as the anterior tubercle, which, with the posterior tubercle of the corresponding transverse process, gives attachment to the intertransverse muscles.

The transverse and costal processes are connected, at a short distance from the body, by a plate of bone, called the costo-transverse lamella, which is deeply grooved superiorly for a spinal nerve, this groove being continuous with the superior vertebral notch. There is thus formed, at either side, an aperture, called the foramen transversarium. This foramen is circular, vertical in direction (except in the case of the axis), and it transmits the following structures: the vertebral artery; the vertebral plexus of veins; and the vertebral sympathetic plexus of nerves. 1 hough the foramen is present in each transverse process, it does not give passage to the foregoing structures in the case of the seventh, lhe vertebral vein, however, may pass through it. In many cases an additional foramen of small size is present on one or both sides, lying behind the. main foramen, and, when this is so, it transmits a small vein. The foramina transversaria of either side, when in position, build up a canal, which is open in each intertransverse space.

The vertebral foramen is situated behind the body, and is triangular, with the angles lounded off. It contains the spinal cord and its membranes, and is larger than in the other regions to allow for the mobility of the neck.

The Atlas.- The atlas is the first cervical vertebra, and is so named ecause it supports the head. Its distinctive characters are the absence o a body and spinous process. It has the form of a ring, narrow in Iront and wide behind, and its component parts are as follows: an anterior arch; a posterior arch; two lateral masses; and a ring.

The anterior arch is a curved plate of bone which connects the antero-internal parts of the lateral masses. It is compressed from before backwards, convex in front and concave behind. The anterior surface presents at its centre a conical prominence, called the anterior tubercle. This gives attachment at either side to a portion of the l'ongus cervicis muscle, and its central part receives the accessory ligament. The posterior surface presents at its centre a circular concave facet, called the odontoid facet, for articulation with the anterior surface of the odontoid process of the axis. The upper border gives attachment to the anterior atlanto-occipital membrane, and the lower to the anterior atlanto-axial ligament.

The posterior arch is serially homologous with the laminae of other vertebrae. It springs at either side from the back part of a lateral

Fig. 82.— The Atlas (Superior View).

The vertebrarterial groove on this bone was converted into a foramen on

both sides.

mass, from which it sweeps backwards and inwards. The part close to the lateral mass at either side is flattened from above downwards. It presents on its upper surface a shallow depression, called the vertebrarterial groove, which lodges the vertebral artery and suboccipital nerve. This groove is sometimes converted into a foramen on one or both sides by a spiculum of bone extending from the back part of the superior articular process to the posterior arch behind the groove. The vertebrarterial groove is serially homologous with the superior vertebral notch of other vertebrae, but, unlike them, it lies behind the superior articular process. The inferior surface of the posterior arch, behind each lateral mass, presents a shallow vertebral notch, which lies behind the inferior articular process. The centre of the posterior arch presents the posterior tubercle, which is the only representative of a spinous process. At either side of this tubercle the rectus capitis posterior minor arises. The upper aspect of the posterior arch give attachment to the posterior atlanto-occipital membrane, and the lowe aspect of the posterior atlanto-axial ligament.

The lateral masses support the superior and inferior articula processes, and laterally the transverse and costal processes spring from them. The anterior surface of each gives partial origin to th< rectus capitis anterior. The medial surface of each presents anteriorl} a tubercle for the transverse ligament. The superior articular processe . are oval and deeply concave, to articulate with the condyles of the occipital bone. Their long axes are directed backwards and outwards so that they converge in front and diverge behind. Anteriorly the} reach as far as the anterior arch, and posteriorly they overhang th( vertebrarterial grooves on the posterior arch to a slight extent, but the} do not extend farther back than about the centre of the ring. The plane

Anterior Arch

of each is sloped downwards and inwards, and the direction of the surface is upwards and inwards. The movement between them and the occipital condyles is one of flexion and extension, or nodding. Sometimes one or both of them may be divided by a groove into two circular facets. The inferior articular processes are circular and almost flat, the plane of each is sloped upwards and inwards, and the direction of the surface is downwards and inwards. They articulate with the superior articular processes of the axis, and the movement allowed is rotation. The articular processes of the atlas, being placed in front of the points of exit of the spinal nerves, do not correspond in position wuh the articular processes of succeeding vertebrae (with the exception

° ± !k e 1 SUperi0r pa ^ r ax * s )- They occupy a position corresponding

wi h the pedicular portions of the bodies of vertebrae, and in this way the superincumbent weight is transmitted to the vertebral bodies.

ie f ran §verse and costal processes spring from the side of each

lateral mass, and, lateral to the costo-transverse foramen, the costotransverse lamella and the anterior and posterior tubercles are more or less fused into one long irregular mass, though the posterior tubercle usually remains conspicuous. The upper surface of this mass at its front part gives origin to the rectus capitis lateralis, and at its back part to the obliquus capitis superior, whilst the lower surface at its back part gives insertion to the obliquus capitis inferior. The foramen transversarium is of large size in order to guard against the vertebral artery being compressed during the rotatory movements of the bone upon the axis.

The ring of the atlas, in the recent state, is divided into two compartments by the transverse ligament. The anterior small division is called the odontoid compartment, and it lodges the odontoid process of the axis. The posterior large division represents the vertebral foramen of other vertebrae, and it lodges the spinal cord with its membranes.

Varieties. — (1) The posterior arch may be incomplete at the centre, the deficiency being bridged over by fibrous tissue. (2) The costal process may be incomplete, the deficiency in the foramen transversarium being filled by fibrous tissue. (3) There is sometimes an additional small foramen on either side, a little behind the foramen transversarium, for the passage of the suboccipital radicles of the vertebral plexus of veins. (4) The groove for the vertebral artery may be converted into a foramen by a bridge of bone, as in the specimen from which Fig. 82 was drawn.

Atlantal Facet

Odontoid Process

Groove for Transverse Ligament

Superior Articular Process

Superior hotch

The Axis. — The axis is the second cervical vertebra, and is so named because its odontoid process, which is the distinctive character of the bone, forms a pivot on which the atlas, supporting the head, rotates. From the presence of this process the axis is sometimes called the vertebra dentata.

The odontoid process ( processus dentatus) springs from the superior surface of the body, and represents the body of the atlas. It is constricted and somewhat circular close to the body, this part being called the neck. Above this it expands into a head, which tapers off at either side by two sloping surfaces, forming by their convergence an antero-posterior ridge, known as the summit. The anterior surface

presents a circular convex facet, called the atlantal facet, for articulation with the odontoid facet on the posterior surface of the anterior arch of the atlas. The posterior surface presents a shallow transverse groove,

Fig. 84. — The Axis (Lateral View).

for the play of the transverse ligament of the atlas. The lateral sloping surfaces on either side of the summit give attachment to the lateral odontoid ligaments, whilst the summit itself gives attachment to the middle odontoid ligament.

The superior surface of the body is occupied by the odontoid process and portions of the superior articular processes. The inferior surface differs from that of other cervical vertebrae only in the greater downward projection* of its anterior lip. The anterior surface presents a median vertical ridge which bifurcates interiorly into diverging lips, enclosing a small triangular surface. On either side of the median ridge the surface is depressed, and gives attachment to a portion of the longus cervicis muscle. The other surfaces of the body present nothing peculiar.

The pedicles are concealed above by the superior articular processes. Each, on its inferior aspect, presents a wide and deep inferior vertebral notch, which is placed in front of an inferior articular process. The superior vertebral notches, which are very shallow, are situated on the upper borders of the laminae, and, like those of the atlas, are placed behind the superior articular processes.

The laminae are massive, and give attachment by their upper borders to the posterior atlanto-axial ligaments, whilst their anterior surfaces, near the lower borders, give attachment to ligamenta flava, as in other vertebrae.

The spinous process is massive. Its direction is backwards, and it terminates in two strong tubercles, separated inferiorly by a triangular cleft. Each of these tubercles gives attachment to some of the deep fibres of the ligamentum nuchae, and to the following muscles from above downwards: the rectus capitis posterior major; the obliquus capitis inferior; and the highest portion of the semispinalis cervicis.

The superior articular processes are situated on the upper surface of the pedicle at either side, the upper surface of the costal process, and a portion of the superior surface of the body, upon which latter it encroaches very near to the odontoid process. The plane of each is sloped outwards and downwards. The surface is slightly , • , . . convex from before backwards

and. circular, its direction being upwards and outwards. They articuia e with the inferior articular processes of the atlas, and the movemen allowed is rotation. The inferior articular processes differ

Fig. 85. — The Axis (Superior View).

from those of most vertebrae only in being situated principally upon the lower borders of the laminae. The superior pair, like all four articular processes of the atlas, being placed in front of the points of exit of the spinal nerves, do not correspond in position with the superior articular processes of succeeding vertebrae, but occupy a position corresponding with the pedicular portions of the bodies of vertebrae.

The transverse processes are very short, and are directed outwards and downwards. Each terminates in a single tubercle.

The costal processes also terminate in tubercles, and the costotransverse lamellae are not grooved superiorly.

The foramen transversarium is directed upwards and outwards, the reason of this obliquity being as follows: when the atlas and axis are in position, each foramen transversarium in the atlas lies farther out than that in the axis. In order, therefore, to obviate any sudden and undue bend in the vertebral artery, the foramen in the axis is directed obliquely upwards and outwards so as to guide the vertebral artery gradually to the foramen in the atlas.

There is nothing peculiar about the vertebral foramen.

Varieties. —(1) The summit of the odontoid process may present a facet, indicating an articulation with the anterior margin of the foramen magnum of the occipital bone, which in such cases presents a prominence known as the middle occipital condyle. (2) The odontoid process may, in very rare cases, remain separate from the body, thus forming the os dentatum. (3) An odontoid process in two halves has been recorded.

The Seventh Cervical Vertebra. —The distinctive character of this vertebra is the great length of its spinous process, which is the only cervical spine that can readily be felt beneath the integument of the neck.

On account of this outstanding prominence the seventh cervical is known as the vertebra prominens.

The spinous process is directed straight backwards, and terminates in a single large tubercular eminence. The other characters of this vertebra to be noted are as follows: the antero - posterior measurement of the body exceeds that of other cervical vertebrae; the transverse process is massive and comparatively long; the posterior tubercle is very distinct, but the anterior is rudimentary, or wanting; the foramen transversarium is of small size, and does not

Fig. 86. — The Seventh Cervical Vertebra (Superior View).

The costal process of the left side was undeveloped in this vertebra.

transmit the vertebral vessels and vertebral sympathetic plexus. The vertebral vein, however, may pass through it.

Varieties. —(i) The costal process may remain separate from the transverse process, thus giving rise to a cervical rib. (2) The costal process may be wanting on one or both sides, in which cases there is no foramen transversarium.

It is to be noted that the sixth cervical vertebra is peculiar in the following respects: the spinous process, like that of the vertebra prominens, terminates in a single large tubercular eminence; and the tubercle of each costal process, known as the anterior tubercle, is of large size, and is called the carotid tubercle (of Chassaignac).

The difference in the arrangement of the articular processes of the atlas and axis to those of the other vertebrae will probably have fixed the reader’s attention, and he may have wondered how they managed to shift their position from the dorsal to the ventral aspect of the nerveroots. To show the advantage of such a change in transmitting the weight of the head directly on to the bodies of the lower vertebrae is no explanation of how it was brought about; but a little examination of the bones will convince him that these so-called articular processes of the atlas and upper surface of the axis are not the buffer-like articular processes or zygapophyses of the lower vertebrae, but modified and enlarged neuro-central lips, and that the true zygapophyses are suppressed, since there is no further need for them.

The Thoracic Vertebrae

The thoracic vertebrae are twelve in number, and their distinctive character is the presence of one or more facets on either side of the bodies for articulation with the heads of ribs. The first, tenth, eleventh, and twelfth (sometimes also the ninth) are peculiar, and require separate descriptions.

A Typical Thoracic Vertebra. —The body is larger than that of a cervical vertebra, but smaller than that of a lumbar. When viewed ^ ove or t >e l° w if is heart-shaped, being broad and hollowed out an< ^ narrow anc i rounded off in front. The posterior depth of the body exceeds the anterior in adaptation to the backward curve of t le vertebral column in the thoracic region. The superior and inferior surfaces present a raised rim round the circumference, due to the original epiphysial plate, and this renders the whole of each surface s .lghtly concave from the periphery towards the centre. The anterior and lateral surfaces merge gradually into each other, and are concave rom above downwards, the entire antero-lateral surface being convex trom side to side and pierced by numerous nutrient foramina. Each ? era surface, close to the vertebral arch, presents two articular emi- acets, superior and inferior, of which the superior is the larger, and is situated upon the pedicular portion of the body, the inferior sma er one emg just in front of the lower part of the inferior vertebral notch. These derm-facets are for articulation with the heads of the an are called the costo-capitular facets. When two vertebrae are in position, the superior demi-facet of the lower vertebra and the inferior demi-facet of the upper form an articular cavity for the head of a rib. The posterior surface of the body is concave from side to

Fig. 88.—The Sixth Thoracic Vertebra (Superior View).

side, and presents nutrient foramina, as in the cervical vertebras. The superior and inferior surfaces are related to the intervertebral discs, and the anterior and posterior surfaces are related to the anterior and posterior longitudinal ligaments.

The pedicles spring from either lateral extremity of the posterior surface of the body, and their upper borders are very nearly on a level with its superior surface. Each pedicle is laterally compressed, and is directed backwards and slightly outwards. The superior vertebral notches are shallow, and each is usually bounded in front by a transverse neuro-central lip. The inferior vertebral notches are deep and wide.

The laminae are short, deep, and compressed from before backwards, their planes being sloped downwards and backwards. The markings for the ligamenta flava are the same as in cervical vertebrae.

The spinous process is elongated and three-sided. Its direction is downwards and slightly backwards, and it terminates in a sloping border ending below in a sharp point. The spinous processes of the middle thoracic vertebrae are imbricated or overlapping, and therefore very vertical.

The articular processes are nearly circular, their surfaces are flat, and their planes are almost vertical. The superior pair project upwards from the junction between the pedicles and laminae, and they look backwards and slightly upwards and outwards. The inferior pair are placed on the anterior surfaces of the laminae, and they look forwards and slightly downwards and inwards.

The transverse processes spring from the junction of the pedicles and laminae, and each is directed outwards and backwards. They are long and club-shaped, being somewhat constricted at their bases, but expanding into knob-like enlargements at their extremities. The anterior surface of the extremity of each presents a circular facet, called the costo-tubercular facet, for articulation with the tubercle of a rib. These facets are often useful in determining the position of the vertebras in the series, because, in the higher ones, they are concave and in front of the transverse process; while, in the lower, they become flat and rise to the upper surface, so that the processes support the lower ribs from below. The posterior surface of the extremity gives attachment to the posterior costo-transverse ligament. The anterior surface of the transverse process faces the posterior surface of the neck of a rib, and gives attachment to the ligament of the neck of the rib. This region corresponds with the costo-transverse foramen in a cervical vertebra. The lower border of the transverse process gives attachment to the superior costo-transverse ligament, which connects it with the crest, or upper border of che neck, of the rib below. The transverse process is serially homologous with a cervical transverse process.

The vertebral foramen is almost circular, and is of smaller size than m the cervical or lumbar vertebrae, since this is the least movable region of the whole column.

Peculiar Thoracic Vertebrae. —These are the first, tenth, eleventh, and twelfth (sometimes also the ninth).

The First Thoracic Vertebra.— The bodies of this and the next vertebra closely resemble those of the cervical region in shape, and °i n t ° f the neuro-central lips are evident. No mistake

s rou d be possible, however, as the characteristic points of all thoracic

vertebras, the facets for the ribs, are present on the sides of the bodies, and in the case of the first there is a whole facet above instead of part of one, since the seventh cervical takes no share in supporting the first rib.

In other respects the first thoracic vertebra is typical.

Fig. 89. — The Peculiar Thoracic Vertebrae and the First Lumbar Vertebra.

The Tenth Thoracic Vertebra.—This vertebra has usually one entire facet on either side, mainly on the pedicle, for the head of the tenth rib. This facet, however, may only be a three-quarter facet, if the ninth thoracic vertebra is normal. It has, usually, a costo-tubercular facet on the extremity of each transverse process for the tubercle of the tenth rib. This facet may be wanting, though its place will be indicated by a roughness. The body and spinous process of this vertebra show indications of the lumbar type, its other characters being thoracic.

The Eleventh Thoracic Vertebra. — This vertebra has an entire facet on the outer surface of each pedicle for the head of the eleventh rib, but there is no facet on the transverse process, which has become short and stunted, the tendency to the club shape being, however, still perceptible. The lumbar type of the bone is more pronounced than in the case of the tenth.

The Twelfth Thoracic Vertebra, like the tenth and eleventh, has a single facet for the head of the rib, but it is usually ragged and irregular instead of being clean-cut and circular; it is also lower down, so that a horizontal line drawn round the centre of the body cuts the facet in the twelfth, but passes below it in the eleventh.

The transverse processes are more set back and have no articular facet. They are trifid, showing three tubercles: a superior one corresponding to the mammillary process of the lumbar vertebrae, a lateral corresponding to the transverse or costal process, and an inferior corresponding to the accessory lumbar process. A distinctive feature is that the inferior articular processes have their facets facing outwards, thus adapting them to the superior articular facets of the lumbar series, which look inwards.

All these points need attention, as any of them may occur in the eleventh as well, and with the greatest care it is sometimes very difficult to distinguish between an eleventh and a twelfth thoracic vertebra.

Individual Thoracic Vertebrae. — It is usually possible to determine the approximate position of any thoracic vertebra by attention to details. The first two have cervical-shaped bodies, while in the third, fourth, and fifth the bodies are very narrow and heart-shaped. After that they rapidly broaden, until the lower ones are as broad from side to side as they are from before backward.

In the upper and lower parts of the series the spines are fairly horizontal, but in the middle, from the fifth to the eighth, where the backbone is least movable, the spines are much more vertical and imbricated.

The position and appearance of the facet on the transverse process, already drawn attention to, also helps materially in distinguishing a high from a low

The Lumbar Vertebrae

The lumbar vertebrae are five in number, and are so named because they occupy the region of the loins. They are the largest of the true vertebrae, and their negative characters are the absence of a foramen ransversarium in the transverse process, and the absence of any kind o, cos hd facet on the side of the body. They increase in size from a ove ownwards, the fifth being the largest, but, as this vertebra has certain distinctive characters, it will be separately described.

Typical Lumbar Vertebra. — The body, when viewed from above 01 e ow, is 1 ney-shaped, being flattened from above downwards, convex transversely over its antero-lateral surface, and slightly concave transversely on its posterior surface. It is wider from side to side than from before backwards. The anterior depth is slightly greater than the posterior, in adaptation to the forward curve of the vertebral column in the lumbar region. There is no facet on either side of the body.

The pedicles are short, strong, and directed backwards. The superior vertebral notches are shallow, the inferior being deep and wide.

The laminae are short, thick, and deep, and their planes are almost vertical.

The spinous process is axe-shaped, its direction being horizontal, and it terminates in a blunt border.

Fig. 90. — The Third Lumbar Vertebra (Superior View).

The articular processes are vertically elongated. The superior pair project upwards from the junction of the pedicles and laminae, and the inferior pair project downwards from the lower borders of the laminae. The superior pair are concave, their planes being vertical, and their direction being inwards and backwards, so that they almost face each other. They stand wide apart, so as to embrace the inferior articular processes of the vertebra above. On the posterior border of each there is a nipple-shaped projection directed backwards and slightly upwards, called the mammillary process {metapophysis) , which corresponds with the superior tubercle of the lower thoracic transverse processes. The inferior articular processes are convex, their planes being vertical, and their direction being outwards and forwards, so that they look away from each other. They are nearer to each other than the superior pair, and are received between the superior pair of the vertebra below.

The transverse processes are comparatively slender, except in the case of the fifth; they are directed outwards and slightly backwards, and they increase in length from the first to the fourth. Each is spatula-shaped, being compressed from before backwards, and terminates in a short round border. It represents the vertebral portion of a rib, and therefore constitutes the costal element of the vertebra. Situated on the posterior aspect of the base of the transverse process, just external to and. below the lower border of the superior articular process, there is a small sharp projection directed downwards, called the

Fig. 91. — The Fifth Lumbar Vertebra (Superior View).

accessory process [anapophysis ), which is the rudiment of the true transverse process. In the case of the fourth and fifth lumbar vertebrae the transverse process becomes shifted on to the body as well as the pedicle.

The lumbar transverse processes (costal elements) of man are serially homologous with the ribs, and also, in the case of the lower thoracic vertebrae, with le anterior tubercles of the transverse process. In the lumbar region each ransverse process has fused with the accessory process (true transverse process), and consequently the foramen transversarium in the transverse process of a cervical vertebra has disappeared.

The vertebral foramen is larger than in the thoracic vertebrae, but angles cerv ^ ca ^ its shape being triangular with rounded

The Fifth Lumbar Vertebra.— The distinctive characters of this vertebra are as follows: (1) it is the most massive of all the lumbar vertebrae, (2) the greater depth of the body in front is more conspicuous than in the others; (3) the transverse processes are massive, conical, and directed definitely upward; and (4) the inferior articular processes are wide apart.

It will be readily understood that attempts at all these characteristics may occur in the fourth lumbar vertebra, though the marked increase m depth of the anterior surface of the body of the fifth is seldom reached in the fourth.

Ossification of the True Vertebrae.

Each true vertebra ossifies in cartilage from three primary, and five secondary, centres. One primary centre is for the principal part of the body, and two are lor the vertebral arch and its processes, including also a small portion of the body at either side adjacent to the pedicle. The centres for the vertebral arch appear about the seventh week of intra-uterine life at the junction of the pedicles and

Fig. 92. — Ossification of the True Vertebra?.

A, cervical vertebra at the third month; B, cervical vertebra at birth;

C, thoracic vertebra at birth.

laminae, and from these ossification invades the neural arch, with its processes, and the adjacent portions of the body. The centre for the principal part of the body appears about the eighth week in the portion of cartilage dorsal to the notochord. Presumably this nucleus is primitively bilateral, but the two unite very quickly to form a dumb-bell-like ossification. When the two fail to unite the body ossifies in two separate parts, or, if one nucleus should be arrested, only one-half of the body ossifies (Turner). At birth a vertebra is composed of three osseous parts, connected by cartilage—namely, the principal part of the body, and the two halves of the vertebral arch, each bearing a small portion of the body. The laminae unite behind in the first year, except in the axis, where the union is delayed until the fourth year, and the vertebral arch joins the body in the third year. The cartilaginous union between the neural arch and the body at either side is called the neuro-central synchondrosis. In the thoracic vertebras the superior costal demi-facets lie behind this, and so they are shown to be placed on the pedicular portion of the body. All vestiges of this synchondrosis have disappeared prior to the sixth year. The secondary centres, five in number, appear about the sixteenth year, and they are consolidated about the twenty-fifth year. One appears at the extremity of the spinous process, one at the extremity of each transverse process, and the other two take the form of epiphysial plates, one on the upper surface and the other on the under surface of the body. In the case of the seventh cervical vertebra, and sometimes one

Fig. 93. — Lumbar Vertebra, showing the Epiphyses. A, the body; B, the neural arch.

or two above it, the costal process has a special centre which appears before birth, and it may develop into a cervical rib, a condition often needing operative interference later on. The transverse process (costal element) of the first lumbar has occasionally a special centre, and in these cases it may be developed into a lumbar rib. The lumbar mammillary processes are ossified from special secondary centres. The fifth lumbar has sometimes four centres for the vertebral arch, two at either side, one of which is for the pedicle, transverse process, and superior articular process, and the other for the lamina, inferior articular process, and one-half of the spinous process. These parts may fail to unite, in which cases the neural arch presents a synchondrosis on either side, situated between the superior and inferior articular processes (Turner). Sometimes the lamime of the fifth lumbar fail to unite, and so a space is left, bridged over by fibrous tissue.

The Atlas— The atlas has three centres of ossification, two for the lateral masses and posterior arch, appearing in the seventh week of intra-uterine life,

and one (sometimes two) for the anterior arch, which does not appear until the first year. The two halves of the vertebral arch usually join towards the end of the third year, there being sometimes a special osseous deposit at the place of junction. The two halves, however, may remain separate throughout life, the interval being bridged over by fibrous tissue.

There is no doubt that the odontoid

Appears in 1st Year

' Appears in 7 th Week (intrauterine)

Fi(j. 94* Ossification of the Atlas.

bodv of the atlas and fLot- +1 , . P rocess (dens) of the axis is the separated

more ventral Later on wh 16 aiTtesrioi* arch of the atlas represents something

the middle fasciculus of the^diafe been studied, it will be seen that

n radiate ligament runs to the ventral surface of the

intervertebral disc, and in some animals meets its fellow of the opposite side so as to form a fibrous bar, which, since it lies below, in the pronograde position, the notochord, is named the hypochordal bar. In many ruminants another ligamentous bar joins the heads of the ribs above or dorsal to the notochord, and so is named the hyperchordal bar. Here we evidently have the morphological explanation, not only of the anterior arch of the atlas, which is the hypochordal bar chondrified and ossified, but of the transverse ligament as well, which, obviously, is the hyperchordal bar, and it only remains to point out that our chief and most convincing argument for regarding the odontoid process as the body of the atlas is that the notochord passes right through it and is continued iip to the skull as the middle check ligament.

The Axis. —Excluding the odontoid process, the axis has three primary centres, like an ordinary vertebra, two for the vertebral arch appearing about the seventh week, and one (sometimes two) appearing in the lower part of the common cartilage of the body and odontoid process in the fourth month. In the upper part of this common cartilage two centres, laterally disposed, appear in the fifth month for the odontoid process, and these unite into one centre about the sixth month. At birth the axis is composed of four osseous parts connected by cartilage—namely, a body, an odontoid process surmounted by cartilage, and two halves of the vertebral arch. The odontoid process joins the body about the fourth year. The two halves of the vertebral arch join each other,

Fig. 95. — Ossification of the Axis.

A, at the fifth month; B, at the fourth year.

and the arch joins the body, in the fourth year. The apical part of the odontoid process has a special centre appearing in the fourth year, and it joins the rest of the process in the twelfth year. The body of the axis has the usual epiphysial plate on the under surface of the body, but there is no such plate on the upper surface. The union between the odontoid process and the body is indicated by a small disc-shaped cavity, centrally placed and persisting until advanced life.

The soft tissue in this cavity, no doubt, represents the intervertebral disc between the axis and the real body of the atlas, and it is situated a good deal lower than the constricted neck of the odontoid process. In many reptiles and in that reptile-like mammal, the ornithorhynchus or duck-bill, the process is separate throughout life, and the separation occurs just where the rudimentary disc is found in man. Again, the double centres for the odontoid are quite in keeping with what would be expected in the body of a vertebra, and the one for the tip is often looked upon as the upper disc-like epiphysis, though its early appearance suggests the possibility of its representing some still more anterior body or pro-atlas.

The Fixed Vertebrae.

The fixed vertebrae are usually nine in number, the upper five of which form the sacrum, and the lower four the coccyx.

The Sacrum.

The sacrum lies below the fifth lumbar vertebra, and is so arranged that in the erect position it forms the roof and part of the posterior wall of the pelvis. It is important to regard this cavity not as the lower part of the abdomen, but as a cave leading backward and slightly downwards from the lower part of the hinder abdominal wall. The sacral vertebrae diminish in size from above downwards, which renders the bone triangular, the base being upwards and forwards.

Fig. 96. — The Sacrum (Anterior View).

The ventral or pelvic surface, which is directed downwards and forwards, is concave from above downwards, and slightly from side to side. It presents along the centre a solid mass, representing the ankylosed bodies and ossified intervertebral discs, which is marked by foui transverse ridges situated at the places of junction. Superiorly it presents a projecting lip, called the promontory . On either side t ere . 15 f d r0 . w anterior sacral foramina , four in number, which < lmimsh m size from above downwards, and are directed outwards and forwards from the intervertebral foramina, by means of which they communicate with the sacral canal. They transmit the anterior primary divisions of the first four sacral nerves.

The lateral masses are situated lateral to the anterior sacral foramina at either side, and each is marked anteriorly by four transverse grooves, which prolong outwards the foramina and lodge the transmitted nerves. The pyriformis muscle arises from the front of each lateral mass by three slips, which are interposed between, and lie lateral to, the

Fig. 97. —The Sacrum (Posterior View).

foramina. The lateral masses are formed by the fusion of the pedicles, transverse processes, and costal elements of the sacral vertebrae.

The dorsal surface, which is directed upwards and backwards, is irregularly convex and narrower than the ventral. In the middle line it presents four eminences, which may be distinct, or fused to form a ridge, representing the spinous processes of the upper four sacral vertebrae. The spinous process of the fifth vertebra is absent, the development of its laminae having been arrested; and there is thus left a triangular opening, which is the outlet of the sacral canal, to be presently described. On either side of the median row of spines there is a solid mass formed by the ankylosed laminae, which forms the sacral groove for the origin of a portion of the multifidus. Lateral to this groove there is a row of foramina, four in number at either side, called the posterior sacral foramina, which are smaller than the anterior, and, like them, diminish in size from above downwards. These foramina open outwards and backwards from the intervertebral foramina (by which they communicate with the sacral canal), and transmit the posterior primary divisions of the first four sacral nerves. It is to be noted that they he directly behind the anterior foramina. Medial to the posterior foramina, and encroaching upon them, there is a row of small projections which represent the articular processes of the sacral vertebrae. The lower pair, which belong to the fifth sacral vertebra, are piolonged downwards as two plates which end in enlargements.

hese are called the sacral cornua, and they are connected with the cornua of the first coccygeal vertebra, usually by ligaments, but somelmes by osseous union. The interval thus bridged over at either side represents a fifth intervertebral foramen, through which the fifth sacral nerve passes. The solid portion external to the posterior oramina at either side is the lateral mass, and it presents a row of four tubercles, each of which is situated lateral to a posterior foramen. These represent the transverse processes of the lower four sacral vertebrae. The boundaries of the triangular outlet of the sacral canal are the spine of the fourth sacral vertebra above, and the imperfect laminae of the fifth sacral and the sacral cornua at either side. It transmits the fifth pair of sacral nerves and the two coccygeal nerves.

The lateral surface is broad above and narrow below. The upper part is divided into two portions—articular and non-articular. The articular division, anterior in position, is covered by cartilage, and is shaped like an ear, on which account it is called the auricular surface. It articulates with the iliac portion of the innominate bone, and extends at least over the first two sacral vertebrae. The non-articrdar division, posterior in position, is rough and irregular for the attachment of the interosseous sacro-iliac ligaments, and it is known as the ligamentous surface. The lower part of the lateral surface corresponds

Fig. 99. — The Base of the Sacrum.

with at least the lower two sacral vertebrae, and may include more or less of the third. It gives attachment to fibres of the sacrotuberous and sacrospinous ligaments and a portion of the coccygeus muscle, whilst the adjacent portion of the posterior aspect gives origin to fibres of the gluteus maximus. Interiorly the lower part is thinned away to a mere margin, and presents a process, called the inferior lateral angle. Below this is the transverse process of the first coccygeal vertebra when that is in position, a notch being thus formed on the side of the fifth sacral vertebra. The inferior lateral angle inclines towards the coccygeal transverse process, with which it is usually connected by a ligament, though in some cases the two processes become ankylosed. There is thus constructed a fifth anterior sacral foramen at either side for the passage of the anterior primary division of the fifth sacral nerve.

The base presents a central and two lateral divisions. The central division corresponds in its characters with the superior surface of a lumbar vertebra. Each superior vertebral notch lodges a fifth lumbar spinal nerve, and the superior articular processes stand wide apart. The lateral divisions of the base are called the alee. Each ala is triangular with the apex directed backwards. The alar surface is depressed, concave from side to side, and convex from behind forwards. It gives attachment to fibres of the iliacus, the lateral lumbo-sacral and anterior sacro-iliac ligaments, and it supports the lumbo-sacral nervous cord and the internal iliac vessels. The ala is formed by the fusion of the pedicle, transverse process, and costal element of the first sacral vertebra.

The apex is transversely oval, and articulates with the first coccygeal vertebra, with the intervention of an intervertebral disc until advanced life, when ankylosis takes place.

The sacral canal is situated behind the bodies of the first four sacral vertebrae, as a rule. It is triangular in the upper part, but somewhat crescentic below. It is closed in front by the ankylosed bodies, and behind by the ankylosed laminae. Along each side it presents four intervertebral foramina for the passage of nerves. These are bounded laterally by the lateral mass, but each opens on the ventral and dorsal surfaces by the anterior and posterior sacral foramina, which represent the limbs of a capital V, the apex of which corresponds with an intervertebral foramen. The superior aperture or inlet is large, triangular, and wide transversely. It represents the vertebral foramen of the first sacral vertebra. The inferior aperture or outlet is comparatively small and somewhat triangular, and is usually situated on the back of the body of the fifth sacral vertebra. It is bounded above by the tubercle which represents the spinous process of the fourth sacral vertebra, and on either side by (1) the imperfectly developed lamina of the fifth sacral vertebra, and (2) the sacral cornu. The outlet transmits the fifth pair of sacral nerves and the two coccygeal nerves. The contents of the canal are the sacral and coccygeal nerves, and the filum terminale of the spinal cord.

The sacrum derives its blood-supply from the lateral sacral and middle sacral arteries.

Articulations .—Superiorly with the fifth lumbar vertebra, infer iorly with the coccyx, and at either side with the innominate bone.

Varieties. (1) Lhe number of sacral segments may be six, or more rarely lour. Increase in the number is usually due to the incorporation of the first coccygeal vertebra, or sometimes the fifth lumbar. The decrease may be due to the fifth sacral vertebra forming a part of the coccyx, or to the first sacral ormmg a sixth lumbar. (2) The bodies of the first and second sacral vertebrae may remain permanently separate, though ankylosis has taken place in all eir other parts. (3) The first sacral vertebra may be normal on one side, u on the other side it may remain separate from the second, and present the c laracters of a fifth lumbar. (4) The number of sacral spines may be reduced rom four to three, two, or one, or they may be entirely absent. As a consequence of this, the sacral canal, which usually opens on the back of the fifth ^ ac !T v . e^ra, ma y 4° so on the back of the fourth, third, second, or first, i-ii a , ln cases it may be entirely open posteriorly. (5) The sacrum is

c j c o much variety as regards the extent of its vertical curve.

Characters of the Female Sacrum.—In the female the sacrum is smoother, shorter, broader, less curved, and is set more backwards than in the male.

Another sexual distinction is that the facet for the last intervertebral disc occupies a much smaller proportion of the base in the female than in the male. In spite of these differences it is by no means always easy to distinguish the sex of the sacrum.

Appears about the 16th Year, and joins about the 25th Year

Appear about 1 the 18th YearJ and join about] the 25th Year.

Appear in the 3rd month .** of inra-uterinelife

Appear about the 6th Month

Appear after the 5th Monthj

Vertebral Arch

Ossification. —The sacrum ossifies in cartilage from thirty-five centres. Each segment has three primary centres, one for the body and two for the vertebral arch. The centre for the body appears in the third month of intrauterine life in the case of the first three, and after the fifth month in the last two. The centres for the vertebral arches appear about the sixth month. The vertebral arches join the bodies, in order from below upwards, from the second to the sixth year. The union of the laminae takes place from the eighth to the twelfth year. It, however, fails in the lowest, and sometimes in those higher up. The anterior parts of the lateral masses of the first three vertebrae, which represent the costal elements, have separate centres, which appear about the sixth month. These join the vertebral arches before uniting with the bodies, the latter union taking place rather later than the union between the vertebral arches and the bodies. Each vertebra has two annular circumferential epiphysial plates, superior and inferior, which begin to ossify about the sixteenth year.

On each side of the sacrum there are two epiphyses, an upper for

the auricular surface, and a lower for the sharp edge below, which appear about the eighteenth year. Consolidation begins about the eighteenth year, and proceeds from below upwards, union taking place earlier between the segments of the lateral masses than between the bodies. In the latter case the ossification invades the intervertebral discs, but in the former it is direct union. The union is complete about the twenty-fifth year, at which period also the lateral epiphyses join the bone.

Fig. ioo. — Ossification of the Sacrum.

A, anterior view; B, first sacral vertebra in early life (superior view).

The Coccyx.

The coccyx is composed as a rule of four rudimentary vertebrae, and it lies below the apex of the sacrum, which constitutes its only articulation. The direction of the bone is downwards and forwards, and its elements diminish in size from above downwards. It is triangular.

The first coccygeal vertebra is compressed from before backwards, broad above, narrow below, concave in front, and convex behind. The superior and inferior surfaces are transversely oval, and the lateral borders are sloped downwards and inwards. Two processes project upwards from the dorsal surface at either side, called coccygeal cornua, which articulate with the sacral cornua, usually by ligaments, but sometimes directly. Each lateral border presents, superiorly, a projection, called the transverse process, which inclines towards the inferior lateral angle of the sacrum, and is usually connected to it by a ligament, which is sometimes ossified.


Levator Ani


Transverse Process f (rudimentary)


— Transverse Process (rudimentary)

The second coccygeal vertebra presents traces of transverse processes and cornua, whilst the third and fourth are reduced to mere nodules.

The muscular attachments of the coccyx are as follows: the gluteus maximus to the back of the upper three segments close to the lateral border; the sphincter ani externus to the tip; the posterior fibres of the levator ani and a portion of the coccygeus to the lateral border, the sacrotuberous and sacrospinous ligaments are partially attached to the lateral border of the first coccygeal vertebra.

The coccyx derives its blood-supply from the inferior lateral sacral and middle sacral arteries.

Varieties. — The number of coccygeal segments may be increased to five, due either to the addition of an extra nodule, or to incorporation of the fifth sacral segment. The number may be reduced to three, due either to incorporation of the first coccygeal segment with the sacrum, or to suppression of one of the nodules. It is often asymmetrical.

Ossification. — The coccygeal vertebrae are cartilaginous at birth. Each vertebra has one primary centre and two secondary centres, the first vertebra having an additional pair of secondary centres.

Primary Centres. — The primary centre for the first vertebra appears from the second to the fifth year ; that for the second vertebra from the sixth to the tenth year, and those for the third and fourth vertebrae about puberty.

Secondary Centres. — Each vertebra has two secondary centres for the epiphysial plates —one on the upper surface, and the other on the under surface of the rudimentary vertebra (centrum or body). In addition, the first coccygeal vertebra has two special secondary centres, one for each cornu.

Union of the four coccygeal vertebrae takes place from below upwards, as in the sacrum. It commences shortly after the eighteenth year, and is not completed until the thirtieth year or later, the last two coccygeal vertebrae to join being the first and second. In advanced life it is not uncommon to find the coccyx ankylosed to the sacrum, thus forming one sacro-coccygeal bone.

The Vertebral Column as a Whole.

The vertebral column supports the other parts of the skeleton, directly or indirectly. Its average length is about 28 inches in the male, and rather less in the female. When viewed from the front, four pyramids are seen. The first extends from the axis to the seventh cervical vertebra, its base being downwards. The second extends from the first to the fourth thoracic vertebra, its base being upwards. The third extends from the fifth thoracic to the fifth lumbar, its base being downwards. The fourth extends from the base of the sacrum to the tip of the coccyx. These pyramids are due to the differences in breadth of the bodies in different parts.

The column presents certain curves, which are arranged in two groups, antero-posterior and lateral. The antero-posterior group comprises four curves, named cervical, thoracic, lumbar, and pelvic. The cervical and lumbar curves have their convexities directed forwards, and the thoracic and pelvic curves have their convexities directed backwards. The lumbar and pelvic curves meet rather abruptly and form a projection, called the sacro-vertebral angle , which is estimated at 117 degrees in the male and 130 degrees in the female. The curves impart springiness or elasticity to the column, and so guard it against shock. The thoracic and pelvic curves are associated with the thoracic and pelvic cavities, the capacity of which they serve to increase. They appear in early foetal life, and are known as primary curves. The cervical and lumbar curves do not appear until after birth, and are known as secondary or compensatory curves. The primary curves are brought about by the greater depth posteriorly of the thoracic and sacral bodies, whilst the compensatory curves are largely due to the intervertebral discs, though in the lumbar region

FiG. 102 .— The Vertebral Column (Lateral View).

(The blue markings represent the facets on the bodies and transverse processes.)

Fig. 103. — The Vertebral Column (Posterior View).

the greater depth of the bodies anteriorly, especially in the fifth lumbar, must also be taken into account.

The lateral group comprises two curves. One is situated in the upper thoracic region, with its convexity directed towards the right side in right-handed persons, and it is to be regarded as due to the greater use made of the right arm. To compensate for this curve there is another slight curve in the upper lumbar region, with the convexity to the left.

When viewed anteriorly, the column presents the bodies, which form the pyramids already described. When viewed laterally, it presents the sides of the bodies, pedicles, intervertebral foramina, and articular and transverse processes. The intervertebral foramina are formed by the apposition of the superior and inferior vertebral notches of contiguous pedicles. They lead outwards from the spinal canal, and each transmits a spinal nerve. They increase in size from above downwards until the sacrum is reached, in which, though hidden at either side of the central mass, they diminish in size from above downwards. In this region each intervertebral foramen opens on the front and back of the sacrum by means of an anterior and posterior sacral foramen, the arrangement thus formed resembling a capital V, the apex being at an intervertebral foramen. On the lateral aspect of the thoracic portion of the column are seen the costocapitular facets, which are twelve in number. The first is situated on the upper part of the side of the first thoracic body. The second to the tenth inclusive are situated on the contiguous margins of the bodies of the vertebrae, each being formed by the small inferior costal facet of the upper body and the large superior costal facet of the lower. The eleventh and twelfth are situated on the sides of the corresponding pedicles. The tenth facet may belong entirely to the tenth thoracic vertebra. The thoracic transverse processes, except the eleventh and twelfth (and sometimes the tenth), are faceted in front at their extremities for the tubercles of the ribs.

When the column is viewed from behind the following parts are seen: the spinous processes; the laminae; the articular processes; the backs of the transverse processes; and the dorsum of the sacrum and coccyx. The cervical spines, except the sixth and seventh, are short, so as not to interfere with backward flexion or over-extension of the neck. The middle thoracic spines are imbricated, and the lumbar spines stand out horizontally. On either side of the spines there is the vertebral groove, which is occupied by the deep muscles of the back, the deepest being the multifidus. This groove is bounded medially by the spines, and laterally by the transverse processes in the cervical and thoracic regions, and by the mammillary tubercles in the lumbar region. The floor is formed by the laminae, and its continuation over the back of the sacrum is known as the sacral groove.

Notice how well the imbricated laminae protect the spinal canal in the cervical and thoracic region. Except between the occiput and atlas it would be extremely difficult to sever the spinal cord by a stab in the back; but in the lumbar region the spinal canal and cauda equina can be reached easily enough between the laminae, and this fact is taken advantage of in ‘ lumbar puncture ' to withdraw cerebro-spinal fluid or inject drugs.

The vertebral canal is situated behind the bodies of the vertebrae, and is formed by the vertebral foramina of all the vertebrae except the fifth sacral and four coccygeal. It commences at the level of the atlas, and it terminates as a rule upon the back of the body of the fifth sacral vertebra. It adapts itself to the various curves of the column, and is large and triangular in the cervical and lumbar regions, small and circular in the thoracic, and triangular in the upper part, but crescentic in the lower part, of the sacral region. It contains the spinal cord and its membranes as low as about the level of the disc between the first and second lumbar bodies, and a copious plexus of vessels. Below the level just mentioned it contains the filum terminate of the spinal cord and the leash of nerves known as the cauda equina, with their coverings. The dura-matral covering or theca comes to an end by being attached to the back of the second sacral body, and the filum terminate passes on to be attached to the back of the fifth sacral or first coccygeal vertebra.

It has been pointed out that in the lower sacral and coccygeal regions the vertebral canal is not enclosed by bone posteriorly. This normal failure to close in may sometimes extend much higher, thus allowing the membranes and cauda equina to bulge backward, a condition known as sacral spina bifida.

a. The Ribs.

The ribs (costae) are twelve in number at either side, and are ananged m two groups, true or sternal, and false or asternal. The true ribs are those which articulate directly with the sternum by their costal cartilages, and they represent as a rule the first seven at either side. I he false ribs are those which have no direct articulation by then* costal cartilages with the sternum, and they represent, as a rule, the last five at either side. The last two false ribs, eleventh an weltth, are called the free or floating ribs , because their costal cartilages stand quite clear of each other and of the tenth. The ribs are e as ic, and increase in length from the first to the seventh, whence hey decrease to the twelfth. The first is the broadest, and the twelfth

QliCTmf rr i° W( i S * J he A r ^^ rec ^ on is a f first downwards, outwards, and y ac wards, then downwards and forwards, and finally inwards.

nh nbs are n °t so oblique as those lower down, the most snrfarpQ nmt fi* With the exception of the first rib, the

, rA ° i 16 °^ bers are ver ^cally disposed posteriorly, but in front

renders most of themXisted. ^ f ° rWards ’ and this circumstance shaft, InfstirnalTtre^ty" 1 ^ ° f a head > neck ' tubercle ’

The head (capitulum) forms the posterior or vertebral extremity, and is slightly expanded. It presents an irregularly flat surface and an anterior margin. The surface is marked by two oblique facets, upper and lower, which are separated by a horizontal ridge.

The lower or primary facet is the larger of the two, and articulates with the large superior or primary costal facet of the lower of the two thoracic bodies with which the head is connected—that is to say, with the thoracic vertebra bearing the same number as itself. The upper facet articulates with the small inferior costal facet on the side of the upper thoracic body, and the intervening ridge gives attachment to the interarticular ligament. The anterior margin gives attachment to the radiate ligament, and the ridge, separating the two facets, to the interarticular ligament.

The neck is about 1 inch long, and is compressed from before backwards. Its anterior surface is smooth and covered by the costal pleura. Its posterior surface, which is rough, faces the anterior surface of the lower thoracic transverse process, or that with which its tubercle articulates, and it gives attachment to the ligament of the neck of the rib. Its superior border forms a sharp lip, called the crest, which gives attachment to the anterior costo-transverse ligament. Its inferior border may show traces of the subcostal groove.

The tubercle is situated on the lateral surface of the rib at the outer extremity of the neck, and presents two divisions, articular and non-articular. The articular division, inferior and slightly internal in position, presents a somewhat oval facet for articulation with that on the front of the extremity of the lower thoracic transverse process. This facet corresponds to the facet on the transverse process of its vertebra, already noticed. In the upper ribs it is convex and looks backward, while in the lower it is flat and looks more downward. I he non-articular division, superior and slightly external in position, gives attachment to the posterior costo-transverse ligament.

The shaft is curved and twisted. It has two surfaces and two borders. The external surface is convex, and its plane is vertical behind, but oblique in front, being here sloped downwards and forwards. Opposite the greatest bend of the rib, called the angle, it presents an oblique ridge, directed downwards and outwards, for a tendinous slip of the sacrospinalis and for the levator costarum. The surface between the tubercle and the angle gives attachment to the longissimus dorsi, and increases in length with each succeeding nb, in the first being absent, since the tubercle and angle coincide, m the sixth about ij inches long, and in the tenth about 2 inches.

ear the anterior extremity (about 2 inches from it) the external surface presents another oblique ridge directed downwards and outwards, known as the anterior angle, where the rib describes a slight curve. The internal surface , which is concave, is covered by the costal pleura. At its lower part it presents the subcostal groove, e ^u eSen ^ described. The superior border is thick and round behind, but thin and sharp in front. Its outer lip gives attachment o an external intercostal muscle, and its inner lip to an internal intercostal a collateral intercostal artery lying between the two musses, the inferior border is for the most part sharp and wiry. T 36 la e / k T 1 - j anc ^ a bove it there is the costal groove, which -T- T^md ^ the tubercle and disappears over the anterior inn^r iT°f teri ° rI ^ l he ? roove belongs to the inferior border. Its 1 P n;i rt f U +i < ^ eC ^ ani ^ S y es attachment to an internal intercostal uscle, whilst the outer hp gives attachment to an external intercostal. The nutrient foramen is situated in the costal groove a little anterior to the centre of the bone, and the canal to which it leads is directed towards the head. It gives passage to a branch of the intercostal artery which lies in the groove. The contents of the groove from above downwards are an intercostal vein, artery, and nerve, and it is important to notice that the outer lip of the groove descends, like a flange, so far that all these structures are protected by it.

The anterior or sternal extremity presents an oval pit, which is almost vertical in direction, for the costal cartilage.

The Peculiar Ribs.—These are the first, second, tenth, eleventh, and twelfth.

The First Rib. — This is the shortest, broadest, and flattest of all the ribs, and its curve is very distinct, but there is no twist. The head is small, and presents a nearly circular facet for articulation with the entire facet on the body of the first thoracic vertebra. The neck is narrow, and compressed from above downwards. The tubercle, which is large, is situated on the outer border at the junction of the neck with the shaft. Being placed opposite the greatest bend of the bone, it takes the place of the angle, and presents the usual articular and nonarticular portions, the former being for the first thoracic transverse process.

The shaft is broad and compressed from above downwards, its surfaces being superior and inferior, whilst the borders are inner and outer. The superior surface close to the anterior extremity gives attachment to the tendon of the subclavius and the costo-coracoid ligament. Farther back there are two oblique grooves, separated to a limited extent by a tubercle or spine for the scalenus anterior. The anterior groove is shallow, and lodges the subclavian vein, whilst the posterior deeper groove is occupied by the third part of the subclavian artery and the lower trunk of the brachial plexus of nerves. Behind the posterior groove, and extending as far back as the tubercle, there is a rough impression for the insertion of the scalenus medius. The inferior surface is flat and covered by the costal pleura. Near the outer border it gives attachment to the internal intercostal muscle of the first space. The internal border , which is thin and concave, gives attachment to Sibson’s fascia. Fully 1 inch from the anterior extremity this border presents a projection, called the scalene tubercle, for the insertion of the scalenus anterior. It encroaches slightly on the adjacent part of the superior surface, and is inclined backwards. The outer border is convex. It gives attachment to the external intercostal muscle of the first space, and a portion of the first serration of the serratus anterior at a point opposite the groove for the subclavian artery, where the outer border is often prominent. The anterior extremity presents the usual oval pit for the first costal cartilage, its direction being horizontal from before backwards. The first rib has no costal groove. In the case of an ill-marked first rib it is always possible to put it in position by laying it on the table. When the superior surface is uppermost the head will touch the table.

The Second Rib. — The surfaces of the shaft of this rib occupy a transitional plane between that of the first and those of the succeeding ribs. It is practically destitute of a twist. The neck is compressed from above downwards, and from before backwards. The distinctive character of the bone is the presence on its supero-external surface, near the centre, of a rough oval eminence for a portion of the first and the second slips of the serratus anterior. Behind this impression the surface gives insertion to the scalenus posterior.

Fig. 105. The First and Second Ribs of the Left Side (Superior View).

The Tenth Rib. — This bone may or may not be peculiar. If the body of the ninth thoracic vertebra has an inferior costal facet, there is nothing peculiar about the head of the tenth rib. If, however, the lower demi-facet is wanting on the ninth thoracic body, the head of the tenth rib has only one facet for that on the body and pedicle of the tenth thoracic vertebra. The tubercle has usually an articular facet for the tenth thoracic transverse process, but this may be wanting. I he angle and subcostal groove are well marked.

The Eleventh Rib. The head of this rib has one facet for that on the pedicle of the eleventh thoracic vertebra. There is a slight tubercle,

destitute of an articular facet, a faint angle, and a more or less wellmarked costal groove. The anterior extremity tapers, and is only tipped with a costal cartilage, which is free.

The Twelfth Rib. This is a very short bone. Its head has one facet for that on the pedicle of the twelfth thoracic vertebra. The tubercle, angle, and costal groove are wanting. The shaft is very narrow, and terminates anteriorly in a pointed extremity, which is merely tipped with a free costal cartilage. The lower border of the shaft has a rough, sharp outline, and gives attachment to a portion of

the quadratus lumborum muscle, whilst the upper border, especially towards the back part, is smooth and round, and has a rather definite knob just above the articular facet, which is rather markedly concave. In putting the bone in position, remember that the thorax is barrel-shaped and that the internal surface also looks upwards.

The ribs are supplied with blood by branches of the intercostal arteries.

Fig. 106. — The Eleventh and Twelfth Ribs of the Left Side (Inferior View).

Appear about the 16th Year, h and join about the 35th Year

Fig. 107. — Ossification of a Rib.

Structure. — A rib is composed of loose cancellated tissue surrounded by compact bone.

Varieties. — (1) The number may be increased to thirteen on one or both sides, and the supernumerar}^ rib may be cervical or lumbar. If cervical, it is developed in connection with the costal process of the seventh cervical vertebra. It may be free, may join the shaft of the first thoracic rib, or it may be attached to the sternum by ligament. If lumbar, it is developed in connection with the costal element of the first lumbar vertebra, is usually very short, and does not articulate with the body of that vertebra. (2) In rare cases the number may be decreased by one, at the expense of the twelfth rib. (3) The ribs are subject to variety in form as follows: (a) the vertebral end of the first thoracic rib may be joined by a cervical rib, or by the vertebral end of the second rib, in which cases the variety known as bicipital rib occurs; (b) the anterior extremity of a rib may be bifurcated; ( c ) adjacent ribs may be connected by small plates of bone.

Ossification. — An ordinary rib has one primary centre and three secondary centres. The primary centre for the shaft appears about the sixth week near the angle. Ossification proceeds so rapidly along the shaft that by the fourth month the shaft is completely ossified. The secondary centres appear about the sixteenth year. One gives rise to the head, and of the other two one is for the rough part of the tubercle and the other for its articular part. The secondary centres join between twenty and twenty-five. Non-articular centres absent below sixth rib. No tubercular centres at all in eleventh and twelfth.

The Costal Cartilages.

The costal cartilages, which are composed of hyaline cartilage, are twelve in number on either side. The outer extremity of each is received into the oval pit on the anterior extremity of a rib, and is there maintained in position by the continuity which takes place between the periosteum of the rib and the perichondrium of the cartilage. The inner extremities of the true ribs articulate with the side of the sternum by means of synovial joints, except in the case of the first, which is directly united to the presternum without the intervention of a synovial membrane. The eighth, as a rule, ninth, and tenth do not reach the sternum, and they articulate with each other by synovial joints, each cartilage being widened at the place of articulation, where it sends downwards a process to the upper border of the cartilage below. In this way interchondral joints are formed between these cartilages, as well as between the eighth, seventh, sixth, and sometimes the fifth. Ihe eleventh and twelfth cartilages are mere nodules tipping the corresponding ribs, and they have no articulation with each other, nor has the eleventh with the tenth. The cartilages increase in length from the first to the seventh, beyond which they gradually diminish to the twelfth. They diminish in breadth from above downwards. The direction of the first cartilage is inwards and downwards, and that of the second horizontally inwards, whilst the succeeding ones, except the eleventh and twelfth, incline more and more upwards as they pass inwards. Prior to middle life the first costal cartilage undergoes superficial ossification underneath the perichondrium, and so a thin shell of bone is formed around it. In advanced life this condition may be met with in the other costal cartilages to a certain extent.

C. The Sternum.

The sternum or breast-bone is situated in the middle line of the anteiioi wall of the thorax, where it articulates on either side with the first seven costal cartilages, and superiorly with the clavicle. It occupies an oblique plane, which is directed downwards and forwards, forming an angle with the vertical of about 20 degrees. It is compressed from before backwards, of unequal width at different parts, and more or less curved from above downwards, the convexity being directed forwards, and being very pronounced in the condition known as ‘ pigeon-breast/ The bone is originally composed of six segments, called sternebrse. The first sternebra forms the manubrium handle ’)

Fig. 108. — The Sternum (Anterior Surface).

or presternum. The succeeding four sternebrae form the body, gladiolus, or mesosternum, and the sixth sternebra forms the ensiform or xiphoid process [xiphisternum ), otherwise known as the metasternum.

The manubrium is irregularly four-sided, and broader above than below. It presents two surfaces and four borders. The anterior surface is convex from side to side, and concave from above downwards. It gives origin at either side to a portion of the pectoralis major, and at its upper and outer part to the sternal head of the sterno-mastoid. Between the latter point and the clavicular depression on the upper border it gives attachment to the anterior sterno-clavicular ligament.

The posterior surface is concave. At its upper and outer part it gives origin to portions of the sternohyoid and sterno-thyroid, the former being the higher of the two, and close to the clavicular depression it gives attachment to the posterior sterno-clavicular ligament. The superior border over its middle portion presents the suprasternal (interclavicular) notch , which gives attachment to fibres of the interclavicular ligament. At either side of this there is a large, oval, concavoconvex articular surface for the clavicle, which is directed upwards, outwards, and slightly backwards, an interarticular fibro-cartilage intervening between the bones. Close to the inner end of each clavicular depression there may be found, on the anterior aspect, an ossicle, known as the episternal bone. The inferior border , which is short and straight, articulates with the body, a disc of fibro-cartilage intervening. In this situation there is a transverse elevation, called the sternal angle (angle of Louis), which serves as a guide to the second rib at either side. Each lateral border slopes downwards and inwards. The upper part presents a triangular depression for the first costal cartilage, and close to the lower part a demi-facet for a portion of the second costal cartilage.

The body presents two surfaces and four borders. The anterior surface is marked by three transverse lines, which indicate the places of junction of the original four sternebrse. At either side it gives origin to a large portion of the pectoralis major. Th eposterior surface presents traces of the highest transverse line, but the lower two have usually become effaced. Adjacent to each lateral border it gives origin to a

Fig. 109. — The Sternum (Lateral View).

portion of the transversus thoracis, as high as the level of the third costal facet. Each lateral border presents a series of costal facets, disposed as follows: close to the upper extremity there is a demi-facet for a portion of the second costal cartilage; opposite each of the three transverse lines there is an entire facet for the third, fourth, and fifth costal cartilages; and on the side of the fourth segment of the body there are usually one entire facet and one demi-facet lying close together, the former being for the sixth costal cartilage, and the latter for a portion of the seventh. Altogether there are usually four entire facets and two demi-facets on each lateral border, the demi-facets being situated one at either extremity. Sometimes, however, the inferior demi-facet is replaced by an entire facet for the whole of the seventh costal cartilage. Each of the upper three entire facets is made up of the contiguous demi-facets of two adjacent sternebras, as in the bodies of most of the thoracic vertebrae. The superior border of the body articulates, as stated, with the manubrium. The inferior border, which is very narrow, articulates with the xiphoid process, an intersternebral disc intervening until about the fortieth year, when osseous union usually takes place.

The xiphoid process (or ensiform cartilage) is subject to much variety as regards condition, direction, and form. It may be entirely osseous, or osseous above and cartilaginous below. Its typical direction is downwards between the seventh pair of costal cartilages, but it may have an inclination forwards, backwards, or even to one side. It is narrow from side to side, and compressed from before backwards. It may terminate in a thin transverse border, in a sharp point, or in a bifurcated extremity. The anterior surface corresponds to the position of the infrasternal depression, and does not come up to the level of the anterior surface of the body. The posterior surface, which is flush with the posterior surface of the body, gives origin at either side to a portion of the transversus thoracis and interiorly it gives origin to a portion of the diaphragm, usually in the form of two fleshy slips. The superior border articulates with the body, and the inferior border gives attachment to the linea alba. Each lateral border usually presents superiorly a demi-facet for a portion of the seventh costal cartilage, but this may be transferred to the fourth segment of the body. In rare cases there may be an entire facet for the eighth costal cartilage, this being constant in early life. The lateral border gives insertion at either side to some of the fibres of the internal oblique aponeurosis, and occasionally, at its upper part, to a portion of the rectus abdominis.

The sternum derives its blood-supply from branches of the internal mammary artery.

Articulations. — With the clavicle and first seven costal cartilages, at either side.

Structure. — The sternum is composed of cancellous tissue covered by a thin layer of compact bone.

Varieties. — (i) The sternum is sometimes characterized by its shortness, breadth, and great depression in its lower part. This condition is liable to be met with in cobblers. (2) A sternal foramen may be present in the body, usually in the third or fourth segment. (3) A sternal foramen may be present in the" xiphoid process. (4) In very rare cases the sternum may be intersected from end to end by a sternal fissure, in which cases the heart and pericardium are left uncovered (ectopia cordis). (5) The costal cartilages may articulate with the sternum asymmetrically. (6) Sometimes the xiphoid process is bifid at its lower extremity.

The Sternum of the Female.—The bone is usually shorter than in the male, the shortness affecting the mesosternum.

Ossification. —The sternum ossifies in cartilage from a variable number of centres. There is usually one centre for the manubrium, which appears in the sixth month of intra-uterine life. Sometimes there are two, placed one

about the other, and there may be as many as six, placed thus . . . The

first segment of the body usually ossifies from one centre, appearing in the

Fig. 110. — Ossification of the Fig. iii.—Development of the SterSternum. num (Modified from Ruge).

seventh month, though there may be two, disposed laterally. The second, third, and fourth segments of the body usually ossify from two centres each, which are disposed laterally and remain separate for some time, but subsequently unite as a rule. There may, however, be only one medial centre for each of these segments. In the second segment they appear in the eighth month, in the third just before birth, and in the fourth during the first year. The xiphoid process ossifies from one centre, which appears in its upper part from the third to the sixth year, though it may be delayed to a later period. The lower three segments of the body unite in order from below upwards, the union commencing about puberty and being completed shortly afterwards. The first segment of the body joins the remainder about twenty five. The xiphoid process unites with the body about forty, but the manubiium usually remains permanently separate unless in advanced life, when it may become ankylosed to the mesosternum.

The sternal cartilage from which the bone is developed consists originally of two elongated strips, each of which bears the cartilages of nine ribs. The strips are separated for some time by a median fissure, but fusion subsequently takes place, and so a single sternal cartilage is formed. The eighth costal cartilage usually loses its connection with the sternum, though it may articulate

permanently with the xiphoid process. The ninth costal cartilage at either side is regarded as dividing into two parts, one of which remains connected with the sternal cartilage and forms with its fellow the xiphoid process, whilst the other acquires a connection with the eighth costal cartilage. If the parts of the ninth costal cartilages, which remain connected with the sternal cartilage, do not unite with each other over their whole extent, a bifurcated xiphoid process is the result. They usually, however, unite wholly, or sometimes in such a manner as to leave a foramen at the centre. A sternal fissure is due to the permanent separation of the two original cartilaginous strips, which, as a rule, unite to form the sternal cartilage. A sternal foramen in the second, third, or fourth segment of the body is due to ossification from two collateral centres failing to meet at the median line.

Sometimes two ossicles, called the suprasternal bones, are met with at either side of the interclavicular notch of the sternum. These are developed in connection with the suprasternal ligaments, which extend between the inner end of each clavicle and the upper end of the sternum. These supra- (or epi-) sternal ossicles probably represent the sternal extremities of cervical ribs.

The Thorax as a Whole.

The thorax constitutes an osseous and cartilaginous cage which lodges the heart and lungs, along with important bloodvessels and nerves, as well as the trachea and oesophagus. It is bounded anteriorly by the sternum, with the costal cartilages and anterior extremities of the first eight or nine ribs; posteriorly by the bodies of the thoracic vertebrae and the vertebral extremities of the ribs from the heads to the angles; and laterally by the ribs beyond their angles. It is barrelshaped, the truncated apex being directed upwards, and it is somewhat flattened from before backwards.

The superior aperture is bounded in front by the upper border of the presternum and the first costal cartilages, on either side by the first rib, and behind by the body of the first thoracic vertebra. Its transverse measurement exceeds the antero-posterior, and it is reniform, due to the forward projection of the first thoracic body. Its plane is oblique, being sloped downwards and forwards, so that the upper border of the presternum is on a level with the disc between the second and third thoracic bodies. The superior aperture transmits the following structures: the apical parts of the lungs and pleurae; the trachea and oesophagus; the vagus, sympathetic, and phrenic nerves; the terminal part of the innominate artery; the left common carotid and left subclavian arteries; and the right and left innominate veins. In early life it also transmits the thymus gland.

A fact which is seldom appreciated is that the anterior surface of the manubrium continues the plane of the upper aperture, and that perpendiculars dropped on the so-called anterior surface of the manubrium and so-called superior surfaces of the first ribs are parallel.

The inferior aperture is of large size, and is bounded posteriorly by the twelfth thoracic body, laterally by the twelfth rib at either side, and anteriorly by a line at either side, connecting the costal cartilages from the twelfth to the seventh inclusive. These two lines constitute the subcostal angle , within which the xiphoid process is situated. The inferior aperture is occupied by the diaphragm, which presents certain openings for the passage of important structures.

In sketching the thorax it is useful to notice that the distance between its lower limit and the apex of the subcostal angle is about equal to the length of the sternum.

Fig. ii2. — The Thorax (Anterior View).

ie cavity, on either side of the thoracic bodies, presents an elongated groove, called the pulmonary groove, which lodges the thick posterior border of a lung. The cavity has the following diameters— namely, vertical, transverse, and antero-posterior. The vertical diameter extends from the superior aperture to the inferior. The transverse diameter extends from the centre of a given intercostal space o ie centie of the corresponding space of the opposite side.

The antero-posterior diameter extends from the anterior to the posterior wall, and is necessarily of less extent in the median line than at either side, on account of the projection formed by the thoracic bodies, its increase at each side being due to the presence of the pulmonary groove. The cavity is increased in all these diameters during inspiration, and diminished during expiration.

The intercostal spaces are eleven in number at either side. They increase in length from the first to the fifth, and are occupied for the greater part of their extent by the external and internal intercostal muscles.

The thorax of the female is rather shorter than that of the male, and is not so much flattened from before backwards.

In early life the thorax is flattened from side to side, and its height is relatively less than in the adult.

For the notochord and the development of the vertebral column, ribs, and sternum, see p. 59.


The notochord forms the primitive basis of the axial skeleton, and around it the bodies of the vertebrae are developed. It is a solid cylindrical rod of cells, derived from the cephalic end of the primitive streak/and it occupies the median line, corresponding to the centres of the bodies of the future vertebrae. It lies along the ventral aspect of the neural tube, which constitutes the primitive tubular nervous system, and along the dorsal aspect of the archenteron, or primitive intestinal cavity. The cephalic end of the notochord is situated on the ventral aspect of the mid-brain, and corresponds to the posterior part of the pituitary region of the base of the future cranium. From this region it extends to the caudal end of the future axial skeleton, where it is continuous with the wall of the neurenteric canal. On either side of it there are the mesodermal somites.

The notochord is of temporary duration, and a considerable part of it is replaced by the bodies of the vertebrae. Certain vestiges of it, however, persist throughout life, these being represented by the central pulp of the in vertebral discs.

Development. — As already pointed out (p. 34 et seq.), the notochord is developed descriptively from the cell-layer which lines the roof of the cavity of the enteron. It is derived morphologically from the front wall of the neurenteric canal, ‘ paid in ’ to the growing roof of the enteron. A longitudinal thickening here, added to from behind as the embryo increases in length, forms a groove which deepens and makes the notochordal groove. This then closes off from the cavity of the enteron by apposition and fusion of its edges, and becomes the ‘ notochordal tube ’; the lumen of the tube opens caudally into the enteron and the lower part of the neurenteric canal, so that all the stages of formation of the chord are to be seen in embryos in which it is still being laid down. When the caudal growth comes to an end, and the number of somites is complete, the tube becomes closed completely. The lumen of the tube exists for a very short time; it is quickly obliterated by a certain amount of swelling of the cells, with some division, the notochord now becoming a solid cylindrical rod. It extends from the end of the tail-process to the original site of the bucco-pharyngeal

Fig. 113. — The Notochord (Cervical Region).

membrane—that is, to a point immediately behind the pituitary body. In other words, its extent corresponds with that of the roof of the enteron. It is for the most part surrounded by the skeletal structures of the vertebral column, but it lies below the occipital region of the basis cranii, in relation with the pharyngeal roof, with which it is connected, leading to the formation of a small recess here, the pouch of Luschka (pharyngeal bursa). It turns up in front of this, and enters the basi-sphenoid, where it terminates in the dorsum sellce.

Although descriptively derived from the roof of the enteron, there is reason to think that its origin should be classed morphologically with that of gastral mesoderm from the same region. This view of its origin is not to be taken as implying that it arises from mesoderm, or is a ‘ mesodermal structure ’ in the usual and restricted meaning of the term. The matter is of great morphological interest, and has been considered in the section dealing with general development; it has not at present any practical anatomical bearing.

Development of the Vertebral Column.

The notochord forms the axis round which the vertebral column is developed, and the formation of the notochord may be regarded as constituting the first or notochordal stage. The notochord and the neural tube, which lies along its dorsal aspect, becomes surrounded by mesenchyme (mesoderm), and this undergoes chondrification and subsequently ossification. As the process of ossification proceeds, a great part of the notochord becomes constricted at regular intervals, where the bodies of the vertebrae are undergoing ossification, and these portions of it ultimately disappear. The parts of it, however, round which the intervertebral discs are formed persist and constitute the central pulp of each disc, as stated.

In addition to the notochordal stage of development, there are three other stages—namely, membranous, cartilaginous, and osseous.

Membranous or Blastemal Stage. —As already described (p. 59 et seq.), the vertebrae are formed in the cell-mass composed of the anterior half of one sclerotome (Fig. 114) joined with the posterior half of the sclerotome in front of it. They are therefore inter segmental in position, and the intersegmental artery (a in the figure) thus comes to lie on the side of the vertebral body. The thick posterior subdivision of the sclerotome —which is therefore the anterior or upper part of the secondary and intersegmental vertebral formation —first comes into relation with the notochord and surrounds it, and then gives extensions dorsally and ventro-laterally; these pass between the adjacent margins of the original segmental somites which gave origin to the sclerotomes concerned, and are hence intersegmental. The dorsal process or vertebral arch is the blastemal rudiment of the vertebral arch. The ventro-lateral or costal process extends on each side toward the lateral sheet of mesoderm. The looser anterior subdivision of the sclerotome, or posterior part of the vertebral formation, also reaches the notochord and helps to cover it, and fills up the intervals between the dorsal processes. It

becomes inseparably associated with the posterior subdivision of the sclerotome m front of it.

The cartilaginous stage begins with the formation of a chondral centre in

e posterior part of each half of a vertebral formation—that is, in the anterior

su c lvisions of the more posterior of the original sclerotomes particularly con ' Centres also appear in the dorsal and ventro-lateral processes, thus

ung carti agmqus neural arches and primitive ribs. These cartilaginous

ex tendmg, become continuous with the cartilaginous body of the

0 ra ‘ . n cas .e °f the rib-process, however, the continuity is only tem nn pjj, a ^i .1 ^ ca ftilagmous rudiment of a rib becomes secondarily separate

frrr-n +u S1 ^ 0r ^ comes dito contact with a ‘ transverse process ’ projecting tiom the neural arch chondrification F F J 5

uol i ucu is omy distinctly formed i

processes grow towards one another, and here the so-called hypochordal bow or l in the upper cervical region, and it may be that the temporary fusion of the processes with the vertebral centra is associated with the want of definition of this structure at a lower level. However this may be, the hypochordal bar only persists entirely in the case of the atlas, where it forms the anterior arch. In the next few cervical vertebrae the bar

Fig. 114. — Transverse Section of Human Embryo during Fourth Week to show Sclerotomes.

Longitudinal horizontal section of same to show subdivision of same: a, intersegmental vessel; N, neural tube. The lower end of the section is the caudal end, and the notochord is divided longitudinally.

is formed more or less distinctly, but is secondarily lost in the upper and ventral aspect of the growing centrum.

The intervertebral discs are formed from the loose mesenchyme between the subdivisions of the sclerotomes. This tissue, although it increases in amount, does not compress the notochord, which enlarges within it and makes the nucleus pulp0 sms of the disc. In the case of the cartilaginous centra, however, their growth entails pressure on the notochord, leading to its atrophy and subsequent disappearance within them.

In the atlas the hypochordal bar forms the anterior arch, connected therefore with the rudimentary costal processes and, secondarily, with the vertebral arch. All these are derived from the anterior part of the vertebral formation, or posterior subdivision of the anterior sclerotome. The usual chondral centres of the centrum are developed in the posterior portion of the vertebral formation, however, but do not fuse with the anterior arch, but with the corresponding centrum of the next vertebra, making its odontoid process. At the other end of the column, also, modifications in the development are found, but these take the form of defects, in accordance with the defective development of the hinder portions of the column. The defects are mainly in the neural arch formations and need not be considered in detail.

The final or osseous stage of the vertebral formation has already been dealt with under the heading * Ossification ’ in the descriptions of the different parts of the column.

Development of the Ribs.

The primitive ribs, represented by the costal processes of the blastemal vertebrae, and of the cartilaginous stage, become the separate secondary ribs when the temporary chondral continuity is resolved. These secondary ribs, in the thoracic region, extend outwards above the broad pericardium and liver, and turn ventrally round these as they grow. In the middle of the second month their distal ends, though they have not as yet grown half-way round the body, are joined together by a blastemal sternal plate ; the upper ends of the two plates are connected by a mesenchymal condensation associated with the ends of the clavicles, and the lower end of each plate is placed indefinitely about the level of the eighth rib. The membranous ribs grow in length, chondrification extending somewhat later. The sternal plates meet over the pericardium, but the ribs over the liver do not close, their ends showing instead a tendency to fuse together away from the mid-ventral line. In the cervical region no true secondary ribs are formed, as the costal processes do not separate from the cartilaginous centra. Distinct chondral centres for the costal processes have been denied for the vertebrae above the seventh, but it is probable that they do appear normally, and join the centra quickly. In the lumbar and sacral regions there occurs also a rapid fusion of the distinct costal centres with the bodies and vertebral arches. In the lumbar type chondrification only affects the basal portions, the rest of the costal process of the blastemal stage remaining, and ossifying, in membrane. In the sacrum the distal ends of the costal processes enlarge and join to form a cartilaginous plate, which constitutes the ateral piece of the sacrum, this fusion does not occur in the lower sacral vertebrae, whose costal processes are reduced, and the reduction is carried farther m the first coccygeal formation.

Development of the Sternum.

The ventral extremities of the upper nine cartilaginous ribs, on either side, ecome expanded, and these portions unite. In this manner an elongated s rip o cartilage is formed on either side, each of which bears nine cartilaginous f hese strips are known as the hemisternal cartilages, and by their union Single Sternal cartilage is formed. This cartilaginous sternum undergoes ssl T ? a lon nn the manner described in connection with the sternum as a bone, •ii th and ninth cartilaginous ribs, on either side, lose their connection

i e emis ternal cartilages, and the portions of these hemisternal cartilages, and nimi 3 C ° 1 ^ 1 n ^^ e( ^ ^y the expanded ventral or anterior ends of the eighth

process (L C s i{orm S p n roces r s) bS ' gm> r ‘ Se ' by their Uni ° n ' t0 th ® Cartilaginous xi I> hoid


In exceptional cases ossification may take place in each hemisternal cartilage independently, and under these circumstances the hemisterna may remain permanently separate, thus giving rise to the extremely rare condition of sternal fissure.

The two halves of the cartilaginous metasternum usually unite along their entire extent. Their distal ends, however, may remain permanently separate, and so give rise to a bifurcated xiphoid process. Again, the two halves may unite in such a manner as to leave a permanent xiphisternal foramen. The presence of a sternal foramen is explained under Varieties of the Sternum.