case, separated to a slight extent from its neighbor by intercellular substance.

By one other repetition of the same process, each

cell producing four, five, or six
young cells, a cluster is formed,
containing from thirty to fifty
cells. These clusters lie in
immediate relation with the
punctum ossificationis; they
are oval in figure (about
of an inch in length by
in breadth), and elongated in
the direction of the long axis
of the bone. The cells com-
posing the cluster lie trans-
versely with regard to its axis.
In the first instance they are
closely compressed, but by
degrees are parted by a thin
layer of intercellular substance,
and each cluster is separated

from neighboring clusters by a broader layer (350 of an inch) of intercellular substance. Such are the changes which occur in cartilage preparatory to the formation of bone.

THE SAME CARTILAGE AT THE SEAT OF OSSIFICATION: the clusters of cells are arranged in columns; the intercellular spaces between the columns being of an inch in breadth. To the right of the figure osseous fibres are seen occupying the intercellular spaces, at first bounding the clusters laterally, then splitting them longitudinally, and encircling each separate cell. The greater opacity of the right-hand border is due to a threefold cause, namely, increase of osseous fibres, opacity of the contents of the cells, and multiplication of oilglobules. In the lower part of the figure some attempt has been made to show the texture of the cells.

Ossification is accomplished by the formation of very fine and delicate fibres within the intercellular substance: this process commences at the punctum ossificationis, and extends from that point through every part of the bone, in a longitudinal direction in long, and in a radiated direction in flat bones. Starting from the punctum ossificationis, the fibres embrace each cluster of cells, and then send branches between the individual cells of each group. In this manner the network, characteristic of bone, is formed, while the cells by their conjunction constitute the permanent areolae and Haversian canals. With a high magnifying power, the delicate ossific fibres here alluded to are seen themselves to be composed of minute cells having an elliptical form and central nuclei. These cells attract into their interior the calcareous salts of the blood, and their nuclei become developed, as I believe, into the future corpuscles of Purkinje. It is possible also that some of the cartilage cells become corpuscles of Purkinje in the fully developed bone.

During the progress of the phenomena above described, the contents of the cells undergo certain changes. At first, they are transparent, then they become granular, and still later opaque, from the presence of amorphous matter mingled with nuclei, nucleoli, and the remains of secondary cells. In the latter state the cells contain an abundance of minute oil-globules, which increase in size as the ossific changes advance, and in the newly-formed osseous areolæ, have attained the ordinary size of adipose cells.

Cartilaginification is complete in the human embryo at about the sixth week; and the first point of ossification is observed in the clavicle at about the fifth week. Ossification commences at the centre, and thence proceeds towards the surface; in flat bones the osseous tissue radiates between two membranes from a central point towards the periphery, in short bones from a centre towards the circumference, and in long bones from a central portion, diaphysis, towards a secondary centre, epiphysis, situated at each extremity. Large processes, as the trochanters, are provided with a distinct centre of development, which is named apophysis.

The growth of bone in length takes place at the extremity of the diaphysis, and in bulk by fresh deposition on the surface; while the medullary canal is formed and increased by absorption from within.

The period of ossification is different in different bones; the order of succession may be thus arranged:

During the fifth week, ossification commences in the clavicle, lower jaw, and upper jaw.

During the sixth week, in the femur, humerus, tibia, radius, and ulna.

During the seventh and eighth weeks, in the fibula, frontal, occipital, sphenoid, ribs, parietal, temporal, nasal, vomer, palate, vertebræ, first three pieces of sacrum, malar, metacarpus, metatarsus, third phalanges of the hands and feet, and ilium.

During the third month, in the first and second phalanges of the hands and feet, lachrymal bone, and ischium.

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[FIG. 24.

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A YOUNG FEMUR. 1, 5. The epiphyses. 4. The diaphysis. 2, 3. Apophyses.]

During the fifth month, in the mastoid portion of the temporal, ethmoid, inferior turbinated, sternum, os pubis, and last two pieces of sacrum.

During the sixth month, in the body and odontoid process of the axis, and in the os calcis.

During the seventh month, in the astragalus.

During the tenth month, in the cuboid bone and os hyoides.

During the first year, in the coracoid process of the scapula; first piece of the coccyx, inferior turbinated bone, last piece of the sternum, anterior arch of the atlas, os magnum, os unciforme, and external cuneiform bone.

During the third year, in the cuneiform of the carpus, internal cuneiform of the tarsus, and patella.

During the fourth year, in the middle cuneiform and scaphoid of the tarsus. During the fifth year, in the trapezium and os semilunare.

During the seventh year, in the second piece of the coccyx.

During the eighth year, in the scaphoid of the carpus.

During the ninth year, in the os trapezoides.

During the twelfth year, in the os pisiforme and third piece of the coccyx. During the eighteenth year, in the fourth piece of the coccyx.

The ossicula auditûs are the only bones completely ossified at birth; the ver

tebræ are not completed until the five-and-twentieth year.

The entire osseous framework of the body constitutes the skeleton, which, in

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the adult man, is composed of two hundred and forty-six distinct bones. They may be thus arranged:

[When the bones which constitute the skeleton, as presented for study, are connected with each other by their own ligaments, it is called a natural skeleton; when, however, they are connected by wires, or other artificial media, it is called an artificial skeleton.]

The skeleton is divisible into: 1. The vertebral column, or central axis. 2. The head and face, or superior development of the central axis. 3. The hyoid arch. 4. The thoracic arch and upper extremities. 5. The pelvic arch and lower extremities.

VERTEBRAL COLUMN.

The vertebral column is the first and only rudiment of internal skeleton in the lower Vertebrata, and constitutes the type of that great division of the animal kingdom. It is also the first developed portion of the skeleton in man, and the centre around which all the other parts are produced. In its earliest formation it is a simple cartilaginous cylinder, surrounding and protecting the primitive trace of the nervous system; but, as it advances in growth and organization, it becomes divided into distinct pieces, which constitute vertebræ,

The vertebræ admit of a division into true and false. The true vertebræ are twenty-four in number, and are classed, according to the three regions of the trunk which they occupy, into cervical, dorsal, and lumbar. The false vertebræ consist of nine pieces united into two bones, the sacrum and coccyx. The arrangement of the vertebræ may be better comprehended by means of the accompanying table:

True vertebræ, 2412 Dorsal, (5 Lumbar.

5 Sacrum,

False vertebræ, 9 {4 Coccyx.

Characters of a Vertebra.-A vertebra consists of a body, two laminæ, a spinous process, two transverse processes, and four articular processes. The body is the solid part of the vertebra; and, by its articulation with adjoining vertebræ, gives strength and support to the trunk. It is flattened above and below, convex in front, and slightly concave behind. Its anterior surface is constricted around the middle, and pierced by numerous small openings, which give passage to nutritious vessels. On its posterior surface is a single irregular opening, or several, for the exit of the venæ basis vertebræ.

The laminæ commence upon the sides of the posterior part of the body of the vertebra by two pedicles; they then expand; and, arching backwards, inclose a foramen, which serves for the protection of the spinal cord. The upper and lower borders of the lamina are rough for the attachment of the ligamenta subflava. The concavities above and below the pedicles are the intervertebral notches. The spinous process stands backwards from the angle of union of the laminæ. It is the succession of these projecting processes along the middle line of the back, that has given rise to the common designation of the vertebral

column, the spine. The use of the spinous process is for the attachment of muscles. The transverse processes project, one at each side, from the laminæ of the vertebra; they also are intended for the attachment of muscles. The articular processes, four in number, stand upwards and downwards from the laminæ, to articulate with the vertebra above and below.

Cervical Vertebra-In a cervical vertebra the body is smaller than in the other regions; it is thicker before than behind, broad from side to side, concave on the upper surface, and convex below; so that, when articulated, the vertebræ lock the one into the other. The lamina are narrow and long, and the included spinal foramen large and triangular. The superior intervertebral notches are slightly deeper than the inferior; the inferior being the broadest. The spinous process is short and bifid at the extremity, increasing in length from the fourth to the seventh. The transverse processes are also short and bifid, and deeply grooved along the upper surface for the cervical nerves. Piercing the base of the transverse process is the vertebral foramen,' which gives passage to the vertebral artery and vein, and vertebral plexus of nerves. The transverse processes in this region are formed by two small developments, which proceed, the one from the side of the body, the other from the pedicle of the lamina, and unite near their extremities to inclose the circular area of the vertebral foramen. The anterior of these developments is the rudiment of a cervical rib; the posterior, the analogue of the transverse processes in the dorsal region. The extremities of these developments are the anterior and posterior tubercles of the transverse process. The articular processes are oblique; the superior looking upwards and backwards; the inferior, downwards and forwards.

A CENTRAL CERVICAL VERTEBRA, SEEN UPON ITS UPPER SURFACE. 1. The body, concave in the middle, and rising on each side into a sharp ridge. 2. The lamina. 3. The pedicle, rendered concave by the superior intervertebral notch. 4. The bifid spinous process. 5. The bifid transverse process. The figure is placed in the concavity between the anterior and posterior tubercle, between the two processes which correspond with the rudimentary rib and the true transverse process. 6. The vertebral foramen. 7. The superior articular process, looking backwards and upwards. 8. The inferior articular process.

THE UPPER SURFACE OF THE ATLAS. 1. The anterior tubercle projecting from the anterior arch. 2. The articular surface for the odontoid process on the posterior surface of the anterior arch. 3. The posterior arch, with its rudimentary spinous process. 4. The intervertebral notch. 5. The transverse process. 6. The vertebral foramen. 7. Superior articular surface. 8. Tubercle for the attachment of the transverse ligament. The tubercle referred to is just above the head of the figure; the convexity below it is the margin of the inferior articulating process.

There are three peculiar vertebræ in the cervical region:-The first, or atlas; the second, or axis; and the seventh, or vertebra prominens.

The Atlas (named from supporting the head) is a simple ring of bone, without body, and composed of arches and processes. The anterior arch has a tubercle on its anterior surface, for the attachment of the longus colli muscle; and on its

Sometimes, as in a vertebra now before me, a small additional opening exists by the side of the vertebral foramen, in which case it is traversed by a second vein.

posterior aspect is a smooth surface, for articulation with the odontoid process of the axis.

The posterior arch is longer and more slender than the anterior, and flattened from above downwards; at its middle is a rudimentary spinous process; and upon its upper surface, near the articular processes, a shallow groove' at each side, which represents a superior intervertebral notch, and supports the vertebral artery (just before it perforates the dura mater) and the first cervical nerve. The intervertebral notches are peculiar, from being situated behind the articular processes, instead of before them as in the other vertebra. The transverse processes are remarkably large and long, and pierced by the foramen for the vertebral artery. The articular processes are situated on the most bulky and strongest part of the atlas. The superior are oval and concave, and look inwards, so as to form a kind of cup for the condyles of the occipital bone, and are adapted to the nodding movements of the head; the inferior are circular, and nearly horizontal, to permit of the rotatory movements. Upon the inner face of the lateral mass which supports the articular processes, is a small tubercle at each side, to which the extremities of the transverse ligament are attached, a ligament which divides the ring of the atlas into two unequal segments; the smaller and anterior, for receiving the odontoid process of the axis; the larger and posterior, to give passage to the spinal cord and its membranes.

The Axis (vertebra dentata) is so named from having a process, shaped like a tooth, upon which the head turns as on a pivot. The body is of large size, and supports a strong process, the odontoid, which rises perpendicularly from its

FIG. 27.

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A LATERAL VIEW OF THE AXIS, 1. The body; the figure is placed on the depression which gives attachment to the longus colli. 2. The odontoid process. 3. The smooth

upper surface. The odontoid process presents two articulating surfaces; one on its anterior face, to articulate with the anterior arch of the atlas; the other on its posterior face, for the transverse ligament; the latter surface constricts the base of the process, which has given rise to the term neck applied to this part. Upon each side of its apex is a rough depression, for the attachment of the alar ligaments; and running down from its base, on the anterior surface of the body of the vertebra, a vertical ridge, with a depression at each side for the attachment of the longus colli muscle. The laminæ are large and strong, and unite posteriorly to form a long and bifid spinous process, which is concave beneath. The transverse processes are rudimentary, not bifid, and project only so far as to inclose the vertebral foramen, which is directed obliquely outwards instead of perpendicularly as in the other vertebræ. The superior articulating processes are situated on the body of the vertebra, at each side of the odontoid process. They are circular and nearly horizontal, having a slight inclination outwards. The inferior articulating processes look downwards and forwards, as do the same processes in the other cervical vertebræ. 7. The superior articular surface. The superior intervertebral notch is remarkably shallow, and lies behind the articular process as in the atlas. The lower surface of the body is convex, and is received into the concavity upon the upper surface of the third vertebra.

facet on the anterior surface of the odontoid process which articulates with the anterior arch of the atlas;

the facet for the transverse ligament

is beneath No. 2, where the constriction called the neck of the odontoid process is seen; the bulk of the process between 2, 3, would represent its head. 4. The lamina. 5. The spinous process. 6. The transverse process, pierced obliquely by the vertebral foramen.

8. The inferior articular process.

The Vertebra prominens, or seventh cervical, approaches in character the upper dorsal vertebræ. It has received its designation from having a very long This groove is sometimes converted into a foramen.

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