Surgical applied anatomy

Preface

Applied Anatomy has, I imagine, a twofold function. On the one hand it serves to give a precise basis to those incidents and procedures in practice that more especially involve anatomical knowledge ; on the other hand it endues the dull items of that knowledge with meaning and interest by the aid of illustrations drawn from common medical and surgical experience. In this latter aspect it bears somewhat the same relation to Systematic Anatomy that a series of experiments in Physics bears to a treatise dealing with the bare data of that science.

The student of Human Anatomy has often a nebulous notion that what he is learning will sometime prove of service to him ; and may be conscious also that the study is a valuable, if somewhat unexciting, mental exercise. Beyond these impressions he must regard his efforts as concerned merely in the accumulation of a number of hard unassim liable facts.

viii Surgical Applied Anatomy.

It should be one object of Applied Anatomy to invest these facts with the interest derived from an association with the circumstances of daily life ; it should make the dry bones live.

It must be owned also that all details in Anatomy have not the same practical value, and that the memory of many of them may fade without loss to the competency of the practitioner in medicine or surgery. It should be one other object, therefore, of a book, having such a purpose as the present, to assist the student in judging of the comparative value of the matter he has learnt ; and should help him, when his recollection of anatomical facts grows dim, to encourage the survival of the fittest.

In writing this manual I have endeavoured, so far as the space at my command would permit, to carry out the objects above described ; and while I believe that the chief matters usually dealt with in works on Surgical Anatomy have not been neglected, I have nevertheless tried to make the principle of the book the principle that underlies Mr. Hilton's familiar lectures on "Rest and Pain."

I have assumed that the reader has some knowledge of Human Anatomy, and have not entered, except in a few instances, into any detailed anatomical

Preface. ix

descriptions. The bare accounts, for example, of the regions concerned in Hernia I have left to the systematic treatises, and have dealt only with the bearings of the anatomy of the parts upon the circumstances of practice. The limits of space have compelled me to omit all those parts of the ''Surgery of the Arteries" that deal with ligature, collateral circulation, abnormalities, and the like. This omission I do not regret, since those subjects are fully treated not only in works on operative surgery but also in the manuals of general anatomy.

The book is intended mainly for the use of students preparing for their final examination in surgery. I hope, however, that it will be of use also to practitioners whose memory of their dissecting-room work is growing a little gray, and who would wish to recall such anatomical matters as have the most direct bearing upon the details of practice. Moreover, it is possible that junior students may find some interest in the volume, and may have their studies rendered more intelligent, by learning how anatomy is concerned in actual dealings with disease.

The illustrations have been executed by Mr. Charles Berjeau, F.L.S., to whose skilful pencil I am much indebted. The nicijority of them have been

X Surgical Applied Anatomy.

copied from familiar sources, from the works of Riidinger, Tillaux, Braiine. Agatz, aiid others.

My best thanks are due to Mr. J. Macdoiiald Brown, F.R.C.S., late Demonstrator of Anatomy at the Owens College, and to Dr. James Anderson, one of the Demonstrators of Anatomy at the London Hospital, for kindly assisting me in revising the proofs, and for many valuable suggestions.

September, 1883.

Part I.-the Head And Neck. Chapter Page

I.— The Scalp 1

IL— TiiE Bony Vault of the Cranium . . . .12

TIL— The Cranial Contents 25

IV.— The Orbit and Evfi 33

v.— The Ear 59

VL— The Nose and Nasal Cavities 70

VII.— The Face 82

VIII.— The Mouth, Tongue, Palate, and Pharynx . 100 IX.— The Neck 118

Part ti.

X.— The Thorax « * « . 14'

PAR'J^ III.-THE upper EXTREMITY.

XL—The Region of the Shoulder 162

XIL— The Arm 202 xii Surgical Applied Anatoi\iv.

CHAPTER PAGE

Xni.— The Region of the Elbow 210

XIV.— The Fork-arm .227

XV.— The Wrist and Hand 235

PART IV.-THE ABDOMEN AND PELVIS.

XVI.— The Abdomen 262

XVII,— The Abdominal Viscera 295

XVIII.— The Pelvis and Perineum 334

PART V.-THE lower EXTREMITY.

XIX.— The Region of the Hip 380

XX.— The Thigh 417

XXL— The Region of the Knee 424

XXII. —The Leg . 450

XXIIL— The Ankle and Foot -....,. 461

PART VI.

XXiV.— The Spine 497

Surgical Applied Anatomy.

i3art ]F.

The Head axd Neck,

Chapter I

the scalp.

The soil parts covering the vault of the

skull may be divided into five layers : (1) the skin, (2) the subcutaneous fatty tissue, (3) the occipito-frontalis muscle and its aponeurosis, (4) the subaponeurotic connective tissue, and (5) the pericranium. It is convenient to consider the term " scalp " as limited to the structure formed by the union of the first three layers above named (Fig. 1),

The skin of the scalp is thicker than it is in any other part of the body. It is in all parts intimately-

—d

Fig. 1.— Diagram of a Vertical Section of the Scalp.

a, Skin ; b, subcutaneous tissue, with vessels: c, aponeurosis ; d, sub-aponeurotic tissue ; e, pericranium ; /, bone ; g, dura mater.

adherent, by means of the subcutaneous tissue, to the aponeurosis and muscle beneath it, and, from this

B

4 Surgical Applied Anatomy. [Chap. i.

glove when it is tightly stretched over the knuckles, and those parts are sharply rapped.

The scalp is extremely vascular, and presents therefore a great resistance to sloughing and gangrenous conditions. Large flaps of a lacerated scalp,, even when extensively separated and almost cut off from the rest of the head, are more prone to live than to die. A like flap of skin^ separated from other parts of the surface, would most probably perish ; but the scalp has this advantage, that the vessels run practically in the skin itself, or are, at least, in the tissue beyond the aponeurosis (Fig. 1). Thus, when a scalp flap is torn up, it still carries with it a very copious blood-supply. Bleeding from these wounds is usually very free, and often difficult to arrest. This depends, not so much upon the number of vessels in the part, as upon the density of the tissue through which these vessels run, the adherence of the outer arterial wall to the scalp structure, and the inability, therefore, of the artery to properly retract when divided.

In all parts of the body where a dense bone is covered by a comparatively thin layer of soft tissues, sloughing of those tissues is apt to be induced by long and severe pressure. The scalp, by its vascularity, is saved to a great extent from this evil, and is much less liable to slough than are the soft parts covering such bones as the condyles of the humerus or the sacrum. But such an effect is sometimes produced, as in a case I saw, where the tissues over the frontal and occipital regions sloughed from the continued application of a tight bandage put on to arrest bleeding from a frontal wound.

The pericranium is but slightly adherent to the bone, except at the sutures, where it is intimately united. In lacerated wounds this membrane can be readily stripped from the skull, and often, in these injuries, extensive tracts of bone are laid bare. Tlic

Chap. I.] The Scalp. 5

pericranium differs somewhat in its functions from the periosteum that covers other bones. If the periosteum be removed to any extent from a bone, the part from whence it is removed will very probably perish, and necrosis from deficient blood-supply result. But the pericranium may be stripped off a considerable part of the skull vault without any necrosis, save perhaps a little superficial exfoliation, following in consequence. This is explained by the fact that the cranial bones derive their blood supply mainly from the dura mater, and are therefore to a considerable extent independent of the pericranium. A like independence cannot be claimed for the periosteum covering other bones, since that membrane brings to the part it covers a very copious and essential contribution to its blood supply. This disposition of the pericranium is also well illustrated by its action in cases of necrosis of the cranial bones. In necrosis of a long bone, the separation of the seqxiestrum is attended with a vigorous periosteal growth of new bone, which repairs the gap left after the removal of such sequestra. In necrosis of the vault of the skull, however, no new bone is, as a rule, formed, and the gap remains unrepaired. The general indisposition of the pericranium to form new bone in other circumstances is frequently illustrated.

Abscess in the scalp region may be situated (1) above the aponeurosis, (2) between the aponeurosis and the pericranium, and (3) beneath the pericranium. Abscesses in the first situation must always be small and comparatively insignificant, since the density of the scalp tissue here is such that suppuration can only extend with the greatest difficulty. Suppuration, however, in the second situation (in the loose tissue beneath the aponeurosis) may prove very serious. The laxity of this tissue offers every inducement to the abscess to extend when once pus has found its

6 Surgical Applied Ana tomy. [Chap. i.

way between tlie aponeurosis and tlie pericranium. SupjDuration in this area may undermine the entire scalp, which, in severe and unrelieved cases, may rest upon the abscess beneath as upon a kind of water-bed. As in scalp w^ounds the aponeurosis is often divided, and as suppuration may follow the injury, it will be seen that the chief danger of those lesions depends upon the spreading of such suppuration to the area of lax connective tissue now under notice. The significance of a small amount of bare bone in a scalp wound is not so much that evils will happen to the bone, but that the aponeurosis has been certainly divided, and the dangerous area of the scalp opened up. Suppuration, when it occurs in this area, is only limited by the attachments of the occipito-frontalis muscle and its aponeurosis, and therefore the most dependent places through which pus can be evacuated are along a line drawn round the head, commencing in front, above the eyebrow, passing at the side a little above the zygoma, and ending behind at the superior curved line of the occipital bone. The scalp, even when extensively dissected up by such abscesses does not peiish, since it carries, as above explained, its blood supply with it. The abscess is often very slow to close, since its walls are prevented from obtaining perfect rest by the frequent movement of the epicranial muscle. To mitigate this evil, and to ensure closing of the sinuses in obstinate cases, Mr. Hilton advises that the whole scalp be firmly secured by strapping, so that the movement of the muscle is arrested.

Abscess beneath the pericranium must be limited to one bone, since the dipping-in of the membrane at the sutures prevents a more extensive spreading of the suppuration.

IIsRinatoinata, or blood tumours of the scalp region, occur in the same localities as abscess.

Chap. I.] The Scalp. 7

The extravasation of blood above the aponeurosis must be of a limited character, while that beneath it may be very extensive. It fortunately happens, however, that the cellular tissue between the a])oneurosis and the pericranium contains but very few vessels, and hence large extravasations in this tissue are uncommon.

Extravasations of blood beneath the pericranium are generally termed cephalhsematomata, and are of necessity limited to one bone. They are usually congenital, are due to pressure upon the head at birth, and are thus most commonly found over one parietal bone, that bone being probably the one most exposed to i^ressure. Their greater frequency in male children may depend upon the larger size of the head in the male fcetus. Such extravasations in early life are encouraged by the laxity of the pericranium, and by the softness and vascularity of the subjacent bone.

In the temporal rcg^ioii, or the region corresponding to the temporal muscle, the layers of soft parts between the skin and the bone are somewhat different from those that have been already described as common to the chief part of the scalp. There is a good deal of fat in the temporal fossa, and when this is absorbed it leads to more or less prominence of the zygoma and malar bone, and so produces the projecting " cheek bones " of the emaciated. The temporal muscle above the zygoma is covered in by a very dense fascia, the temporal fascia, which is attached above to the temporal ridge on the frontal and parietal bones, and below to the zygomatic arch. The unyielding nature of this fascia is well illustrated by a case recorded by Denonvilliers. It concerned a woman who had fallen in the street, and who was admitted into hospital with a deep wound in the temporal region. A piece of bone several lines in length was found loose at the bottom of the wound.

8 Surgical Applied Anatomy. [Chap. i.

and was removed. After its removal the finger could be passed through an opening with an unyielding border, and came in contact with some soft substance beyond. The -case was considered to be one of compound fracture of the squamous bone, with separation of a fragment and exposure of the brain. A bystander, however, noticed that the bone removed was dry and white, and a more complete examination of the wound revealed the fact that the skull was Tininjured, that the supposed hole in the skull was merely a laceration of the temporal fascia, that the soft matter beyond was muscle and not brain, and that the fragment removed was simply a piece of bone which, lying on the ground, had been driven into the soft j)arts when the woman fell.

Abscesses in the temporal fossa are prevented by the fascia from opening anywhere above the zygoma, and are encouraged rather to spread into the pterygoid and maxillary regions, and into the neck.

The pericranium in the temporal region is much more adherent to the bone than it is over the rest of the vault, and subpericranial extravasations of blood are therefore practically unknown in this part of the cranial wall.

Trcpliiiiing:. — This operation is frequently performed in the temporal region, its object being to reach extravasations of blood from the middle meningeal artery. This artery crosses the anterior inferior angle of the parietal bone at a point 1^ inches behind the external angular process of the frontal bone, and 1| inches above the zygoma.

In cutting down to the bone in the temporal region the foUowin^^c structures are met with in order : (1) The skin ; (2) branches of the superficial temporal vessels and nerves; (3) the fascia continued down from the epicranial aponeurosis ; (4) the temporal

Chap. I. j The Scalp. 9

fascia ; (5) the temporal muscle ; (6) tlie deep temporal vessels; and (7) the pericranium. In trephining the skull generally, the comparative thickness of the cranial wall in various parts should be borne in mind (page 2-4), and the large arteries of the scalp should be avoided if possible. The trephine should not be applied over the frontal sinuses, which are often of large size in the aged, and should, when possible, keep clear of the sutures, owing to the frequent exit of emissary veins at or about suture lines. Especially must the superior longitudinal sinus be avoided, which runs backwards in the middle line, and the lateral sinus, whose course is represented by a line drawn horizontally from the occipital protuberance to a point about one inch behind the external meatus of the ear, where it turns downwards to groove the mastoid process.

The zygoma may be broken by direct or indirect violence. In the latter case the violence is such as tends to thrust the upper jaw or malar bone backwards. When due to direct violence, a fragment may be driven into the temporal muscle, and much pain caused in moving the jaw. In ordinary cases there is little or no displacement, since to both fragments the temporal fascia is attached above and the masseter below.

The vessels and nerves of the scalp.— The supraorbital artery and nerve pass vertically upwards from the supraorbital notch, which is situate at the junction of the middle with the inner third of the upper orbital margin. Kearer the middle line the frontal artery and supratrochlear nerve ascend. This artery gives life to the flap that in rhinoplasty is taken from the forehead to form a new nose. The temporal artery, ^vith the auriculo-temporal nerve behind it, crosses the base of the zygoma just in front of the ear. The branches of this artery, especially the lo Surgical Applied Ana tomy. [Chap. i

anterior brancli, are often very tortuous in the aged^ and afford early evidence of arterial degeneration. Arteriotomy is sometimes practised on the anterior brancli of this vessel. The superficial temporal vessels are very liable to be the seat of cirsoid aneurism, as, to a less extent, are the other scalp arteries. Cirsoid aneurism is more often met with in the superficial temporal arteries than in any other artery in the body. The posterior auricular artery and nerve run in the groove between the mastoid process and the ear, and the occipital artery and great occipital nerve reach the scalp just internal to a point midway between the occijDital protuberance and the mastoid process.

Certain of the emissary veins are of great im- ])ortance in surgery. These veins pass through apertures in the cranial wall, and establish communications between the venous circulation (the sinuses) within the skull and the superficial veins external to it. The principal emissary veins are the following : 1. A vein passing through the mastoid foramen and connecting the lateral sinus with the posterior auricular vein or with an occipital vein. This is the largest and most constant of the series. The existence of this mastoid vein serves to give an answer to the question, Why is it a co]nmon practice to aj)ply leeches and blisters behind the ear in certain cerebral affections 1 2. A vein connecting the superior longitudinal sinus with the veins of the scalp through the parietal foramen. 3. A vein connecting the lateral sinus with the deep veins at the back of the neck through the posterior condylar foramen (inconstant). 4. Minute veins following the ninth nerve through its foramen, and connecting the occij)ital sinus with the deep veins of the neck. 5. Minute veins passing through the foramen ovale, foramen lacerum and carotid canal to connect the cavernous sinus with (respectively) the

Chap. I.] The Scalp. ii

pterygoid venous plexuFi, tlie pliaryngeal plexus, and tlie internal jugular vein.

Then, again, many minute veins connect the veins of the scalp ^vitll those of the diploe. Of the four diploic veins, two (the frontal and anterior temporal) enter into surface veins (the supraorbital and deep temporal), and two (the posterior temporal and occipital) enter jnto the lateral sinus.

Lastly, there is the well-known communication between the extra- and intra-cranial venous circulation effected by the commencement of the facial vein at the inner angle of the orbit. In this communication the angular and supraorbital veins unite with the superior ophthalmic vein, a tributary of the cavernous sinus.

Through these various channels, and through many probably still less conspicuous, inflammatory processes can spread from the surface to the interior of the skull. Thus we find such affections as erysipelas of the scalp, diffuse suppuration of the scalp, necrosis of the cranial bones, and the like, leading by extension to mischief within the dij^loe, to thrombosis of the sinuses, and to inflammation of the meninges of the brain. If there were no emissary veins, injuries and diseases of the scalp and skull would lose half their seriousness. Mischief may even spread from within outwards along an emissary vein. Erichsen reports a case where the lateral sinus was exposed in a compound fractui'e. The aperture was plugged. Thrombosis and suppuration within the sinus followed, and some of the pus, escaping through the mastoid vein, led to an abscess in the neck.

The scalp iserves, especially such as are branches of the fifth pair, are often the seat of neuralgia. To relieve one form of this afl'ection the supraorbital nerve has been divided (nevirotomy) at its point of exit from the orbit, and a portion of the nerve lias been resected (neurectomy) in the same situation.

The lyiiipliatics from the occipital and posterior parietal regions of the scalp enter the suboccipital and mastoid glands ; those from the frontal and anterior parietal regions go to the parotid glands, while some of the vessels from the frontal region join the lymphatics of the face, and end in the submaxillary glands.

Chapter II. The Bony Vault Of The Cranium

Position of the sutures. — The bregma, or point of junction of the coronal and sagittal sutures, is in a line drawn vertically upwards from the external auditory meatus, the head being in normal position (Fig. 2). The lambda, or point of junction of the lambdoid and sagittal sutures, lies in the middle line, about 2j inches above the occipital protuberance. The lambdoid suture is fairly represented by the upper two-thirds of a line drawn from the lambda to the apex of the mastoid process on either side. The coronal suture lies along a line drawn from the bregma to the middle of the zygomatic arch. On this line, at a spot about on a level with the external angular process of the frontal bone, and about 1^ inches behind that process, is the pterion, the region where four bones meet — the squamous bone, the great wing of the sphenoid, the frontal and parietal bones. The summit of the squamous suture is just two inches above the zygoma.

In the normal subject all traces of the fontanelles and other unossified parts of the skull, disappear

Chap. II.] Bony Vault of the Cranium

before tlie age of four years. The anterior fontanelle is the last to close, while the posterior is already filled at the time of birth. It is through or about the anterior fontanelle that the ventricles are usually tapped in paracentesis in cases of hydrocephalus. In this operation the trochar is either entered at the sides

Fig. 2.— The Cranial Sutures. {After Gavoy.)

of the fontanelle at a sufficient distance from the middle line to avoid the sinus, or is introduced through the coronal suture at some spot other than its middle point. It may be noted that in severe hydrocephalus the coronal and other sutures of the vault are widely opened.

The condition known as cranio-tabcs, a condition assigned by some to rickets and by others to inherited syphilis, is usually met with in the vertical part of the occipital bone, and in the adjacent parts of the parietal bones. In this condition the bone is greatly thinned in spots, and its tissue so reduced that the affected district feels to the finger as if occupied by parchment, or, as some suggest, by cartridge paper. It is, on the other hand, about the site of the anterior fontanelle that certain osseous deposits are met with on the surface of the skull in some cases of hereditary syphilis (Parrot). These deposits appear as rounded elevations of porous bone situated upon the frontal and parietal bones, where they meet in the middle line. These bosses are separated by a crucial depression represented by the frontal and sagittal sutures on the one hand, and the coronal suture on the other. They have been termed '^ natiform " elevations by M, Parrot, from their supposed resemblance, when viewed collectively, to the nates. To the English mind they would rather suggest the outlines of a " hot-cross bun."

It is necessary to refer to the development of tUe sliiill in order to render intelligible certain conditions (for the most part those of congenital malformation) that are not unfrequently met with. Speaking generally, it may be said that the base of the skull is developed in cartilage, and the vault in membrane. The parts actually formed in membrane are represented in the completed skull by the frontal and parietal bones, the squamo-zygomatic part of the temporal bon«, and the greater part of the expanded portion of the occipital bone. The distinction between these two parts of the skull is often rendered very marked by disease. Thus there are, in the museum of the Pk^oyal College of Surgeons, the skulls of some young lions that were born in a menagerie, and that, in consequence of mal-nutrition, developed certain changes in their bones. A great part of each of these skulls shows considerable thickening, the bone being converted into a porous structure ; and it is remarkable to note that these changes are limited to such

Chap. 11.] Bony Vault of the Cranium.

parts of the skull as are formed in membrane, the base remaining free.

Among the more common of the gross malformations of the skull ako, is one that shows entire absence of all that part of the cranium that is formed in membrane, while the base, or cartilaginous part, is more or less perfectly developed.

Meniiig'occle is the name given to a congenital tumour that consists of a protrusion of a part of the cerebral membranes through a gap in an imperfectly developed skull. When the protrusion contains brain, it is called an encephalocele, and when that protruded brain is distended by an accumulation of fluid within the ventricles, it is called hydrencephalocele. These protrusions are most often met with in the occipital bone, and next in frequency in the fronto-nasal suture, while in rarer cases they have been met with in the lambdoid, sagittal, and other sutures, and have projected through normal and abnormal fissures at the base of the skull into the orbit, nose, and mouth. Their frequency in the occipital bone may be in some way explained by a reference to the development of that part.

This bone at birth consists of four separate parts (see Fig. 3), a basilar, two condylar, and a tabular or expanded part. In the tabular part, about the seventh week of foetal life four nuclei appear, an upper and a lower pair.

These nuclei are to some extent separated by fissures running inwards from the four angles of the bone, and those that run in from the upper and lateral angles persist for some time after birth. Meningoceles of the occiput are

Fig. 3.— The Occipital Bone at Birth, i6

Surgical Applied Anatomv. [Chap. ii.

occur through one always in the middle line, and or other part of the vertical fissure. Since the upper part of the vertical fissure closes later than the lower part, it follows that the abnormal gap is more often above than below the occipital protuberance. If below the protuberance, the foramen magnum, may be opened up, or, on the other hand, the gap may involve the entire bone in the vertical direction. The rig. 4. — Back View of Abnormal Skulls, showing Occipital and

Parietal Bones.

A, The "OS epactal " ; B, parietal fissures ; c, the sagittal fontanelle.

lateral fissures may persist, and may simulate fractures, for which they have, indeed, been mistaken ; or they may be so complete as to entirely separate the highest part of the occipital bone from the remainder. The bone so separated is the os epactal of the French (Fig. 4 A).

Parietal fissui'es. — In the developing parietal bone, fibres concerned in ossification radiate towards the periphery from a nucleus about the centre of the

Chap. II.] Bony Vault of the Cranium. 17

bone. An interfibrillar space, larger tlian the rest, is seen about the fifth month to separate the loose osseous fibres which abut .on the posterior part of the sagittal border from the stronger fibres which form the rest of this border (Pozzi), This is the lyarietal fissitre. It usually closes and leaves no trace, but it may persist in part as a suture-like fissure, and be mistaken for a fracture (Fig. 4 b). If the fissure persists equally on the two sides an elongated lozenge-shaped gap is left, the sagittal fontanelle. It is situate about an inch in front of the lambda (Fig. 4 c).

Necrosis is fairly common on the vault of the skull, and most often attacks the frontal and parietal bones, while, for reasons that are not very obvious, it is rare in the occipital bone. The external table is frequently necrosed alone, it being more exposed to injury and less amply supplied with blood than is the internal table. From the converse of these reasons it happens that necrosis of the internal table alone is but rarely met with. Necrosis involving the entire thickness of the bone may prove very extensive, and in a case reported by Saviard, practically the whole of the cranial vault necrosed and came aAvay. The patient was a woman, and the primary cause of the mischief was a fall u])on the head when drunk.

Fractures of the skull. — It is not easy to actually fracture the skull of a young infant. The skull as a whole at this age is imperfectly ossified, the sutures are wide, and between the bones there is much cartilage and membrane. Moreover, the bones themselves in early life are elastic', and comparatively soft and yielding. If a blow be inflicted upon the vault in a young child the most probable efi'ect, so far as the bone itself is concerned, is an indenting or bulging in of that bone unassociated with a fracture in the ordinary sense. In this particular relation, the skull of an infant is to that of an old man as a cranium of thin tin would be to a cranium of strong earthenware. The yielding character of the young child's skull is well illustrated by the gross deformity of the head that certain Indian tribes produce in their offspring by applying tight bandages to the part in infancy. In the E-oyal College of Surgeons' museum are many skulls of " flat- headed " Indians, that show to what an extreme this artificial deformity may be carried. Gueniot also asserts that much deformity of the head may be produced in infants' by the practice of allowing them to always lie upon one side of the body. Here the deforming agent is simply the weight of the brain.