Section I.
COMMENCEMENT AND TERMINATION OF OSSIPICATIOI*.
During the early period of the existence of the embryo, the bones consist of a viscid fluid ; they then become soft and gelatinous ; afterwards cartilaginous ;' and lastly, osseous. Some writers tliink ihttt the ani- mal substance, which in Ihc first instance compo^tcs the basis of bune, is mucilaginous ; but others contend that it is gelatinous, and as the bodies of young animals always contain a considerable proportion of jelly, I am inclined to concur in this opinion.
It is difficult to define the exact time when ossiRcation oommences; according to Beclard, it begins about n month after conception ; whilst Meckel says, that this process docs not really commence' until towards the eighth week, although tlie cartilages which supply the place of the future bones, of which they have the form,' appear at the fourth week. The different bones do not begin to appear at the same time, but in a successive ordtT, which has been minutely detailed by some writers. The long bones, with a few exceptions, am developed before the flat ones, and the latter before tlie short bones.- Thus the clavicle, the ribs, the inferior jaw, and the great' bones of the extremities, appear before the os nccipiliR and the os frontis ; and some months before tlie carpal and tarsal bones, a few of which contain no osititic centre even at the time of birth. It has al.so been observed, that the bones which are near the centre? of the nervous and vascular systems are formed at am early period.
The following is a general scale of the commencement
of ossification in the 'arious parts of the skeleton. The
2 D
4
40S DBVELOPHBNT OF DIPFBBENT DONBS.
process begins at the end of the first month,* in the clavicle, and successively in the inferior maxilla, tlie fe- mur, t])C tibia, the humerus, the superior maxilla, and in the hones of the fore arm, in the latter of which it commences towards llie thirty-fifth day. The fibula, th« scapula, and the palate bones appear towards the for- tieth day, and in a (avf days after ossification be^as in the following parts : viz., in the tuberosity of the o« occipitis, the os frontis. the arches of the upper vertebne, the ribs, the great wing of the sphenoid bone, the z>'go- ma, and the phalanges of the fingers. About Uic saine time the earthy substance is deposited in the bodies of the dorsal vertcbrEe, Uie ossa nasi, the ossa mnlarum, the ilium, the metacarpal bones, the phalanges of the toes, the condyleH and basilar process of the os occipitis, the squamous portion of the temporal bone, the parietal bone, and the Tomeri in all the« bones the process be^DS after the middle of the seventh week. Towards the middle of the ninth week it commences in the body of the splienoid bone, in the bodies of the tqiper sacnU verlehrcc, and in the ring for the memhrana lympnni. In the middle of the third month, ossification is mani- fested in the labyrinth, and towards its end in ischium and internal pterygoid process ; and in the tnii die of the fourth montJi it is seen in the osstcutn auiU At the middle term of gestation osseous matter is dqioi^ "^ sited in the pul)is, the os colcis, the smallest phalanges of the toes, the lateral parts of the tethmoid bone, nod in th(- os.sa spnngiosa ; and a little later in the first pieces of thf sternum. Towards the sixth month, the process commences in U»e body and odontoid proccs* of the
• Bnriaid, Auk On. p. 490.
y
tui^H
iDOi^H
.. TBRMtNATION OP OS91PICATIOH.«ll 403
second vertebra, In the lateml pnrtt of the first sacral vertelira, and afterwards in the astragalus ; and at the seventh month in the crista galli of the tethmoid bone. Towards the close of the fottal existence, the earthy sub- stance is dupositud in the os cuboidca, the first portion of the coocyx, and in the fore part of the nng of the atlas. At the age of one year, the coracoid process begins to be ossified, and the same may be said of the magnum and unciform of the carpus, and of the tirst cuneiform of the tarsus. The patella is formed in tiic third year. The second and Uiird ossa cuneiformia and other bonra of the tarsus and carpus, are formed successively between the fourth and the twelfth yean,
;i'<The process of ossification is not simultaneously com- pleted ; and it is remarkable that bones which appear very early in the embryo, are not perfected till a late period ; thus the condyles of the os femoris, which bone b so precocious in its formation, are not consolidated with the shaft before the age of eighteen or twenty years ; whilst many of the short and irregular bones in which the process is more tardy at the commencement, are completed much earlier. The os fronds frequently remains divided into two parts after the other bones are formed, so that the frontal suture does not disappear till the twenty-fifth year, and occasionally it is perceptililc in adxTmccd age.
«i Section II. r
DIFFERENT MODES OF OSSIFICATION.
Tt has been ascertained by the ingenious and laborious researches of several modem physiologists, that the
2d 2
404
DIFFBRBNT MODEA OF OaSlFICAttON.
P
ossiftc matter is deposited under three different foriris • -1. In the soft and gelatinous condition nf bone. 2. la' the substance of cartilage. 3. Setween two membranes. It has been contended by some high authorities that' cartilago is a nt-cessary antecedent to bone ; but thia ', opiuiuD has bet'U rcfutt^ by tbe experiments of Mr. Howship, who has shewn, that the lirat rudiments of | ossification in the long bonus, appear before the ctoIu- ■, tion of any cartilaginous structure.* Tliose observ'R- 1 tions have been confirmed by Serres and Beclard, who suppose that in those bones which are first devrfoped, or in those parts of bones in which the process takes place at an early period, the osseous substance is (le-l posited in tbe fluid form ; wbiUt in those bones which arc formed at a remoter period, the cartilaginous or in- termediate state which they assume, is rather a pro-J visional condition than a stage of ossification — a tem- ; porary structure fur the purpose of pprfitrming the i functions of bone, and not a necessary antecedent to ossific process,
I shall now proceed to describe the three modes of o*. j iiific'ation.
I . In the diaphysis of the long hones and in the centre of the l)roa<l ones which are early de%'eloped, there is ani appearance of osseous matter before any cartilage can baj detected ; thi.s matter, which is arranged in the form ■ short hollow cylinder, is secreted, according to Mn^ Howsbip, from the arteries of the periosteum, and con-| Bequently is deposited from witliout, inwards. It b' stated by Beclard that the earthy substance is deposited in a fluid condition and at the same time with the
HciUal and Chiinrg. Ttuu. toL vi p. 264,
BAETUY DEPOSITION IN CARTILAGS. 406
animal matter, in the organized tissue which secret^aji it. Its aubsuqucnt solidity is probably owing to the con- tinued (Ifpusition of phosphate of lime, nnd to the absorp- tion of the fluid parts. Dr. Bastock is doubtful if the soft matter in which the osseous cylinder is formed, be not itself the fiiture cartilage, merely in a soft Btale, united to a large proportion of water.* It is difficult to decide this question ; but as there is no sufHcient proof to shew that tJie tluid substance is only cartilage in an imperfect condition, we should not be justified in rejecting Mr. Howship's conclusion.
- In a rather more advanced age of the embryo, the mode of ossitication is changed, in order that it maj proceed more speedily. At this period, which is about the eighth week after conception, the temporary car- tilages appear, in the centre of which the osseous matter [& deposited.
/, We learn from the observations of Mr. Howship, to whom we are indebted for a minute account of these cartilages, and the changes they experience by the progress of ossification, that when they aiy first formed they contain several irregular cavities, which are after- wards converted into canals ; they are lined by a vasctdar membrane, and art- filled with a considerable quantity of gelatinous mutter. These jmssoges gradually decrease in number and size, so that in a cliild eleven months old there are but few of them ; at the age of eleven years the canals are further diminished, and at seventeen years it is with great difficulty that any trace of them can be found.
The cartilage in the beginning contains minute
• PhytioloBT* to'- "• P- !"■
406 CBNTBI! OP OSSIFICATION.
T^sselB, wniiA ctary only a colourless fluid ; but as the' proceiis H(l'Rnces the arteries arc enlur^-d so as to adinic I red blood, by which the carliloge is tingt-d. Several'] arteries appear, and by their communication produce k I vasculiir plexus, from which particles of tliv phosphitte'l of lime bciug dL-posllcd, u centre of ossijicatioti is formird.-j These particles, when they become first apparent andfi coherent, constitute an assemblage of very fine and thia fibres, which ore moulded into the form of short tubes^ and extend according to the length of the bODC. Thtf cartilage now grows opaque, yellow, and brittle, and( will no longer bend ; the small osseous nucleus may be' felt, and, when touched with a sharp point, is ea&iljrl known by its gritty feel. Other centres of os!<iHcutioil' are successively formed, always being foretold by the I spreading of the arteries, and by the arrival of rcd| blood.»
The first point of ossification, which is invariably i formed in the interior of the cartilage, and nci-er at ibij surface, continually increases by the addition of ncW Biatter; and, in proportion as the earthy phosphate ill deposited, the cartilage is removed by the alworbentsj till at length, the bone being perfectly formed, it dis^ appears.
These two processes, according to the doctrine of j Mr. Hunter ,t arc proceeding at the same time, aaA] mutually assist each other ; the secretory ^-essels bring^ , ing supplies to the bone, and the absorbents ciirr)-ingj away the original prticles so as to give it a propefl form. /Uthough the actions of the arteries and lym-J
■ Btll'k Aofttomy, vol. L p. 184. t Tmu. of ■ Society for tb* iniprovemeDt of Med. aud Chlr. Koowl, vol. ii. p. 261.
osairiCATioN bbtwein membranes. 407
phatics correspond ia point of time, yet they differ as to the seat of tlieir relative activity, the greater quantity of osseous matter being deposited on tUe outer surface of tl^e bone, wliilst the absorpUoa is curried on at the centre ; so that when the external part acquires its proper de^e of hardness the interior is either formed iiittj a complete cavity, or is hollowed into an immense Qtmiber of cells producing the spongy substance that has been described.* As the original cartilage contains no celLs, it docs not possess any medullary membrane ; but after the cavities arc produced the internal periosteum K developed.
.<3. The third mode of ossification, viz. that between membranes, exhibits peculiarities which are not met witli in either of Uie former processes. The bones of the cranium, which are formed in this manner, begin to appear about the seventh week idler conception, at which time the pericranium and dura mater are very vas- ctUur. The phusphute of lime is first duposited in minute granules, which ore dispersed in unec[ual ma.sses and without regularity between the membranes ; they afterwards coalesce and form fibres, which have a ra- diated disposition firom the centre towards the edges. The intervals between the fibres are filled with a gtairy and colourless fluid, which is similar in itji sensible qualities to the substance found in the canals of tlie temporary cartilages. The surfaces of the hones are also covered with a gelatinous and reddish fluid, which contains a great number of the finest capillary arteries variously disposed.f
The short or thick bones arc ossified in the same manner as the extremities of the long ones. They are
• Six p. 389. t UowUup, L c. p. 380.
ECCINTRIO FORMATION OF BONE.
r
preceded ia their formation by cartilagts, which are at fimt homogeneous and full ; they arv ibcn hollowed iotc cavities and canals similar to those already deticribccl,!] and are at length converted into bone.
The patella and tlie os&a »e.sBn)oidea, are formed ii a tissue which is at first fibrous and aiterwardii cartila- ginous. He mixed or irregular bones participate hy\ their formation, as by their cxU-ronl figure and intenMJi[ structure, in the characters of the three other classes, i
The growth of the bones is in most in&tanocs aecclo-i rated, in consequence of each of them poeiicssing »;%'eraL] distinct points of ossification. Many that arc placed on the median line are formed by two lateral halves, which are united at a later pericni ; ex. gr. the archc of the vertebnc, the frontal, Uio body of the sphenoid, the occipital, the inferior jaw, and the central pieces oCj the sternum. It has been discovered by Serres even in those central parts of the median bones, as Ihl bodips of the rertebrie, and tlie body of the ys hyoic in which it was formerly thought that ossilicatton bef in the middle, and extended to the sides, the process} really commences by two lateral points which afterwarda] coalesce on the median line
In some bones there are several primitive points ' ossification, which are sooner or later consotidatud tonJ gether ; they are met with in the vertebra, os occipitisftj OS sphenoides, os temporis, sternum, sacrum, &c.
The most important circumstances connected y this phenomenon of ossification, are observed in tbfti long and in a few of the short bones, which have ac-j
- Sib Iti Loii dc t'niUot[«nie. The tlmve it a vny mBukahk la or Um: KccaUic mode of foRDsUon kinady ilIudHl to at p. ??.
BPIPHY9H3. 409
cessory centres of ossification, called epiphyses. The great bones of the thigh, the arm, the leg, and the fore arm, have at least one epiphysis at each end ; but the smaller hones, as thone of the metacarpus, metft* tarsus, and fingers, have only one epiphysis, and the same may be said of the cla^-icle and of the os calcis. The large bones have cartilaginous edges, wliich have been callud by some anatomists their marginal qiiphyses; they arc very apparent in the scapula and os innomina- turn. The epiphyses, whicli begin to be formed about fifteen days before birtli, do not disappear till the eighteenth or twentieth year, and sometimes not till the twenty-fifth year. They are separated fi-om the sliaft by a layer of cartilage, which is of considerable thickness in the beginning, but as the process approaches its com- pletion, the intervening sulistance becomes very thin, and is at length entirely removed.* The large pro- cesses, as the trochanter, are likewise separated fi*om the shaft by a portion of cartilage ; these detached processes have been called by some writers, apophyses. *The growth of bones takes place in the direction of their height and of their circumference, so that the new sub- stance is not only added at their extremities, but also pe- netrates into the mass which previously existed. The increase in length was thought by Duhamel to depend on a slow and gradual extension of the parts of a bone ; but the rapid elongation that occurs before the consolidation of the qiiphyses, probably results from the addition of -■
• The «xiiluii«« o( the epiphyiM io childcen. sxplatoi the peculiar fne^ hire wliich lomctiinM occurs in Uw articular cxtiemiliis of the long bono. Id t)iu accldeal, ulled iliaifaii'i, the cplptiyu* U torn off; bal then ii no crepitiu, because lite laoosted cartiUee does nol [traduce thai grating, go mo- tion, which it toch a <)tBgno)ti<; mailt cf the cooimon fracture. ,
410
llYPOTllBSia OP PAVKNTKR.
ossilic rnnlter at its extreniitiui. This opinion is sup- ported by the follotring experiment perfanned by Huo> tcr ; he bored two holes in the tibia of a young pig, precisely tu'o inches asunder ; some time after, vtUva the bone had increased in length, the unimal was killed, and it was found that the distanm between the opemogs waa exactly the same.