bone ("step-ladders"). The bone meshes of the encroaching
bone are also shaped in correspondence with the cartilage cap-
sules, that is, they are long and narrow.

Formation of bone through the medium of cartilage. The
successive changes in this species of bone development have
been best described by Klein. According to him the hyaline
cartilage that is destined to prepare the way for bone is covered
with perichondrium, consisting like the periosteum of two
layers. This membrane does not at first contain mature fibrous
tissue, but merely the rudiments of it, under the form of spin-
dle-shaped corpuscles ; its internal layer, however, is early pro-
vided with spherical corpuscles, the future osteoblasts, and is
rich in vessels.

Subsequently this osteogenetic envelope puts out processes
(periosteal processes, Virchow) that penetrate into the carti-
lage-capsules, which, melting as the external growth makes its
7



98 MANUAL OF HISTOLOGY.

way inward, develop communications between the capsules, so
that in this way a cartilaginous network is formed that is filled
with the arborescent tissue. This change in the cartilage,
which is characterized by absorption and rarefaction, is called
chondro-porosis.

At a more advanced stage the cartilage around the oldest
channels has become transparent in places, while the walls are
irregular, because portions of calcified trabeculse project into
them. These irregular spaces are called primary marrow
cavities. Now upon the walls may be seen, not the cartilage-
corpuscles, but the osteoblasts, which are proceeding to develop
concentric layers of osseous tissue.

When this process has been completed, the osseous tissue
will be found to have replaced the calcified. cartilage, and true
bone has been formed. But this action may be no sooner
completed than absorption will again commence, and at first in
the last or most internal layer of the Haversian system. This
process is essential for the development of the central marrow
cavity. After an Haversian system has been removed, the
matrix will also disappear.

Now, while this cavity is filling up with marrow a gradual
development of bone is taking place from the periosteum,
which slowly encroaches upon the bone whose formation we
have just described.

This last stage results in the formation of adult bone.
When it has been completed all the first formed bone has
been absorbed before it. This periostea! or metaplastic bone
is at first spongy, as is all new bone ; in the fulfilment of its
task it next appears to form compact bone, and then part of
this latter is rarefied, as, for example, along the wall of the
central cavity. Thus, as we have already seen, compact bone
is formed from spongy, and spongy from compact. The peri-
pheric or interstitial lamellae are either the remains of calcified
and unabsorbed trabeculse, or perhaps the walls of other Haver-
sian systems forming sides of the bony network.

Formation of bone from membrane. This second method
of bone-formation is seen in the bones of the skull and face.
The steps are precisely similar to those already described. The
inner layer of the periosteum, which is lined with osteoblasts,
produces both matrix and bone corpuscles by a process of bud-
ding. The change first begins at the points of ossification.



THE CONNECTIVE SUBSTANCE GKOUP. 99

At first the bone is spongy, but later absorption takes place
osteoporosis. Around some of the marrow- tubes concentric
lamellae are formed, and in this way a Haversian system de-
velops. The unabsorbed portions of the trabeculae are thought
to constitute the lamellae known as the intermediary. Com-
pact tissue is thus formed from spongy. This theory, which
has been placed in its present acceptable light by Klein, is very
simple and appears to accord with observation, and explains
all the phenomena. Yet those' who have believed in the direct
transmutation of cartilage into bone are still in the field.
Kolliker maintains that both views are correct.

According to this last named author the differences between primary or
primordial and the tegumentary or secondary bones are, from a morphological
point of view, sharp and complete. The former are ossifications of the carti-
laginous skeleton.

The tegumentary are never cartilaginous at first ; the primordial bones, on
the other hand are, without exception, formed from cartilage. The method and
manner in which bony tissue is formed is the same in both bones. The pri-
mordial skeleton in the lower vertebrates ossifies only in part from the peri-
chondrium, in part perichondrally, and, in part, endochondrally.

According to Kassowitz, in the tuberosities and spines of the bones the
periosteal processes of the periosteum, which develop the bone, are primarily
cartilaginous, the fibrillated tissue being converted into hyaline cartilage,
which is at first calcified and then undergoes direct conversion into bone.

According to the experiments of Strawinsky a transplanted periosteum will
develop either bone or cartilage, when the conditions are favorable. The con-
ditions of nutrition determine which it shall be. When the supply is best,
cartilage is formed ; when poorest, bone.

The earliest evidences of ossification were seen by this observer between
the fourth and fifth days. The formation of vessels preceded that of bone.
Absorption commenced between the second week and the second month. The
new formation of periosteum is partly derived from the border of the wound
and partly from the Haversian canals, which contain a small amount of connec-
tive tissue.

Development of bone and absorption. It has been seen
that these two processes go on hand in hand. As soon as the
periosteum has commenced to deposit new layers of bone on
the surface of the primary spongy bone, absorption takes place
along the marrow canal. First of all, as we have already said,
the innermost of the concentric lamellae yield. In this way
the Haversian canals are widened and become Haversian spaces,
as they were at first ; then the interstitial lamellae, and finally



100 MANUAL OF HISTOLOGY.

the spaces disappear, and in place of them there is a single
dilated central cavity.

Howship's lacunce are the pits or lacunae seen in bone
beneath the periosteum. They usually contain a multinuclear
corpuscle (giant-cell), which is in some way related to absorp-
tion, and, therefore, has received the name osteoclast (Kolli-
ker). It has been surmised (Klein) that they are the agents by
which an acid is formed that dissolves the lime-salts. Whether
they are developed out of the osteoblasts or not is a matter of
uncertainty.

All the steps, both in development and absorption of bone,
have been carefully studied and placed upon a most satisfac-
tory foundation (Lieberkuhn and Bermann). The absorption
of bone has also been actually proved by measurements of the
bones in children (Schwalbe). By comparing the bones of the
third and fourth years of life, it was found that the marrow
cavity had enlarged in the latter, while the compact bone had
diminished in thickness. The change commenced at the sixth
month. This physiological process is closely allied to the
pathological one exhibited in rachitis ; in the latter the de-
velopment of bone from the periosteum has the character of
foetal bone, but the formation of the lamellsD is slow and
incomplete.

It has been claimed that the growth of bone takes place
by an expansion of the intercellular substance (Strelzoff), but
this is denied (Kolliker, Wegener, Schwalbe, and others).
The ossein appears to increase somewhat, but it is at the ex-
pense of the bone- corpuscles, which are thereby diminished in
size.

Formation of callus. The method is the same as in the de-
velopment from periosteum. A corpuscular blastema is devel-
oped from the periosteum and intermuscular tissue. This
presses in between the fibres and bundles of the loose con-
nective tissue, pressing them asunder, assuming considerable
volume. This new tissue is hyaline cartilage. In from three
to six weeks it ossifies, being in part directly transformed into
bone, in part mediately, i.e., through the agency of medullary
spaces and osteoblasts. Where the extremities of the bone are
widely separated there is a formation of bone in the medullary
spaces of the broken ends of the bones. The pre-existing bone-
corpuscles have no part in the new-formation. This compact



THE CONNECTIVE SUBSTANCE GROUP. 101

bone thus formed will be absorbed in a few months, in its
internal portions, by rarefying ostitis, so that the marrow cavi-
ties of the broken diaphysis will be in communication.



BIBLIOGRAPHY.

The student is referred, for further particulars, to Klein's Atlas of Histology, Ban-
vier's Traite technique d'histologie, Strieker's Manual of Histology, and also to the
following recent writers :

SCHAEFER. Pract. Histology. 1872.

LIEBERKUHN and BERMANN. Ueber Kesorption der Knochensubstanz. 1877.

AUFRECHT. Ueber Riesenzellen in Elfenbeinstiften. Med. Centralblatt, No. 26.

Jahresb. d. Fortschritte der Anat. und Phys. 1878.
ARNOLD, J. Virchow's Archiv. Bd. 71, p. 17. 1877.
VON EBNER. Sitzungsbericht der Wiener Akad. III. Abtheil. Bd. 75. Hofmann

und Schwalbe's Jahresb. 1878.

KASSOWITZ. Med. Centralblatt, No. 5. Hofmann und Schwalbe's Jahresb. 1878.
SCHWALBE. Sitzungsb. dermed. naturwiss. Gesellschaf t zu Jena. 1877. H. und S.'s

Jahresb. 1878.

STRAWINSKY. Ueber Knochenresorption. H. und S.'s Jahresbericht, p. 109, 1878.
LITTEN, M., and ORTH, J. Berliner klin. Woch. No. 51, p. 743. 1877.
KOLLIKEB. Entwickelungsgeschichte. V. und S.'s Jahresb. 1878.



CHAPTER VIII.

THE TEETH.

FROM the standpoint of descriptive anatomy, every tootli is
composed of three parts : (1) the crown, that portion which
stands above the level of the mucous membrane of the gum ;
(2) the neck, a constricted part at the level of the gum ; and (3)
the root, which terminates in one or more fangs, and is firmly
embedded in the alveolar process of the jaw. Each fang also
is pierced from below by a canal, which extends up into the
crown, and is filled by a soft material rich in nerves and ves-
sels, called the pulp, which has the special province of sup-
plying nutriment to the dense tissue about it.

From a histological point of view, every tooth may be di-
vided into : 1, enamel ; 2, dentine, or ivory ; 3, cement, or true
bone. The enamel forms the covering for the crown, the cement
for the root ; but they meet at the neck, and there the cement
slightly overlaps. The ivory or dentine lies intermediate be-
tween the outer coatings and the pulp.

The enamel. This substance, which is the hardest met with
in the body, consists of a series of long polyhedral columns
grouped in bundles and disposed mostly at right angles to the
surface of the dentine which lies beneath it. Each column or
pillar is a hexagonal prism, having a diameter varying between
nrJiro and -^5^ inch. When viewed in cross-section these col-
umns look like a tesselated pavement. They are not, however,
closely applied to one another, but have interspaces which are
said to be filled with a homogeneous substance or fluid.

All of the groups of columns do not stand vertical to the
dentine ; some are parallel to it, and thus are interwoven with
the vertical ones. This crossing of the fibres produces an
alternation of light and dark bands (Fig. 38, 1). But there are
other systems of markings. In the same figure are wavy



THE TEETH.



103



lines running parallel to the surface. These are the " brown,
parallel stripes of Retzius" They pursue a somewhat curved
course. No unity of opinion exists about their significance,
one (Hertz) attributing them to deposits of pigment, another
(Von Bibra) to the pres-
ence of the oxide of iron.
Still other striae are ob-
served, and are thought
to represent the zigzag
or spiral course of the
enamel prisms. It is
observed that when the
prisms are isolated, which
can be accomplished by
immersion in a dilute hy-
drochloric acid solution,
they have a somewhat
spiral form, and have
bulging sides and cross
markings, the signifi-
cance of which will be
alluded to at another
place.

Near the line of the
dentine there are spaces
between the prisms which
are continuous with the
cavities in the dentine.
These are called the in-
terglobular spaces of
Czermak. They also oc-
cur at irregular intervals
in the dentine.

% T n Trrmno* cnVnrtc section, magnified 15 diameters. 1, enamel with decussating
111 7 IDjeC . and parallel string ; 2, dentine with Schreger's lines ; 3, eel

there is a delicate mem-
brane covering the sur-
face of the enamel. It is composed of laminated epithelial
scales, and corresponds to the corneous layer of the skin, of
which, indeed, it represents the vestiges.

The dentine or ivory (Fig. 38, 2) consists of a dense and
hard matrix impregnated with the salts of lime. It contains




FIG. 38. Premolar tooth of the cat, in situ. Vertical



and parallel striae ; 2, dentine with Schreger's lines ; 3, ce-
ment ; 4, periosteum of the alveolus ; 5, inferior maxillary
bone. (Waldeyer.)



104



MANUAL OF HISTOLOGY.



numerous passages having, like the enamel prisms, a direction
at right angles to the surface of the bone. These passages, the
dentinal canals, are united with one another laterally by
minute oblique branches, and form undoubtedly open channels
of communication between the pulp cavity and the spaces be-
tween the enamel prisms in the crown and the bone lacunae of

the cement in the fang. Each canal
-^=^^^E^^^ is lined with a particularly delicate
and resistant membrane, the den-
tinal sheath of Neumann.

Upon the internal surface of the
dentine, or the external surface of
the pulp- tissue, is the layer of
odontoblasts (Schwann). These cor-
puscles, according to Waldeyer,
have long branching processes ex-
tending in three directions, inward
into the pulp-cavity, outward
through the dentinal channels,
forming the dentinal fibres of
Tomes, and laterally so as to form
connection with adjacent corpus-
cles. On the outer surface of the
dentine the canals connect with
the interglobular spaces of Czer-
mak, and they in turn are con-
tinuous with interstices between
the enamel prisms. The dentinal
tubules never appear to be in di-
rect communication with the enam-
el spaces, but only mediately, as
has been described. These cavi-
ties are filled with protoplasmic
material. Those immediately adjoining the cement are small
in size, and form what is known as the granular layer of
Tomes or PurJcinje.

Dentinal globules (Fig. 39, 2) is the name given to certain
spheroidal masses that are regarded (Waldeyer) as calcified
remains of the corpuscles in the spaces. The contours of these
masses correspond in outline with those of the interglobular
spaces.




FIG. 39. Canine tooth of man, present-
ing a portion of the transverse section of
the root : 1. cement with larsre lacunae and
parallel striae ; 2, interglobular substance ;
8, dentinal tubulea. Magnified 300 diame-
ters. (Waldeyer.)



THE TEETH. 105

Beneath the cement the intercommunication of interglobu-
lar spaces and bone-lacunae is well shown. The interglobular
substance is apt to be present in layers ; the lines which are
then called the incremental lines of Salter, are supposed to
show that there has been growth by successive stages. The
lines of Schreger (Fig. 38, 2) are also waving parallel lines ; they
are thought to be due to the curvature of a series of adjacent
fibres. In some instances vascular channels have been found
in the dentine, which has acquired the name osteo or vaso-
dentine. In pathological conditions masses have also been
found containing bone-lacurise. They have been called odonto-
mata by Yirchow.

The cement is true bone-tissue, containing lacunae and
canaliculi, and in them the bone-corpuscles with their pro-
cesses. The matrix is also subdivided into lamellae. The peri-
osteum of the gum dipping down into the bony socket from
the surface of the gum forms a coating over the cement. Oc-
casionally Haversian canals and blood-vessels are seen where
the cement is thick (Salter). Sharpey' s fibres may also be seen,
according to Waldeyer.

The pulp is a substance that belongs to the connective-
tissue series. Adjoining the dentine are two layers of corpus-
cles. The nearest are long cylindrical bodies whose oval nuclei
are distant from the dentine. Wedged in between them, and
forming a layer intermediate between them and the pulp, are
peculiar branched corpuscles of a spindle or pyramidal shape.
According to Klein, these latter send processes into the den-
tinal tubules, while, according to Waldeyer and Boll the odon-
toblasts send the fibres, and are also connected to one another
by lateral processes. The pulp tissue is very rich in non-
medullated nerves ; their prolongations penetrate between the
odontoblasts, but it is a matter of question whether they enter
the dentinal canals.

Capillaries are abundant and form close networks in the
pulp. The lymphatics are said to accompany the blood-vessels
and to be surrounded by endothelial sheaths.

Development of t?ie teeth. Waldeyer, whose views on the
teeth are the most complete and satisfactory extant, makes the
following succinct statement :

"The anatomical model of a tooth of a vertebrate animal
is a large papilla of the mouth or of the pharyngeal mucous



106



MANUAL OF HISTOLOGY.



membrane, which in consequence of chemical and histological
conversion of its constituents has acquired a remarkable degree
of hardness, and according to whether the connective- tissue
substance of the papilla participates in the hardening or not,
two large groups of teeth are distinguished dentinal teeth and
horny teeth. The horny teeth are by far the most simple in





FIG. 40. Vertical section of the inferior maxilla of a hu-
man foetus, measuring 11 ctms. from the vertex to the. coccyx.
Magnified 25 diameter?. 1, dental groove ; 2, remains of the
enamel germ ; 3, enamel organ presenting externally epitheli-
um, as also where it forms the enamel germ of the papillae of
the dental sacculus ; 4, secondary enamel germ : rudiment of
the permanent tooth ; 5, dental germ : 6. lower jaw ; 7,
Meckers cartilage. (Waldeyer.)



FIG. 41. 1, various forms of
odontoblasts, with the three kinds
of processes ; 2. three enamel
cells, with a few cells of the stra-
tum intermedium attached ; 3, an
enamel cell, with a hmall portion
of enamel ; 4, fragments of ena-
mel fibres from young and soft
enamel ; 5, old '.-namel fibres with
transverse Ptrias and rounded ex-
tremities. (Waldeyer.)



their structure. They appear as more or less developed papil-
lae covered with a thick horny investment. They are never
continuous with portions of the skeleton, but constitute the
transition to other horny formations, as hairs, stings, etc."

"In the dentinal teeth the connective-tissue matrix of the
papillae plays a most important part in the hardening process,
which here proceeds in a manner precisely similar to the ossi-
fying process, except that no true bone is formed, but only an
allied substance, of much harder consistence, and differing
more or less in histological structure, termed dentine. The epi-
thelium of the tooth papillae either atrophies to a rudimentary



THE TEETH.



107



horny investment the cuticula (membrane of the enamel) or
it becomes elongated in a remarkable manner into long, petri-
fied prisms, which collectively invest the dentine and are
known as the enamel."

Preparations for the development of the teeth take place at
a time when the epithelium of the mucous membrane of the
mouth is found growing downward, like a solid peg, with a
rounded extremity.

This has been called the primary enamel organ. As a next
step, the material which is to give form to the tooth pushes
upward as a papillary growth, and meeting
the epithelial peg, pushes in or invaginates
its rounded extremity. This is the tooth
papilla, and as it pushes upward ihspri-
mary enamel organ becomes the secondary
enamel organ, or the enamel cap. We have
now two tissues which are embedded in the
soft embryonic substance, that happens at
this early period to be gelatinous. That
portion of it immediately surrounding the
papilla and cap is called the tootJi-sac.

The papilla, which becomes highly vas-
cular, is covered, on its outer surface, by
the odontoblasts, a layer of columnar epi-
thelial corpuscles, which elongating, are
transformed directly into the dentinal sub-
stance at their outer extremity.

According to Kolliker and others, they
excrete the dentine. The former view seems
to have the most weight of argument in its
favor, but it seems less likely that the odon-
toblasts both make the matrix and send
fibres into the tubulse. The view of Klein
already given seems to be preferable, and in conformity with
what we know of other connective substances.

The separation of the tooth-sac from the mucous membrane
is effected by the gelatinous tissue, which, gradually closing in
the neck of the sac, finally cuts it off. The epithelium of the
enamel cap is abundant and of various kinds ; into it push a
number of papillary processes downward from the gelatinous
tissue. Later the enamel cap is changed into three membranes.




FIG. 42. Longitudinal sec-
tion of a milk tooth from the
foetal sheep, carried through
the margin of the dentine
pulp and adjoining portion of
the enamel organ. Magni-
fied 200 diameters. 1, dental
sacculus; 2, external epithe-
lium and stratum interme-
dium here united to the in-
ternal epithelium or enamel
cells ; 3, after the disappear-
ance of the enamel pulp ; 4,
young layer of enamel de-
tached from the enamel cells ;

6, dentine ; 6, odontoblasts :

7, part of the dentine pulp.
(Waldeyer.)



108 MANUAL OF HISTOLOGY.

The middle membrane is a peculiar cellular network, formed
by the transformation of the middle epithelium layer into a
network of cells, below which there is a deposit of a hyaline
material. The inner membrane is formed of cylindrical epithe-
lial bodies, which are called enamel-cells ; outside of them are
one or more layers of polygonal cells ; they form the stratum
intermedium of Hannover. The outer membrane is composed of
several layers. Finally, the middle membrane disappearing, the
outer and inner membranes are brought into close apposition.

Development of the enamel. This is formed by the enamel-
cells (inner epithelium, Kolliker), presumably in the same
way as the dentine by the odontoblasts. There is a direct con-
version of the outer extremities of the enamel-bodies into en-
amel. Kolliker, Hertz, and Kollmann, however, regard the
enamel as an excretion from the enamel-cells. The former
view appears the more natural, especially as the enamel-prisms
are continuous with the enamel-cells, having the same form and
shape. The successive stages of growth, it is believed, give
rise to the transverse markings.

Whether or not, in the interstitial substance of the enamel,
there are corpuscular elements (Boedecker), is a matter that
will require further investigation. The outer membrane even-
tually gives rise to the cuticle covering the enamel.

The development of the cement takes place precisely as
bone is produced, viz., from the periosteum, or, which is the
same in this instance, from the fibrous tissue of the tooth-sac,
the periodontium.



BIBLIOGRAPHY.

The following systematic works and journal articles may be consulted :
RETZIUS. Miiller's Archives. 1837.
NASMYTH. Med.-Chir. Trans. Vol. 22. 1839.
KftLLiKER. Man. of Human Histology. 1853.
WBNZEL. Arch. d. Heilkunde. 1868.
HENLE. Anatomic. 1871.

STRICKER. Manual of Histology. Am. Ed. 1872.

TOMES. Manual of Dental Anatomy, Human and Comparative. Lond., 1876.
OWEN. Comparative Anat. and Phys. of Vertebrates. 1866.
BOEDECKER. Dental Cosmos. XXL, 409 416. Phil., 1879.
HEITZMANN. Microscopic Anat. of Human Teeth. Med. Rec., N. Y., 1879. XV.,

187.
KLEIN. Atlas of Histology. 187980.



CHAPTER IX.

GENEKAL HISTOLOGY OF THE NERVOUS SYSTEM.

WE may gain a clear conception of the nervous system in
its general outlines by remembering that it consists essentially
of a series of delicate cords which, on the one hand, proceed
from the nucleated bodies of the gray matter, conveying voli-
tional impulses to the periphery of the organism ; or, on the
other hand, of sensitive peripheral extremities that take up the
impression of external objects and carry them back to the cen-
tral gray substance.

In either case both the conducting cords and the central