In the hand book of facial growth, Donald Enlow proposes a series of anatomic concepts that account for the plan of the human face . Three of those concept are examined in this article , which presents measurements of 253 adult female primates from thirty-two species. As part of a system of craniofacial counterparts, Enlow proposes that the breadth of the mandibular ramus should equal the breadth of the pharynx.
The relationship between ramus breadth and PNS-Ba in the primates studied strongly supports this hypothesis. A second concept tested concerns the relationship between prognathism and maxillary arch length.
The relationship is not as strong as in the previous case, and some species are more prognathic than required for the size of their dentition. It is concluded that arch length and prognathism have an important biologic relationship but that two features can vary with some independence.
The 3rd concept tested is the relationship between prognathism and interorbital breadth.
Here the relationship is weak, and it is concluded that the interorbital breadth it is not significant in setting a structural limit to the amount of facial prognathism.
1. The Plan of the Human Face
ACHIEVED BY: Dr.Maen Dawodi
2. The Plan of the human face :
A test of three general concepts
3. In the hand book of facial growth, Donald Enlow proposes a series of anatomic
concepts that account for the plan of the human face .Three of those concept
are examined in this article , which presents measurements of 253 adult
female primates from thirty-two species. As part of a system of craniofacial
counterparts, Enlow proposes that the breadth of the mandibular ramus
should equal the breadth of the pharynx.
The relationship between ramus breadth and PNS-Ba in the primates studied
strongly supports this hypothesis. A second concept tested concerns the
relationship between prognathism and maxillary arch length.
The relationship is not as strong as in the previous case, and some species are
more prognathic than required for the size of their dentition. It is concluded
that arch length and prognathism have an important biologic relationship but
that two features can vary with some independence.
4. The 3rd concept tested is the relationship
between prognathism and interorbital
breadth.
Here the relationship is weak, and it is
concluded that the interorbital breadth it
is not significant in setting a structural
limit to the amount of facial prognathism.
5. • In Chapter 4 of the Hand book of
Facial Growth, Enlow Lists ten
concepts provide a basis for
explaining much of the anatomic
and functional organization of the
human face and the reasons for
differences in facial morphology
among mammals.
• While all of the concepts appear
reasonable and can be supported
by illustrations, many have never
been tested .The purpose of this
study is to use the comparative
morphology of non human
primates to test three specific
principles proposed by Enlow
6. • The first specific test concerns the relationship
between the anteroposterior dimensions of the
pharynx and mandibular ramus.
• As part of a more general discussion of
craniofacial counterparts, Enlow relates these
structures as follows
The Pharynx relates specifically to the middle cranial
fossa. Because of the human cranial floor flexure, the
size of the middle cranial fossa in man determines
the horizontal dimension of the pharyngeal space .
The dimensions of the middle cranial fossa should be
equaled by the breadth of the mandibular ramus.The
function of the ramus is to span the pharynx and
middle cranial fossa in order to place the lower arch
in occasions with the upper.
7. • The second concept to be tested is the
relationship between prognathism and
maxillary dental arch length
Enlow describe this relationship as follows
The reduction of the nasal region
associated with orbital convergence and
olfactory –anterior cranial fossa rotation
must necessarily also be accompanied by a
more or less equal reduction in maxillary
arch length , because the floor of the nasal
chamber is also the roof of the mouth.
8. • Because the Interorbital segment is the root of the
nasal region , a decrease in this dimension reduces
the structural (and also physiologic) base of the
bony nose . A wide nasal base can support a
proportionately longer snout. A narrow nasal base
however reduces the architectural limit to which
the bony part of the nose can protrude and the
snout is thereby shorter
• Finally, Enlow also proposes a structural
relationship between interorbital breadth and
prognathism
9. In order to have a comprehensive and systematic survey
of higher primates , the species selected include one
from every genus of Old World monkeys, NewWorld
monkeys , and apes , except for the small South
American Callithricidae.
MATERIALS & METHODS
Primate skeletal material in the collections of the
American Museum (Smithsonian Institution),The
Museum of Comparative Zoology (Harvard University),
and the British Museum of Natural History were
examined. Measurements from a total of 253 adults
female primates , consisting of 3 to 10 specimens of
each of 32 species , are included in the study .
10. • Adults were defined by the eruption of third molars into occlusion. Zoo specimens and
animals with excessive dental wear were excluded .The following variables were
measured on each specimen.
• The following variables were measured on each specimen:
• 1.Prognathism = Defined as the distance from orbitale to prosthion,parallel to occlusal
• plane.
• 2.Ramus breadth =The anteroposterior dimension of the ramus at the level of the
• occlusal plane.
• 3.Interorbital breadth =The minimum distance between the medial borders of the orbits.
• 4.PNS-Ba = Posterior nasal spine to basion
• 5.Maxillary arch length = Measured in the midline from the incisal edge of the maxillary
• central incisor to a line connecting the distal marginal ridges of the
• left and right third molars
11. RESULTS
• The mandibular ramus as a counterpart to the pharynx
• For all 32 species , the mean breadth of the mandibular
ramus is 26.1 mm, and the mean distance from PNS to
basion is 37.9 mm , a difference of 11.8 mm.The smallest
difference between measurements is 7.3 mm. for
Mandrillus, and the largest is 19.5 mm for Pongo
(orangutan) . In 23 of the 32 species the PNS-Ba dimension
is between 8.5 and 13.5 mm larger than the breadth of the
ramus .
12. RESULTS
• For the 32 species ,the
correlations coefficient
between ramus breadth and
PNS-Ba is 0.97 when the two
variables are plotted against
each other (Figure 1 shown)
• All points are rather close to
the regression line. When
PNS-Ba is used to predict
ramus breadth , no species is
more than 20% from its
expected value and all but 6
are within 10% of what would
be predicted by the
regression equation.
13. • Prognathism and maxillary arch length
• The mean length of the maxillary tooth row is 44.5 mm.,
and the mean prognathism is 23.9 mm , a difference of
20.6mm.The smallest difference between prognathism and
arch length is in the baboon , Papio anubis , in which the
tooth row is only 3.4mm greater in length than the
prognathism.
• When oriented parallel to the occlusal plane, almost the
entire dentition of this species is in front of the orbits .The
greatest difference between arch length and prognathism is
36.6 mm in the gorilla. For this species , about 40% of the
maxillary arch is under and behind the orbits , rather than
projected infront of them.
• The correlation between arch length and prognathism for
these 32 species is 0.92 .The regression equation resulting
from an attempt to predict tooth row length from
prognathism is shown in the figure 2 . 14 of 32 species differ
by more than 10% from predicted values
14. • Interorbital breath and prognathism
• The mean value of the interorbital breadth in the 32 species is 8.8 mm, with a range of 3.2
to 24.3mmThe mean prognathism is 23.9mm , with a range of 6.9 to 67.4 mm. Unlike the
previous two comparisons , these two measurement do not bear a constant relationship
to other .
• The species with the greatest interorbital breadth (24.3) is the gorilla, which has a
prognathism of 53.6 mm. However , orangutan , with a similar prognathism of 51.9mm ,
has an interorbital breadth of 11.1 mm, and Mandrillus , with a prognathism of 51.2mm ,
has an interorbital breadth of Callicebus , which has only 7.1 mm of the prognathism.The
most prognathic species Papio anubis , has an interorbital dimension of 8.9 mm , only 0.1
mm. greater than the average for all species
• The correlation coefficient between interorbital breadth and prognathism is 0.46.
Transformation of the data to logarithms , which is often done to improve the fit of this
type of data , increases correlation to 0.56 .
15. • When prognathism is used in a
regression equation to predict
interorbital breadth , 28 out of
32 species are predicted
incorrectly by more than 10%
and six by more than 50%
(figure 3 shown)
16. • DISCUSSION
• As proposed by Enlow , the first concept tested in this study involves a relationship
between
• 3 Structures :
• 1.Middle cranial fossa
• 2.The pharynx
• 3.The mandibular ramus.
• The specimens available to us did not allow any measurement of internal cranial
dimensions , so our test of this concept involves only the pharynx and the ramus
• In human beings , the posterior wall of the pharynx is demarcated by the pharyngeal
tubercles , which are several millimeters anterior to basion.
• In many non human primates , however , these tubercles are difficult to define, and the
angular relationship between the pharynx and the mouth makes the tubercles less
significant as a mark of pharyngeal depth .
• We consider PNS-Ba to represent a reasonable reproducible approximation for pharynx
depth in these highly diverse species.
17. • DISCUSSION cont.1
• For the second and 3rd concepts Enlow interchangeably uses “nasal chamber” , “nasal
region”, “bony nose” ,and“snout” as the feature related to maxillary arch length (concept 2)
or interorbital breadth (concept 3).We have quantified this feature as the distance that the
face protrudes in front of the orbits . Our measurement could be named in several different
ways , including “ prognathism” or snout length , and appears to us to be closely related to
the facial characteristics addressed by Enlow .
• The relationship between the ramus breadth and PNS-Ba reported in this study is a striking
confirmation of the counterpart model of craniofacial morphology as proposed by Enlow and
his co workers .The fact that the ramus dimensions and PNS-Ba differ in absolute size is not
of importance .
• Measurement of ramus breadth varies at different positions on the ramus ; PNS-Ba is not an
exact substitute for the pharynx , and soft tissues would modify the relationship between
skeletal dimensions .
• In addition to the high correlation coefficient , the average difference of 11 t0 12 mm
between ramus breadth and PNS-Ba is usually constant , particularly considering the range
of the skull size for these species
18. • DISCUSSION cont.2
• Callicebus torquatus ,with an average body weight of about 0.7kg. and a skull
length of about 66mm , has a difference of 11.0 mm between PNS-Ba and
ramus breadth.The gorilla at over 92 kg. in body weight and 223 mm. in skull
length , has a PNS-Ba ramus breadth difference of 13.5 mm.
• This degree of similarity is unusual in comparative metric studies over large size
range .
• The relationship between the ramus breadth and prognathism can be cast as
either supporting or contradicting Enlow’s model .
• The correlation coefficient between these two variables is high, and clearly
there is a biologic relationship between prognathism and arch length. Species
with long maxillary dental arches have a strong tendency to be more
prognathic . However , it is also evident that the organization of the face can be
altered in a variety of ways to accommodate a long dental arch.
• Some species are more like Homo sapiens in having dental arch arranged at
least partially “under” the orbits , while others, particularly baboons , have
almost the entire dentition positioned in front of the skull base and orbits .
19. • DISCUSSION cont.3
• There is some evidence of a relationship between prognathism and root size ,but a
simple cephalometric inspection of a prognathic species , such as Papio anubis , indicates
that the degree of prognathism is greater than that necessary to accommodate the roots
of the teeth.(figure shown)
• A consequence of this variability between prognathism and tooth size is that it is difficult
for one to argue that human beings have flat faces because the dentition has become
reduced during the later phases of hominid evolution .
• Differences in the morphology of primate faces indicate that the size of the dentition and
prognathism can evolve with some independence.
20. • DISCUSSION cont.4
• The hypothesis that interorbital breadth limits the size of the nasal region was first proposed
by Enlow in 1968 and was discussed at length by Enlow and McNamara.
• The results of the present study do not support this hypothesis. A correlation of 0.46
indicates that approximately 20% of the variability in interorbital breadth and prognathism is
related .
• Both prognathism and Interorbital breadth are correlated with the overall size of a species. In
comparing different species , we find that most skull dimensions increase as the overall size
of the skull becomes larger.
• The correlation of interorbital breadth with body weight for these thirty –two species is 0.81,
and the correlation of prognathism with body weight is 0.64.Thus , even if prognathism and
interorbital breadth have no functional relationship to each other, it is that they would be
correlated to some extent , because each is correlated to a 3rd variable –overall size.
• For a study using animals that vary more than 100-fold in body weight , a correlation of 0.46
between two anatomically related skeletal features is unusually low.
• There is some evidence , from both comparative morphology and biomechanical modeling ,
that the interorbital region aids in absorbing and transmitting forces of mastication
21. • DISCUSSION cont.5
• Hylander notes that the role of the interorbital region may depend upon detailed
morphologic features .
• If interorbital breadth is wide because of extension of the nasal process of the maxilla, forces
generated at the maxillary teeth could be transmitted to the frontal bone , but if the
interorbital structure consists primarily of the nasal bones , w/o a large nasal processes of the
maxilla , force transmission through this area will be lower.
• In either case , it cab be concluded that the structural base that support a protruding muzzle
is not determined by the interorbital breadth . It may be that prognathic primates can use the
entire width of the face in the orbital region as an architectural base for a snout .
• The reduced interorbital breadth and reduced prognathism of man are features that could
have evolved independently , instead of as structural and physiologic consequences of each
other .
22. • DISCUSSION cont.6
• In interpreting the results of this study , it should be recognized that whether or not the
concepts tested are confirmed depends in part on specific species selected for analysis.
• For example , baboons are unusual in facial form, and a broad survey of different mammals
instead of only primates ( or a study of primates excluding baboons ) might have yielded
different findings .Nevertheless , there is good justification for including a typical species ,
such as baboons , for they result in a more stringent test of the concept under consideration.
• The results also depend upon the precise definitions of measurements . However , the
different versions of essentially similar measurements would be highly correlated with each
other and would produce results with same general implications.
