The internal anatomy of the face

Preface To The Second Edition

Many years ago the author began to investigate the variations from the standard typical anatomy of the text-books which constantly presented themsehes in his surgical practice. During the past twenty years hundreds of skulls have been sectionized and studied. This investigation completely overturned the author's conception of what was meant by the term typical. There is, doubtless, a typical or typal form for each bone, but it is not often found in nature. If we were to photograph a thousand temporal bones, for example, and make a composite of the entire number, the composite would properly be accepted as figuring the typal temporal. It is possible, though doubtful, that of the thousand bones, two or three could be found which would exactly correspond with the typal bone so pictured. This, in the writer's view, is strong testimony that the typal bone is ideal; that the actual is a \'ariant. It is with these variants that the surgeon and dentist have practically to deal.

The author's investigations of the anatomy of the head have convinced him of the need for similar systematic study of the anatomical structure of the other parts of the body. For it can scarcely be doubted that the departures from the normal noted in the bon\- structures of the head and face will be found associated with equal variations of the other structures. It further shows that the text-l)ook by itself is insufficient for the thorough study of anatomy; that the only authentic book of anatomy is the body itself; that, therefore, the use of text-books must be supplemented by the intimate stud\- of the body.

In the preparation of this edition the text has been thoroughly and carefully revised to meet the requirements of those making special studies upon, or operating in, the region of which it treats. New vi PREFACE TO THE SECOND EDITION

matter has been added to the extent of about i8o pages, including chapters on the teeth with their nerve and blood supply; on the distribution of the trigeminal nerve; on the uses of frozen sections; on the inter-relations between the nasal cavity and its accessory sinuses and cells. Wide and narrow dental arches have been further considered, and an extended chapter has been added in which impacted teeth, modern, ancient and prehistoric skulls and teeth have been compared and noted. With certain modifications the Basle nomenclature has been generally adopted.

The writer desires to acknowledge the encouragement and assistance received from Dr. Thomas C. Stellwagen, Dr. Edward C, Kirk, Dr. Arthur Hopewell-Smith, Dr. Herman Prinz, Dr. Rodrigues Ottolengin, Dr. Truman W. Brophy, Dr. Robert H. Ivy, Dr. A. H. Ketcham, Dr. James D. McCoy, and many others who have kindly furnished specimens or other material which have aided so materially in the preparation of this volume.

Matthew H. Cryer.

Philadelphia, 1916.

CHAPTER I. Introductory

CHAPTER H.

General Considerations

Development of the Face

20 21

CHAPTER HI.

The Mandible or Lower Jaw Inflammatory Changes Necrosis and Regeneration .

Fractures of the Mandible

Neuralgia

Secondary Deposits

IVIandibular Triangle

The Mandibular Articulation

23 48 49 59 64 64 67 68

CHAPTER IV.

The Maxilla 76

Studies of Certain External Surfaces of the Skull 81

General Comparisons between the Width of the Upper Dental Arch, the Floor of

THE Nasal Fossa, and the Size of the Maxillary Sinus 94

CHAPTER V.

The Teeth 101

ICruption of Teeth 118

Various L.lustrations of Jaws and Teeth 120

The Vascular Supply of the Teeth 134

The Sensory Nerve Supply of the Teeth and Face 139

Sympathetic Ganglia Connected with the Trigeminal Nerve 154

Local Anesthesia of the Teeth, Surrounding Tissue, and Parts of the Face Presided over by the Trigeminal Nerve 163 viii COX TEXTS

CHAPTER VI.

Teeth Influenced by Irregular Eruption 166

Retarded Eruption or Impacted Teeth 166

Supernumerary Teeth 173

CHAPTER VII.

The Xasal Cavity and its Accessory Cells and Sinuses .... 196 Pathological Condition of the Nasal Cavity 208

CHAPTER VIII.

The Maxillary Sinus 217

CHAPTER IX.

The Frontal Sinus 248

CHAPTER X.

The Ethmoid.\l and other Cells which have Their Final Outlet in the Nasal Cavity 259

The Orbit.\l Processes 260

The Sphenoid.\l Sinuses 260

Cell of the Crista Galli 262

CHAPTER XI.

Variations in the Anatomical Structures of the Face .... 263

CHAPTER XII.

The Relation between the Mouth, Tongue, Pharynx, and Nasal Chamber . 285

CHAPTER XIII.

MODIFIC.VTION OF THE NORMAL ShAPE OF THE BONE THROUGH ABNORMAL FoRCES . 304

Supernu.merary Premolar Teeth 323

CHAPTER XIV.

The Influence of Muscular Action 330

CHAPTER XV.

Hypertrophy of the Gums and Alveolar Process 342

CHAPTER XVI.

The Relation of the Two Jaws 345

THE IXTHRXAL ANATOMY 01- Till- FACR.

CHAPTER I.

INTRODUCTORY.

Carefully conducted studies of numerous dissections prove conclusively that many of the stereotyped descriptions of the internal anatomy of the face are not justified by the facts; and that, therefore, the hard and fast rules for surgical procedures founded on these descriptions do not adequately cover the ground. In pursuance of these studies, hundreds of sections of the facial region have been cut and examined. The lesson they teach is that the accepted descriptions are to be received as only general truths, and that the>' cannot be depended upon or followed literally as a guide for the surgeon or dentist. The results of these investigations afford a basis for the explanation of the failure of many operations conducted on the lines of the accepted anatomical descriptions; such failures have been regarded as merely the natural percentage of unsuccessful treatment; whereas, they have probably been due to variations in the parts clearly within the limits of normality. This will be demonstrated in the following pages by many illustrations.

Anatomical Variations. — Xo man who spends any considerable portion of his time in the study of anatomy — that is, in actual dissections— can fail to note how great is the number of anatomical variations which he meets. So common are these that it cannot be said with exactness what are typical and what are atypical conditions. In other words, anatomy as a study is not to be classed among the exact sciences. It is not meant by this that there is not such a basis of anatomical science that general rules cannot l)e laid down, but the more closely the subject is studied the more variations as to details

IS INTRODUCTORY

are recorded. From the mandible of an animal, a femur, or even a tarsal bone, the nature of the associated bones, their sizes, positions, and forms can be satisfactorily deduced. Admitting this, however, there are still as many variations in the internal anatomy as there are differences in the external appearances. Especially is this true of the anatomy of the human head, as it is modified by climate, race, age, disease, occupation, and many other conditions.

Climate. — Climate and environment have a great influence in modifying the development of the bones of the head, as is demonstrated in the differing formations of the skulls of the great races of the world; and more markedly in branches of the same race living under diverse climatic and social conditions.

Age. — The changes produced by age are very marked. The skull consists of bones of both cartilaginous and membranous origin. In the fetus and infant these bones are soft and yielding; they receive deposits of certain salts of calcium, becoming harder and harder as age advances until the degeneration of senility sets in. In the jaws, constant changes are caused by the development, eruption, and loss of teeth and the consequent alterations of the alveolar process.

Disease. — Disease causes profound changes in the bony structures as well as in the other tissues of the body (see Fig. 45). In the presence of some disorders of the nutritive system — such for example, as rickets, either the bones may fail to become infiltrated with a sufficient quantity of lime salts, which would have the effect of leaving them soft and yielding; or, on the other hand, an undue proportion of calcareous material may be incorporated into the bones, with the opposite effect of making them hard and unyielding, thus modifying the physiological functions with which they are concerned.

Occupation, Diet, etc. — Occupation will modify the shape and character of the face and head, especially in youth. Those persons who are studious, and pass an indoor life, are likely to have a more delicate development of the face, with a larger brain-case than those who are brought up to a laborious outdoor life. The comminution of coarser foods will develop the muscles of mastication and their bony attachments. Numerous other facts might be cited to show the

DIAGSOSTIC IMPORTAXCK 19

influences of personal habit upon the course of anat(^mical development.

Asymmetry. — There are also variations in the same individual in the shape, size, and markings of the two sides of the face. In the bilateral bones such as the frontal, sphenoid, vomer, ethmoid, and mandible, one side is usualh' found to differ fnjm the other. In the homonymous bones, as the maxilla?, the malar, the lachrymal, the turbinate, and the palate bones, the same variations are observed. This being the case, it will be readily understood that the internal openings and spaces, viz., the mouth, the nasal chambers, the orbits, the maxillary-, frontal, and sphenoidal sinuses, the ethmoidal and other cells, will differ accordini;ly.

Diagnostic Importance. — It is clear that variations of the nature referred to must have a direct bearing on the diagnosis of morbid conditions for which there is no evident explanation, and even more so on the performance of operations for their relief. A knowledge of these variations will point the way to an understanding of many otherwise obscure and doubtful lesions. It will also show why, for example, following stereotyped ideas, the surgeon seeking to open into the antrum will occasionally enter the nasal cavity instead. It would seem that to the surgeon, and more especially to the dentist, such information is a necessity.

The main object of this volume is to present a digest of these revealed facts relating to the internal anatomy of the face — facts which have an important bearing on all surgical operations in\olving this region, and especially on the work of the dentist and the rhinologist. With this in view, the aim will be to call attention to misconceptions of the actual conditions; to correct errors which, having found currency, have been commonly accepted and more especially to enforce the idea that a sla\ish following of typical descriptions is likely to lead to disaster.

Chapter II. General Considerations

Anatomical Structures. — The anatomical bony structures of the facial region include the framework, superficially, of the external face and, more deeply, the walls of the various cavities and air spaces of the internal face. As with other bones, they consist of a cortical outer wall inclosing cancellated tissue, the latter being extremely fine and delicate in many cases, in some instances becoming so attenuated as to be almost lost. The exterior cortical parts are covered with a true periosteum, while the interior surfaces, those looking toward the internal cavities, as the mouth, nasal cavity, the frontal, maxillary, and sphenoidal sinuses, and the ethmoidal cells, are covered by a mucoperiosteum. From these characteristics, the former are known as non-mucous, and the latter as mucous or mucoid surfaces. It is important to consider the difference in these surfaces in the treatment of some of the diseases of the bones.

The dense exterior or non-mucous surface is roughened at various points by the actions of the attached muscles. The exterior cortical portion varies in thickness according to the amount of work to which it is subjected or the protection it has to afford. The greatest thickness is found in the mandible, the active bone of mastication, which occupies a position in the face where it is peculiarly exposed to the effects of external forces, such as blows, etc.

The inner or mucous surfaces, while dense and compact, are thinner, smoother, and more delicate. They are marked by depressions for the lodgment of the mucous glands, by grooves for the lodgment of the nerves and vessels, and also by elevations due to the attachment of the muscles.

Cancellated Structure. — The cancellated tissue found between the plates of cortical Ijone varies in thickness and compactness according

DEVEWPMEXT OF THE FACE 21

to the (IcMisit)', the i)()siti()n, and the functions of tlu' bone. '1 he arrangement of the trabecuUe is an interlacing network. T(; give bulk to the bone where required, and to diffuse shock, constitute the functions of the cancellated tissue. Through it pass the nerves and vessels to supply local structures and, by means of bon\- canals or tubes, the more distant parts. The bones of the head contain man\' canals and foramina for this last-named i^urjDose, thus differing materially from the other bones of the body. This is a fact of surgical importance. For when these bones have become altered either by the breaking down of the tissue or by abnormal growths encroaching uj^on the foramina or canals, the functions of the nerves and vessels are interfered with, thus affecting not only adjacent tissues, but parts of the face and body remote from the seat of the lesion, causing abnormalities in the area of distribution, as atrophy, neuralgia, etc.

DEVELOPMENT OF THE FACE.

The bones of the brain case are in an advanced stage of development before the facial bones commence to be built. To such an extent is this so that the dermoid structures are nearly in contact with all of that portion of the head below and anterior to the notochord. At this time there is no opening into the alimentary canal.

The facial bones arise from the under surface of the brain case, from certain processes that push outward and downward, leaving a layer of dermoid tissue on their inner as wxll as their outer surfaces. This dermoid tissue becomes the mucous or epidermal lining of the mouth, the nasal cavities, and all internal surfaces of the face.

It is also from this dermoid tissue that the teeth and alveolar i)r{)- cesses take their origin, and not from the developing buds or processes that form the remainder of the bones of the face; so that when the teeth are lost and there is no function for the alveolar ]:)rocess to perform, it is also lost. It is for this reason, also, that when the dermoid tissues are attacked by systemic diseases such as syphilis, scarlet fever, etc., the teeth and alveolar processes are involved simultaneously with the skin, as are also the bones of the face, which were covered

22 GENERAL CONSIDERATIONS

originally with dermoid tissue. It is quite possible that pyorrhea alveolaris may also be a manifestation of a dermoid disease.

When the dermoid appendages are entirely lacking at birth, and do not develop later on, the alveolar process will also be lacking.

The processes in front which pass down and forward are called the frontonasal; those on the side, the maxillary and mandibular; those situated deeply within the face are known as the spheno-ethmoid prolongations. The general tendency of these buds, forming the upper and lower jaws is to send processes toward the median line w'hich form a union with their fellows of the opposite side. The bonds of these unions vary accordingly to circumstances depending upon their position, function and the age of the individual.

CHAPTER IT I.

thp: mandible or lower jaw.

Thk niaiuliblc dcxelops Ironi the first pair of the \iscc-ral or branchial folds called the mandibular plates, which in early embryonic life advance from the sides of the base of the cranium and meet al the median line, forming the symphysis menti.

The mandible is symmetrical in its general shape, although one side may and usually does differ from the other. It presents for study r-v c D

Fig. 1. — Four mandibles ranging from birth to eighteen months: .4, at birth; B, at three months;

C, at six months; D, at eighteen months.

a body which is horizontal in direction, with two rami extending upward to the articulation in the anterior portion of the mandibular fossa of the temporal bones. The angle (gonion) formed by the union of the lower border of the jaw and the posterior border of the ramus, varies considerably- at different periods of life. Eigs. i, 2, 3 and 4 are views of the external cortical surfaces of the normal lower jaw at various ages, showing progressive changes in the angle between the rami and the body of the bone as life progresses. At birth {A, Fig. i) the angle is very obtuse, but as the teeth develop and erupt, it becomes less and less obtuse until about the time the last of the permanent teeth are erupted it is almost a right angle, as shown in Fig. 2.

Fig. 2. — Side view of a typical mandible at maturity.

As the muscles grow and increase in strength, that portion of bone to which they are attached also increases in size, giving the ramus a square appearance, especially in strong muscular persons (see Fig. 8).

After full maturity, as time passes, the muscular action becomes less and less, and the tuberosities, etc., throughout the body become

Fig. 3. — Mandible of aged person, showing a great change from that of adult life.

smaller. The same condition takes place at the angle of the jaw, and it becomes more obtuse as age advances. When the jaws can be kept apart with good teeth and normal, complete mastication, there is not so much change in the angles.

THE MAXDIRLE OR LOWER JAW l^o

As the tc'C'lh l)t'(()nu' abraded, or when they are extracted, the alveolar process is resorbed, the horizontal i)lanes of the jaws api)roach each other more closeK', and the an^le ai^ain becomes obtuse (see Fig'. 3).

The teeth and their alveolar i)rocesses are placed on the ui)i)er portion of the l)()d>- of the bone. The third molar is partially posterior to the anterior margin of the ramus, and a line can l)e drawn between the first and second premolars to the mental foramen.

Fig. 4. — Mandible of an aged person showing the mental foramina in the top of the body of the jaw.

The incisor teeth should have neither lingual nor lal)ial inclination. Fig. 2 shows a fairly typical mandible: as is also shown in the skull marked Fig. 77.

The mental process gives prominence to the chin and lower part of the face. It belongs to man only, and is always associated with the genial tubercles. In the monkey there is a depression instead of the tubercles. The space between the halves of the body of the mandible in the region of the premolar teeth is much wider in man than in the other mammalia, thus giving more freedom for the action of the tongue. The mental process, the genial tubercles, and the freedom of the tongue are severally and collectively concerned in the production of articulate speech. The writer also considered that the presence of a well-developed mental process contributes to a greater command of articulate speech than can be attained by individuals in whom it is small or underdeveloped; and as it extends outward beyond the line of the teeth, it — the mental process — gives the origin for the muscles that control the action of the lower lip, and when properly developed holds the latter in position to conform with the upper lip, thus perfecting the labial sounds.

In considering the manner in which deaf people can communicate orally by observing the action of the lips, and the fact that the deaf and blind can read what is being said, merely by gently placing the fingers upon the lips, one is inclined to the opinion that the lips are an important factor in the communication of thought as well as in the production of articulate speech.

The orbicularis oris muscle and its associates must have great freedom and power of action in order to produce the numberless varieties of motion necessary to this end. The facial nerve thus becomes one of the nerves of speech, as it not only controls the lips, but also the cheeks and some of the muscles of the soft palate.

In order that these oral muscles shall have a free and balanced action, the lower portion of the orbicularis and its associated inferior muscles must be carried forward to a line equal to that of the upper portion, thus as before mentioned, the mental process also becomes an important factor in articulate speech.

Mandibles of Different Races. — In comparing mandibles of different races and also of the same race great variations in general characteristic shape and size are found. The following figures illustrate a few of these variations:

Fig. 5 is a side view of an Indian mandible found in one of the buttes in western Kansas. It is a very powerful jaw, the angle is almost acute, 1 06 degrees, the distance from the condyloid process, which acts

MANDIBLES Of DIFFERENT RACES as the fulcrum, to the coronoid i)rocx^ss, the place of attachment of the temporal muscle, is 50 nun., which i^ives i^reat jiower to the mandible.

50 MM

Fig. 5. — A'powerful mandible of an Indian found in buttes of western Kansas. (Loaned by

Dr. Ketcham.)

Fig. 6. — A mandible of a South African negro, a member of the Fan tribe.

Fig. 6 is from a mandible of a South African negro a member of the Fan tribe (see Fig. 78). The body is fairly typical, except that the angle is nearly a right angle. The characteristic features of this mandible are caused by the position of the teeth and their alveolar processes, which are set forward on the body of the jaw. It will be

Fig. 7. — A mandible from a skull, showing prognathous jaws. (See Fig. 79.)

noticed that the third molar is in advance of the ramus about the width of a molar, and that a line drawn downw^ard between the premolar teeth would pass across the body of the jaw a full width of a molar tooth in advance of the mental foramen. The canine and

Fig. 8. — A manclible of a heavy skull. (See Fig. 80.)

incisor teeth are placed in front of the jaw proper, with considerable labial inclination of the incisors. The mental process is not so promi-

MAXDIBLES OF DIFFERENT RACES nent as in Fig. 2, due in part lo the carrying f(jr\varcl (jt the teetli and alveolar process.

Fig. 7 is made from a skull (see Fig. 79) showing prognathous jaws. In Fig. 6 the prognathism is evidently caused by the malposition of the teeth and their alveolar processes upon the body of the bone. In Fig. 7 the principal cause of prognathism is in the relation of the ramus to the body, which carries the body of the jaw so far forward that the mental process is much more prominent than in Fig. 6 and the anterior teeth have lingual inclination.

Fig. 8 is made from the mandible of a very heavy skull with massive teeth in excellent alignment (see Fig. 80) and when articulated

Fig. 9. — A mandible of a peculiar skull. (See Figs. 363, 364 and 365.)

with its skull there is fairly good occlusion. The marked feature of this mandible is the relation of the ramus to the body of the bone; its external angle is nearly a right angle, being 103° on the right side and 100° on the left.

Fig. 9 is made from a mandible of a ])eculiar skull (see Figs. 93, 363. 364 and 365). It is a great contrast to the mandible shown in Fig. 8. The angle of the ramus with the body of the. bone is 139°. Age should be taken into consideration in this comparison, as the angle usually increases as age advances.

Fig. 10 is made from the mandible of a skull ha\ ing a \ery flat face (see Fig. 82). There is a slight anterior occlusion. The angle of the jaw is obtuse — viz., 133 degrees.

Fig. II is made from the mandible of a Chinese skull (see Fig. 8i). The rami are rather square and unite with the body of the bone at an

Fig. 10. — A mandible from a skull of a flat-faced person. (See Fig. 82.)

angle of io8°, which makes the mandible short. On the left side of the mandible there is an impacted lower third molar.

Fig. 11. — A mandible of a skull of a Chinese. (See Fig. 81.)

The following diagrams, each reduced one-half, give the measurements and angles of the mandibles in Figs. 2, 6, 7, 8, 9, 10 and 11:

MANDiniJ'S OF nil- F EKES T RACES

'M

124* ANGLE 75MM

Fio. 12.— Mandible, Fig. 2.

M ENTAL FORAMEN

MENTAL FORAMEN

MENTAL FORAMEN

I'U;. 18.— Mandible, Fig. 11.

The following diagrams illustrate the triangles of mandibles in skulls of various ages from eight months to old age:

ISO* 42 n M Fig. 19. — Age eight months.

^lT 48 MM

Fig. 20. — Age fifteen months

Fig. 21. — Age eighteen months.

Fig. 23. — Age four years.

Fig. 25. — Age seven to eight years.

so MM

Fig. 22. — Age two years.

Fig. 24. — Age five years.

Fig. 26. — Age eight to nine years.

MS" 70MM

Fig. 27. — Age thirteen years.

Fig. 28.— Age eighteen years.

77 r-iM

Fig. 29.— Adult. Fig. .^0.— Old age.

Fi(;s. 19 to 30. — Diagrams of mandible, reduced one-half.

THE BODY OF THE J AW

The Body of the Jaw. — The body of the- jaw in Iransxcrsc section, shows a r-sliai)C'(l cortical or dense l)()n\- strnctnrc, tlic amis of the U terminating in the pkUes of the alveoku- process — outer and inner — which are composed of a mocHhed cortical bone with no dehnite line of demarkation between them and the body of the bone proper; the

Fig. M. — Anterior lateral view of upper and lower jaws, with the external cortical portion of bone covering the roots of the teeth removed, exposing the cancellated tissue, the roots, and the cribriform tube. (Mandibular canal.)

function of the body, however, is quite different from that of the

alveolar process.

The bone projXT is covered with a true periosteum, the alveolar

process with mucoperiosteum. the latter being thick and dense and

containing many mucous glands. It is commonh- known as gum 3 tissue. The space between the arms of the U is filled with fine trabeculae forming the cancellated structure. The roots of the teeth are imbedded within this cancellated structure, each root being surrounded by thin, compact bony tissue. Lamina dura,Mvhich approaches the cortical bone in density, but is cribriform (sieve-like) in character (see Figs. 31, 32 and 34).

Fig. 34 is an upper view of the mandible with the teeth removed, showing single sockets for the ten anterior teeth and double sockets for the six molars. The shapes of the sockets as shown correspond

Fig. 32. — Mandiljle wilh the cortical portion of bone removed from the body.

with the transverse section of the various teeth at the level of the margins. The septa between the sockets are cribriform in character.

Cribriform Tube. — Through the cancellated tissue passes the mandibular canal, which is, however, more accurately described by the term, "cribriform tube of the mandible." The function of this tube is to afford a protective passage for the mandibular nerve and the bloodvessels.

The cribriform tube passes downward and forward from the man-

' A. Hopewcll-Smith, Dental Cosmos, August, 1913, p. 769.

CRIBRIFORM TUBE dibular furanien, at first along the inner cortieal portion, then, after it leaves the ramus, gradually crossing over through the cancellated tissue toward the outer cortical portion and downward toward the border of the I -shaped space. As it ai)proaches the mental foramen, its course is near the outer cortical portion and along the lower border of the cancellated tissue, passing beneath the foramen to its termination near the roots of the incisor teeth. This tube can be removed from a normal jaw or isolated as shown in Fig. 33, taken from a specimen in which the cortical and cancellated tissues have been cut away,

Fig. 33. — Cribriform tube (mandibular canal) of the lower jaw isolated.

exposing the cribriform tube. Figs. 31, 32 and T,2t show that the cribriform tube is an independent structure, not merely a canal through the bone. In Fig. 31 it will be noticed that a portion of the outer wall of the tube has been removed, while in Fig. 32 the wall is left intact, showing its tubular form. The outer wall of the tube in the region of the second and third molars is extremely well shown in Fig. 31.

As the tube passes along the jaw its cribriform character becomes more and more marked until, beneath the first molar tooth, it becomes so opened, probably by a sort of stretching process coincident with the growth of the bone, that the tube-Uke formation is almost lost, as is well shown in Fig. 31. Further forward it again resumes its original character. This main cribriform tube gives off lesser branch tubes which afford passage for the nerves and vessels to the substance of the bone; also, in more or less curved course to the roots of each tooth. The branch tube for the accommodation of the nerves and vessels

Fig. 34. — View from above of mandible from which all the teeth have been removed, showing the cribriform character of the septa of the sockets of the teeth.

to the mental foramen, is usually given off slightly anterior to the foramen, passing backward from the main tube to the foramen. This is almost invariably the rule — namely, that the tube to the mental foramen is in the form of a return or recurrent canal, mental canal, though occasionally it passes from the main tube as it approaches the foramen. The recurrent tube is well shown in Figs. 31 and 32. In the former, the anterior wall of the mental foramen has been cut