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mass is found which contains some remnants of the notochord. Elsewhere this structure is pressed out of existence and there is no further use for it when the cartilaginous vertebrae are once formed. One other series of structures must be mentioned though they do not

25 24 2 7

From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 5. â€” Ossification of Vertebrae.

Cervical Vertebra. Centre for body. Superior epiphysial plate. Anterior bar of transverse process

developed by lateral extension from

pedicle. Neuro-central synchondrosis. Inferior epiphysial plate.

Lumbar Vertebra.

6 Body.

7 Superior epiphysial plate.

8 Epiphysis for mammillary process.

9 Epiphysis for transverse process.

10 Epiphysis for spine.

1 1 Neuro-central synchondrosis.

12 Inferior epiphysial plate.

Dorsal Vertebra.

13 Centre for body.

14 Superior epiphysial plate, appears

about puberty; unites at 25th year.

15 Neuro-central synchondrosis does not

ossify till 5th or 6th year.

16 Appears at puberty; unites at 25th

year.

17 Appears at puberty; unites at 25th

year.

18 Appears about 6th week.

Axis.

19 Centre for transverse process and

neural arch; appears about 8th week.

20 Synchondroses close about 3rd year.

but the ventral ends of the ninth and sometimes the eighth probably remain as the xiphisternum, indeed a fibrous band is sometimes seen joining the caudal end of that structure to the ninth rib. The fusion of the two parallel bars begins at their cephalic ends and sometimes is interrupted toward the caudal end, thus leading to cleft or perforate sternum. At the cephalic end of each sternal bar, close to the place where the clavicles articulate, is an imperfectly separated patch of cartilage which usually fuses completely with the presternum, though sometimes it remains distinct and may later acquire a separate centre of ossification and so form a separate episternal bone on each side. If the sternum is to be regarded as the fused ventral ends of the thoracic ribs, the episternal elements are probably the remnants of the ventral ends of the seventh cervical ribs. The question of the morphological meaning of the sternum and surrounding parts cannot be settled entirely by a study of their development even when combined with what we know of their comparative anatomy or phylogeny. Professor A. M. Paterson (The Human Sternum, London, 1904) takes a dif- ferent view from the foregoing and regards the sternum as derived from the shoulder girdle. To this point of view we shall return in the section on comparative anatomy.

The last stage in the development of the axiaf skeleton is the ossification of the cartilage; bony

21 Centre for summit of odontoid pro- centres appear first in each half of the neural arches

cess; appears 3rd to 5th year, fuses of the vertebrae arid a little later (tenth week) 8th to 1 2th year. double centres are deposited in the centra though

22 Appears about 5th or 6th month ; these are so close together and fuse so rapidly that

unites with opposite side 7th to 8th their double nature is often only indicated by their month. oval or dumb-bell-like appearance. The bone in the

23 Synchondrosis closes from 4th to 6th two halves of the neural arch spreads and fuses in

the mid dorsal line, and later on joins the ossified

epiphysial plate; appears centrum ventral to the facet for the rib. This point puberty, unites about 25th of junction remains as a narrow strip of cartilage for

a long time and is known as the neuro-central r double centre for body; suture or synchondrosis. The head of the rib

24

year. Inferior about year. 25 Single

appears about 5th month. Atlas. 26 Posterior arch and lateral masses developed from a single centre on either side, which appears about 7th week.

therefore articulates with the developmental neural arch instead of the centrum. About the age of puberty secondary centres or epiphyses appear at the tips of the transverse and spinous processes and as thin plates just above and below the body (see fig. 5 â€” 2 and 3). These are fully united by the . â€¢. ,, . , . twenty-fifth year. In the lower two cervical verte-

27 Anterior arch and portion of superior ferae / here i/ often a separate centre for the part

articular surface developed from correspond i ng t0 t h e rib, while the lumbar have an

single or double centre, appearing extra epiphysis for the mamm iHary process. The

during 1st year. atlas has one centre for each side of the dorsal part

Dorsal Vertebra. of the arch and one (probably two fused) for the

28 Epiphysis for transverse process; ventral part, which has already been referred to as

appears about puberty, unites about a hypochordal bar. In the axis, in addition to the 25th year. ordinary centres, there is one for each side of the

29 Epiphysis appears" about puberty; odontoid process and one for the tip (see fig. 5â€”

unites about 25th or 27th year. 20, 21, 22). The sacral vertebrae have the usual

30 Centre for neural arch on either side ; centres, except that the anterior part of the lateral

appears about 6th or 7th week, the mass (costal element) has a separate centre and laminae unite from birth to 15th that there are two extra centres on each side of month. the whole sacrum where it articulates with the ilium

Centre for body; appears about 6th (see fig. 6).

week, unites with neural arch from The ribs ossify by one primary centre appearing

31

5th to 6th year.

play any great part in human development. In the intersegmental tissue ventral to each of the intervertebral disks a transverse rod of cells, known as a hypochordal bar, is formed which connects the heads of two opposite ribs. In man the greater number of these either disappear or form the middle fasciculus of the stellate ligament which joins the head of the rib to the intervertebral disk, but in the case of the atlas the rod chondrifies to form the anterior (ventral) arch which is therefore intersegmental, while the segmental body of the atlas, through which the notochord is passing, joins the axis to form the odontoid process. These hypochordal bars are interesting as the last remnant in man of the haemal arch of the vertebrae of fishes (see subsection on comparative anatomy). In the cervical region the ribs are very short and form the ventral boundary of the foramen for the vertebral artery. They are so short that little movement occurs between them and the rest of the vertebra, hence no joints are formed and the rib element becomes fused with the centrum and transverse process, leaving the vertebrarterial canal between. Sometimes in the seventh cervical vertebra the rib element is much longer and then of course more movement occurs, and instead of fusing with the rest of the vertebra it remains as a separate cervical rib with definite joints.

The sternum is developed according to G. Ruge by a fusion of the ventral ends of the ribs on each side thus forming two parallel longi- tudinal bars which chondrify and eventually fuse together in the mid line. The anterior seven or sometimes eight ribs reach the sternum,

about the sixth week and by secondary ones

for the tubercle and head. The sternum is ossified

by centres which do not appear opposite the attachment of

the ribs but alternately with them, so that although the original

From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 6. â€” Ossification of Sacrum â€” a,a, Centres for bodies; 5,5, Epiphysial plates on bodies; c,c, Centres for costal elements; d,d, Centres for neural arches ; e,e, Lateral epiphyses.

cartilaginous structure is probably intersegmental the bony segments are segmental like those of the vertebral centra. As seven ribs articulate with the sternum six centres of ossification between them might be looked for, but there is so little room between the points of attachment of the sixth and seventh ribs that centres do not occur