Elementary human physiology

Front matter

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Preface

Twenty years ago I wrote for Messrs W. & R. Chambers a small Manual on Animal Physiology. The present work is based on that book, but the chapters have been almost wholly rewritten, and they have been rearranged to suit the Syllabus of the First or Elementary Stage issued by the Department of Science and Art. The publishers have been able to place at my disposal a larger number of illustrations than are usually met with in an elementary work ; and I would point out to both teachers and pupils that much valuable information may be derived by a careful study of the descriptions of the woodcuts. To aid in testing the advancing knowledge of the pupil, numerous questions have been introduced, usually at the close of a chapter, and reference is made to the paragraph where an answer may be found.

The general scope of the work and a statement of my notions as to how Elementary Human Physiology may be effectively taught in a school, will be found in the following extracts (with emendations) from the preface to the book on Animal Physiology.

It is now universally admitted that an acquaintance with the principal phenomena manifested by living beings, and more especially with the structure and mode of action of the different parts of the human body, is one of the most important branches of knowledge for men and women in all conditions of life. It is, therefore, no longer necessary to advocate the study of Physiology as a part of general education.

The object of the present text-book is to aid in this work. It professes to give such an account of the general mechanism and functions of the human body as ought to be known by every well-educated person. While aiming at brevity and clearness, the author has not attempted to avoid the use of technical terms, because such terms frequently express in a word a meaning which can only be conveyed in familiar language by circumlocution, and with the risk of vagueness. Many of the best of these terms have been handed down to us for generations, and they have become clear and definite symbols of thought. To attempt to teach a science without them will simply result in giving the pupil vague and superficial notions, so that if he should be obliged again to study the subject with the view of entering one of the professions for which it forms a special branch of training, he will find that he has to learn the science anew. Many technical

4 PREFACE.

terms, however, have been explained, and their derivation given, in the Index.

To aid both in teaching and in learning, it will be observed that the subject has been broken up into numerous divisions and subdivisions, the headings of which have been so printed in diverse type as to catch the eye, and thus impress the memory.

To teach physiology efficiently, it will be a great assistance to the teacher to have the following appliances :

(1) A human skeleton, or the skeleton of any of the higher animals, such as a monkey, a dog, a cat, or a rabbit. Any of these may be obtained by applying to any well-known bookseller or surgicalinstrument maker.

(2) A good microscope, capable of magnifying about 300 diameters. The one recommended by the author is No. IV. of Carl Reichert of Vienna, with objectives Nos. 3 and 7, at a cost of about ;^3, los. By means of this instrument, which ought to be provided by the authorities as part of the educational appliances of every school, a teacher can demonstrate the simpler tissues and fluids found in the body, and he will also be able to interest his pupils by showing infusoria, the humbler forms of plant life, &c., many of which may be found in the water of every stagnant pool by the roadside. '

(3) A set of diagrams. These may be made by copying in Indian ink on a large scale, on cartridge paper, the woodcuts in this work. Photographs may also be easily taken of any of the illustrations, and lantern slides prepared. An occasional lantern demonstration, after the pupils have gone over the part of the subject to be illustrated, will be of great value.

(4) Dissected preparations. These may readily be procured by dissecting a rabbit. Skin the front part of the body, and also a limb ; clear off the loose tissue covering the muscles of the limb ; separate the muscles from each other ; expose the chief nerves and vessels in the limb ; open the chest and abdomen, so as to be able to point out the appearance and position of the chief organs ; and open the head, so as to expose the brain.

By such objective methods, after a little practice, the work of the teacher will become easy, and sound notions of anatomical structure and of physiological action will be taught.

JOHN G. M'KENDRICK.

Contents

PAGE

INTRODUCTORY 7

DIVISION I.— GENERAL PHYSIOLOGY.

Chapter I. General View Of The Anatomy Of The Body

The body contains fluids and solids. The fluids are very abundant, existing not only in certain vessels or tubes fitted for their reception, but permeating the solid parts. Without these fluids the solid parts of the body would die. -The solid parts consist chiefly of hard resisting parts termed the bones, of softer structures forming muscle or flesh, and of the various organs of the body, such as the brain, the lungs, or the heart.

The Locomotive Apparatus consists of two kinds of organs — the bones and the muscles. The bones, which are hard firm structures, form levers joined to each other by firm or movable articulations or joints, which often permit the bones to move on each other with great facility. The muscles constitute the chief part of what is usually called flesh, and they possess the property of contracting or shortening so as to move the bones to which their ends are attached. The bones may be called the passive, and the muscles the active, organs of locomotion.

A. THE PARTS OF THE SKELETON.

1. The Skeleton as a whole. — The whole of the bones in their natural position form the skeleton, which may be divided into the trunk and the limbs. The entire skeleton of an adult consists of two hundred bones, namely :

ELEMENTARY HUMAN PHYSIOLOGY.

CERVICAL VERTEBRA-

DORSAL ERTEBR>G

Fig. 1. — Human SJceleton viewed in front.

GENERAL VIEW OF THE ANATOMY OF THE BODY. I3

1. The spine or vertebral column, including the sacrum

and coccyx 26

2. The cranium 8

3. The face 14

4. The hyoid bone, sternum, and ribs 26

5. The upper extremities 64

6. The lower extremities 62

The trunk consists of, first, the spine or vertebral column (figs. 1, 2), a flexible stalk formed of a number of distinct bones articulated one below the other ; second, the head, containing the brain and organs of sense, and consisting also of numerous bones ; and third, the thorax or chest, composed of detached arches called the ribs, and which are connected in front, by cartilage or gristle, with a single bone, the sternum. The thorax contains the principal organs of respiration and circulation.

The limbs, four in number, are termed superior and inferior. In man, the inferior limbs support the trunk, while the superior bear the hands, which serve as organs of prehension ; but in the lower mammalia, as in the horse or dog, all four support the trunk. The superior limbs are divided into the shoulder, the arm, the forearm, and the hand. The inferior limbs comprise the haunch or pelvis, the thigh, the leg, and

DR

Fig. 2. — Axis of Spine, A, A :

C, cervical (neck) vertebrae^

D, dorsal (back) vertebrae ;

L, lumbar (loin) vertebrae :

S, sacrum ; Coc, coccyx.

ELEMENTARY HUMAN PHYSIOLOGY.

the foot. In birds, the anterior or superior limbs constitute wings.

2. The Vertebral Column. — This consists of twenty-four free bones called vertebrae, and of two bones at the lower extremity, each composed of several vertebrae, termed the sacrum and coccyx. Superiorly, it supports the skull ; laterally, it has attached to it the ribs, through which it supports the weight of the upper limbs; and at its lower extremity it rests on the bones of the pelvis, which

Fig. 3. — Dorsal Vertebra viewed from above :

I, body : 2, pedicels ; 3, laminae ; 4, ring ; 5, spinous process ; 6, transverse process : 7, articular process for next vertebra ; f, facet for head of rib ; d, facet for tubercle of rib.

Fig. 4.

Section of Intervertebral Disc :

transmit the weight of the body to the lower limbs. It also affords support and protection to the spinal marrow by enclosing it in a canal of bone. The bones of which it is composed are bound together by ligaments or bands, and by elastic discs of a fibrous and gristly substance called the intervertebral discs (fig. 4, b). There is thus secured great strength combined with flexibility. A certain amount of movement occurs between individual vertebrae, more especially in the cervical region.

3. General Characters of a Vertebra. — Each vertebra has more or less the form of a ring, and presents a body (fig. 3, i), which is placed anteriorly ; a ring, 4 — containing the spinal cord and its membranes — which is formed by the body, i ; the pedicels, 2 ; the laminse, 3 ; and the spinous process, 5. Attached to the ring we find the transverse processes, 6, placed one on each side, and the articular processes, also one on each side, 7, two superior and two inferior, for connection with the adjoining vertebrae.

4. Characters peculiar to groups of Vertebrae. — The vertebrae are divided into three groups — cervical, in

Fig. 5. — First Vertebra or Atlas: Fig.6. — Second Vertebra or Axis:

A, ring for spinal cord ; B, ring for r, odontoid process; 2, articulation for

odontoid process of axis ; dotted line atlas ; 3, canal for vertebral artery ;

indicates ligament ; i, articulation 4, spinous process ; 5, articulation for

for occipital bone ; 2, transverse pro- third cervical vertebra, cess : rt, canal for vertebral artery.

the neck ; dorsal, in the back ; and lumbar, in the loins. Of the first there are seven ; of the second, twelve ; and of the third, five (fig. 2). The first cervical, which supports the head, is called the atlas (fig. 5), and the second, the axis. The atlas has no body, and development shows that

Fig. 7. — Cervical Vertebra : I, body : J, ring ; 3, bifid spinous process ; 4, transverse process ; 5, canal for vertebral artery.

Fig. 8. — Lumbar Vertebra :

i, body ; 2, transverse process ; 3, spinous

process ; 4, articulation for next verfor vertebral artery. tebra.

the process called the odontoid process of the axis (fig. 6, i)

i6

ELEMENTARY HUMAN PHYSIOLOGY.

is in reality the body of the atlas connected with the body of the axis. The atlas, bearing the head, rotates round this process when the head is moved from side to side. The general characters of a cervical vertebra are seen in fig. 7, the two chief points being the notched spinous process, 3, and the canal in the transverse process for the vertebral artery, 5.

The chief character by which a dorsal vertebra may be recognised on the body or

Fig. 9. — Front of Sacrum: Jg the I , upper end : 2, lower end for presence transverse process of an articulatina;

coccyx : vi — V5, vertebrae * ^

united ; a, foramina or surfacc for the head Or angle of the

holes for sacral nerves; 3, ^b (seC fig. 3, C, d). The lumbar articulation of pelvic bone. ^ *^ n\ ^ , . , ,.

vertebrae (fig. 8) have their bodies more massive than those of the dorsal, and the spinous

D Fig. 10. —Side View

of Human Skull :

a, frontal bone ; 5, parietal bone : c, occipital bone ; d, temporal bone (squamous portion) ; £, sphenoid bone ; ^ malar bone ; ffj nasal bone ; /i, superior maxillary or upper jaw-bone ; /, inferior maxillary or lower jaw-bone.

BD, height of cranium ; GO, length of cranium ; BN, basi-nasal length ; BA, basi-alveolar length. (These measurements are supposed to be made in a straight line from point to point.}

processes are large, flat, and point directly backwards instead of downwards (fig. 2, L).

5. The Sacrum (figs. 1, 2, and 9) is placed below the last lumbar vertebra, above the coccyx, and between the pelvic bones (fig. 1). It consists in early life of five vertebrae, which in the adult are united into one bone. The coccyx, at the lower extremity of the vertebral column, consists of four rudimentary vertebrae, which diminish in size from above downwards (figs. 2 and 18).

The average length of the vertebral column is about 28 inches, and it presents four curves in its course, the convexity being forwards in the neck and loins, and backwards in the back and pelvis (fig. 2).

6. The Skull is supported on the vertebral column, and is formed of a number of bones, all of which, with the exception of the lower jaw, are firmly fixed together by surfaces termed sutures. It is divided into two portions — tlie cranium and the face.

The cranium protects the brain; the face surrounds the nose and mouth, and contains several of the organs of sense. The cranium (fig. 10) is composed of eight bones — namely, the occipital, c; the two parietal, b; the frontal, a; the two temporal, d; the sphenoid, e ; and the ethmoid. The sphenoid forms part of the base of the skull (a little in front of 5, fig. 11), and the

Fig. U.— Base of Skull :

i, superior maxillaries ; 2, 2, palate ; 3, vomer ; 4, wing of sphenoid ; s, basilar process of occipital ; 6, foramen magnum ; 7, foramen ovale ; S, temporal ; 9, 11, styloid process of temporal; 12, mastoid process.

i8

ELEMENTARY HUMAN PHYSIOLOGY.

ethmoid is found (immediately above and behind N in fig. 10) between the orbital plates of the frontal bone, and enters into the formation of the orbits and the nasal cavities. The /ace is composed of fourteen bones, of which twelve are' in pairs, the two superior maxillary (fig. 10, A, and fig. 11, ., i), the malar (fig. 10,/), the nasal (fig. 10, g), the

9.8.,'^,

Fig. 12.— The Ribs, in situ : I and 2 are the upper and the middle parts of the sternum or breast-bone ; 3, its ensiform cartilage ; 4, the first dorsal, and 5 the ]a£t (or twelfth) dorsal vertebra ; 6, the first rib ; 7, its head ; 8, its neck, resting against the transverse process of the first dorsal vertebra ; g, its tubercle ; 10, the seventh or last true rib; 11, the costal cartilages of the true ribs ; 12, the last two false ribs or floating ribs.

palate (fig. 11, .<, 2), the lachrymal (a little to the right of ^ in the orbit, fig. 10), and the inferior turbinated in the nose ; and two single, namely, the vomer, a bone forming a partition between the two nostrils (fig.

II, 3), and the inferior maxillary (fig. 10, i).

7. The Cranial Cavity is seen on sawing off the roof of the skull. The walls consist of two layers of compact tissue, the duier and inner tables, and between these a cellular structure known as the diploe.

The tipper part of the cranial cavity forms an arch, and the lower is divided into three parts having different levels, called the anterior, the middle, and the posterior fossae, in which the anterior and middle lobes of the cerebrum and the cerebellum rest. The base is perforated by numerous openings for the passage of nerves

GENERAL VIEW OF THE ANATOMY OF THE BODY.

and blood-vessels. The most notable of these openings is the foramen magnum (fig. 1 1, e), for the passage of the spinal marrow and of certain blood-vessels.

8. The Thorax, or chest, consists of the dorsal vertebrae, the sternum or breast-bone (fig. 12), the ribs, and the cartilages connecting these with the sternum, known as the costal cartilages. The sternum, 1-3, is situated in the median line at the fore-part of the thorax, and is connected with the rest of the trunk by the costal cartilages of the seven highest pairs of ribs, n, n.

The ribs are twelve in number on each side. They are long slender curved bones which extend forwards from the spine, some of them joining the breast-bone or sternum. The seven upper ribs, which join the sternum by cartilages, are termed the ' true ' ribs ; while the lower five, which do not join the sternum, are termed the ' false ' ribs (see fig. 12, 12). Each rib has a double attachment to the backbone posteriorly; byits head, which unites with the body of the vertebrae (fig. 3, c), and by a rounded prominence, called the tubercle, with the transverse process of the vertebra (fig. 3, ct). The ligaments binding the ribs to the vertebral bones are seen in fig. 13. The costal cartilages

Fig. 13. — A Front View of the Articulations of the Ribs with the Spinal Column ;

I, i, dorsal vertebrae ; 2, i, intervertebral cartilages : 3, 3, anterior common ligament; 4, neck, and 5, head of rib ; 6, 7, 8, flat bundles of ligamentous fibres (removed in the lowest rib shown in the figure) ; 9, articulation between the tubercle of the ribs and the transverse vertebral process.

are continuations of the ribs. They give elasticity to the framework of the thorax. In advanced life they become impregnated with earthy matter, partially lose their elasticity, and thus diminish the force and depth of respiration.

9. The Bones of the Upper Extremity. — The upper extremity consists of the shoulder, the arm, the fore-arm,

and the hand. The bones of the shoulder are the scapula (fig. 1) or shoulder-blade, and clavicle or collar-bone (fig. 1) ; in the arm, is the humerus (fig. 1) ; in the forearm, are the radius and ulna ; and in the hand, three groups of bones — the carpus, metacarpus, and digital phalanges.

The scapula (fig. 14), placed on the upper and back part of the thorax, is attached directly to the trunk only by the clavicle, and from it the

^. , , „ . ,r. r ^^ humcrus is suspended. The Fiff. 14. — Posterior View of the , . , .' , ,. , . ,

Left Scapula: '^'''"''' '« ^ l°"g cylindrical

The parts designated by the figures i, =, bonC placed On Cach sidc of

4, 6, 8, IP, II, 12, are sufficiently de- the neck, and Connecting the

scribed in the text; 3 is the superior . , -_ -, *\ .

border; 5. the external or axillary StCrnum With 12 (fig. 14), the

border ; 7, the inferior angle ; 9, the acromion prOCeSS of the internal or vertebral border ; 12, the \ /c 1 r- \ mi

acromion process; 13, one of the SCapula (fig. 15, c). The

holes for the passage of an artery into humerUS (fig. 15, K) is an

the bone ; 14, the coracoid process. ■ _ r 1.1 1 • j • i i

imperfectly cylindrical bone, extending from the shoulder to the elbow-joint In the figure it is seen as it is placed when the arm is hanging down and the palm turned forwards. It consists of a head which articulates with the scapula at 6 (fig. 14), of a shaft, and of a lower end, which supports the radius and ulna. The two bones of the fore-arm are seen in fig. 15. They consist of the radius, r, which is the external of the two bones of the fore-arm, and the tdna, u, which is the internal. The radius articulates above with the humerus, and below with two of the bones of the carpus, or wrist. The ulna articulates with the humerus and the radius, but is not directly connected with the carpal bones, a thin, fibro-cartilaginous disc being interposed between its lower end and the

Fig. 15. — Bones of the Human Arm : h, humerus ; r, radius ; tt, ulna ; w, wrist-joint; hd^ hand: f, scapula ; c^ clavicle or collar-bone.

-Posterior Aspect of , Left Hand.

For description, see text.

cuneiform bone (fig. 16, c). Wlien the arm and hand hang downwards, the palm being directed forwards, the position is called supination ; but when in the same position the back of the hand is directed forwards, the position is called pronation. These movements are effected by the rotation of the radius on the lower end of the humerus. The. carpus, or wrist, consists of eight short feLfiMENTARY HUMAU fHYSIOLOCV.

bones, arranged in two rows. Enumerated from the radial or thumb side, they are (fig. 16), in the first row, the scaphoid, sc; the semilunar, /; the cuneiform, c; and pisiform, / ; and in the second row, in the same order, the trapezium, / ; the trapezoid, t^ ; the os magnum, m ; and the unciform, u. The metacarpus, forming the palm, consists of five shafted bones which support the fingers, Wj, ^2, OTg, m^, and m^. The digital phalanges are fourteen in number, three for each finger, except the thumb, which

Fig. 18.— Pelvis :

I, lower lumbar vertebrae ; 2, 3, 4, sacrum : 6, 6, innominate bones ; 5, coccyx : g, acetabula ; f^ symphysis pubis ; I i Ijk k, brim of pelvis.

Fig. 17. — Outer Aspect of Right Os Innominatum or Iliac Bone :

A, ilium ; B, ischiiim ; C, pubis ; i, crest of ilium; 2, anterior superior spine ; 3, anterior inferior spine ; 4, posterior superior spine ; 5, posterior inferior spine ; 6, acetabulum ; 7, symphysis pubis : 8, descending ramus of pubis ; 9, obturator foramen ; 10, ascending ramus of ischium ; 11, tuberosity of ischium.

has only two. In each instance, the proximal phalanx, /j, is longer than the second, p^, and the second longer than the third, p^.

10. The Bones of the Lower Extremity. — The lower limb is divided into the haunch or hip, the thigh, the leg, and the foot The haunch-bone on each side, with the sacrum wedged in between, and bearing the coccyx

General vie* of the anatomy of the body. 23 at its lower extremity, forms the peivis, which transmits the weight of the body to the lower limb. It is usually called the imiominate bone, or os coxm (fig. 17). The pelvis, at basin (fig. 18), contains the urinary and generative organs and the lower end of the alimentary canal. Its upper opening is termed the inlet, and its lower the outlet pf the pelvis. In the erect position (fig. 1), the pelvis is so inclined that the plane of the brim forms an angle of from 60° to 65° with the horizontal.

The line of pressure of the weight of the body on the sacrum is downwards and forwards towards the junction of the two innominate bones termed the symphysis pubis (fig. 18,/), and the Fig. 19.— Front View Fig. 20.— Front View pressure is com- of the Right Femur : of Right A, Tibia,

municated to the heads of the thighbones, which are lodged in deep depressions termed the acetabula (fig. 17, e; fig. 18, 9; and fig. 1). The form and size of the pelvis differ in the two sexes, as it is broader, more expanded, and shallower in the female than in the male.

The femur, or thigh-bone (fig. 19), the largest bone of the skeleton, articulates, 2, above with the acetabulum of the OS innominatum (figs. 17, a, and 18, 9). In the erect position it inclines inwards and slightly backwards, and it is divisible into a shaft, having at one end a head, 2, attached to it by a neck, 3, and bearing two rough promi-

:, shaft ; 2, head; 3, neck; 4, greater, and 5, lesser trochanters ; 6, external,

1 and 7, internal tuberosity : 8, articulation for tibia and patella.

and B, Fibula : A — I, shaft ; z, articulation with femur ; 3, articulation with astragalus. B — 4, shaft : 5, head ; 6, lower end. Observe forkshaped cavity at lower end for astragalus.

ELEMENTARY HUMAN PHYSIOLOGY.

nences for the attachment of muscles, called respectively the great, 4, and the small, 5, trochanters ; and at the other, or inferior extremity, a broad, irregularly shaped surface called the external, 6, and internal, 7, condyles, for articulation with the tibia.

In the female, from the greater breadth of the pelvis, the thighbones converge more towards their lower extremity than in the male.

'Y\\&patella or kneepan (fig. 1) is situated in front of the knee-joint, ^nd may be regarded as a mass of bone developed in the tendon or sinew belonging to the great muscle in front of the thigh by which the leg is extended on the thigh.

The bones of the leg are two in number, the inner termed the tibia or shin-bone (fig. 1, and fig. 20, A), and the outer, the The tibia alone com-

Fig. 21. — Upper View of Left Foot : :, the astragalus, its upper articular surface ; 2, its anterior extremity^ which articulates with 4, the scaphoid bone ; 3, the os calcis, or heel-bone ; 4, the scaphoid bone ; 5, the internal cuneiform bone ; 6, the middle cuneiform bone ; 7, the external cuneiform bone ; 8, the cuboid bone ; g, the metatarsal bones of the first and second toes ; 10, II, the first and second phalanges of the great toe ; 12, 13, 14, the first, second, and third phalanges of the second toe.

fibula, or clasp-bone (fig. 20, B), municates the weight of the trunk to the foot. It articulates inferiorly with one of the bones of the ankle termed the astragalus (fig. 21, i). The fibula is much more slender than the tibia.

The foot is divided into the tarsus, the metatarsus, and \he phalajtges. The tarsus consists of seven bones — namely (fig. 21), the OS calcis, or heel-bone, 3; the astragalus, which receives the weight of the body from the leg, i ; the cuboid, so named from its shape, on the outer side of the foot, 8 ; the scaphoid or navicular bone, 4, and the three cuneiform or wedge-shaped bones intervening between the scaphoid and the metatarsals, s, 6, 7 ; the metatarsal bones, 9, are five in number, and they bear the phalanges of the toes, 10, II, 12, 13, 14.

The foot as a whole is admirably adapted for receiving the weight of the body, and for affording stability. It is arched from behind forwards, the posterior support of the arch being formed by the heel, and the anterior by the

Section through the lower end of the tibia a, and through- the astragalus b, the heel-bone c, the scaphoid bone </, the internal cuneiform bone e, and the bones of the great toe^

balls of the toes — indeed, the arch may be regarded as having a single pier behind and a double in front (fig. 22). II. Adaptation of the Skeleton of Man to the ERECT POSITION. — The skull of man is nearly balanced on the vertebral column (fig. 2), whereas in the lower animals it is suspended, as it were, from the extremity of the column, and is sustained by an elastic ligament (ligatnentum iiuchc£), which runs from the spinous processes of the vertebrae to a protuberance on the occipital bone. The body is balanced

26 ELEMENTARY HUMAN fHYSlOLOGV.

on one or both of the lower limbs, which can be extended in a straight line at the knee-joint. The foot of man alone possesses an arched instep, and the great toe is not intended for grasping, as in the monkeys, but for supporting weight and giving elasticity to the step. The great length of the femur enables the body to be balanced as in crouching, and the great breadth of the pelvis enables the equilibrium to be maintained even during considerable lateral movements. The spinal column, by becoming broader inferiorly, is fitted to sustain weight, and by means of its curvatures elasticity and strength are secured, while a wide range of movement is permitted. In man, the articular facet on the scapula for the head of the humerus looks outwards, so as to allow a free play of the upper extremity at the shoulder-joint. Mobility, lightness, and delicacy of movement are permitted by the structure of the upper limb, as contrasted with strength and firmness in the lower, as may be seen on comparing the shoulder, elbow, and wrist, with the hip, knee, and ankle.

THE JOINTS.

12. General Description.— A joint is the union of any two segments of the skeleton through the intervention of a structure or structures of a different nature. Bone forms the fundamental part of all joints ; strong bands or ligaments hold the bones firmly together ; and in joints in which there is free movement, we find the ends of the bones covered by cartilage or gristle, and the joint lined by a smooth membrane termed a synovial membrane. This membrane secretes a thin, watery fluid, synovia, which lubricates the surfaces and diminishes friction. Joints are divided into three classes : (i) those in which the parts are continuous and there is no synovial membrane between the bones, as seen in the construction of -the skull (fig. 10) ; (2) those in which there is an intervening substance between the bones in the form of a disc or cushion of fibro-cartilaginous substance, allowing a certain degree of mobility, as in the articulations between the bodies of