Fig. LIV.Anterior view of the sternum.
Anterior view of the sternum.
99. Its position is oblique, being near the vertebral column at the top, and distant from it at the bottom (fig. XLV. 6). Its margins are thick, and marked by seven depressions, for the reception of the cartilages of the seven true ribs (fig. LIV). Its anterior surface is immediately subjacent to the skin, and gives attachment to powerful muscles, which act on the superior extremities: its posterior surface is slightly hollowed in order to enlarge the cavity of the thorax (fig. LV.).
Fig. LV.Posterior view of the sternum.
Posterior view of the sternum.
100. The thorax is bounded at the sides by the ribs, which extend like so many arches betweenthe spinal column and the sternum (fig. XLV. 7, 8). They are in number twenty-four, twelve on each side, of which the seven upper are united to the sternum by cartilage, and are called true ribs (fig. XLV. 7); the cartilages of the remaining five are united with each other and are not attached to the sternum; these are called false ribs (fig. XLV. 8): all of them are connected behind to the spinal column (fig. XXXV.).
101. The ribs successively and considerably increase in length as far as the seventh, by which the cavity they encompass is enlarged; from the seventh they successively diminish in length, and the capacity of the corresponding part of the cavity is lessened. The direction of the ribs from above downwards is oblique (fig. XLV. 7, 8). Their external or anterior surface is convex (fig. XLV. 7, 8); their internal or posterior surface is concave: by the first their strength is increased; by the second the general cavity of the thorax is enlarged (fig. XLV. 7, 8). Their upper margin is smooth and rounded, and gives attachment to a double layer of muscles, called the intercostal, placed in the intervals that separate the ribs from each other (fig. LIX.). Along the lower margin is excavated a deep groove, for the lodgment and protection of the intercostal vessels.
102. The ribs are connected with the spinal column chiefly by what is termed theanterior ligament(fig. LVI. 1), which is attached to thehead of the rib (fig. LVI.), and which, dividing into three portions (fig. LVI. 1), firmly unites every rib to two of the vertebræ, and to the intervertebral substance (fig. LVI. 1). This articulation is fortified by a second ligament (fig. LVI. 2), also attached to a head of the rib, termed theinterarticular(fig. LVI. 2), and by three others, one of which is attached on the fore part, and the two others in the back part, to the neck of the rib (fig. LVII. 1).
Fig. LVI.Ligaments connecting the ribs to the spinal column.1. anterior ligaments; 2. interarticular ligament; 3. ligamentsof the necks of the ribs.
Ligaments connecting the ribs to the spinal column.1. anterior ligaments; 2. interarticular ligament; 3. ligamentsof the necks of the ribs.
The cartilages of the seven superior ribs are attached to the sternum by a double layer of ligamentousfibres, termed theanterior and the posterior ligaments of the sternum(fig. LVIII.). So strong are the bands which thus attach the ribs to the spinal column and the sternum, that the ribs cannot be dislocated without fracture. "Such at least is the case in experiments upon the dead body, where, though the rib be subjected to the application of force by means of an instrument best calculated to detach its head from the articulation, yet it is always broken."
Fig. LVII.1, &c. Ligaments connecting the ribs to the vertebræ behind.
1, &c. Ligaments connecting the ribs to the vertebræ behind.
While thus firmly attached to their points of support, the ligaments, which fix them, are so disposed as to render the ribs capable of being readily moved upwards and downwards: upwards in inspiration; downwards in expiration; and it is by this alternate action that they enlarge and diminish the cavity of the thorax in the function of respiration.
Fig. LVIII.Ligaments joining the cartilages of the ribs to the sternum.
Ligaments joining the cartilages of the ribs to the sternum.
103. Such are the boundaries of the cavity of the thorax as far as its walls are solid. The interspaces between these solid portions at the sides arefilled up by muscles, principally by those termed the intercostal (fig. LIX.); below, the boundary is formed by the diaphragm (fig. LXI. 2); while above, as has been already stated (69), the cavity is so contracted as only to leave an opening for the passage of certain parts to and from the chest.
Fig. LIX.A view of the muscles calledIntercostals, filling up thespaces between the ribs.
A view of the muscles calledIntercostals, filling up thespaces between the ribs.
104. The inner surface of the walls of the thorax, in its whole extent, is lined by a serous membrane, exceedingly thin and delicate, but still firm, called the pleura. The same membrane is reflected overthe organs of respiration contained in the cavity, so as to give them an external coat. The membrane itself is every where continuous, and every where the same, whether it line the containing or the contained parts; but it receives a different name as it covers the one or the other: that portion of it which lines the walls of the cavity being called the costal pleura (fig. LXI.a), while that which covers the organs contained in the cavity is termed the pulmonary pleura (fig. LX. 5, 1).
105. A fold of each pleura passes directly across the central part of the cavity of the thorax; extending from the spinal column to the sternum, and dividing the general cavity into two. This portion of the pleura is called the mediastinum, from its situation in the centre of the thorax, and it so completely divides the thoracic cavity into two, that the organs on one side of the chest have no communication with those of the other; so that there may be extensive disease in one cavity (for example, a large accumulation of water,) while the other may be perfectly sound.
106. The main organs contained in the cavity of the thorax are the lungs with their air tube; the heart with its great vessels; and the tube passing from the mouth to the stomach (fig. LX.).
107. The two lungs occupy the sides of the chest (fig. LX. 5). They are completely separated from each other by the membranous partition just described, the mediastinum. Between the twofolds of the mediastinum, namely, in the middle of the chest, but inclining somewhat to the left side, is placed the heart, enveloped in another serous membrane, the pericardium (fig. LX. 2, 1).
108. The lungs are moulded to the cavities they fill; whence their figure is conical, the base of the cone being downwards, resting on the diaphragm (fig. LX. 5,b); and the apex upwards, towards the neck (fig. LX. 5).
109. That surface of each lung which corresponds to the walls of the chest is convex in its whole extent (fig. LX. 5); on the contrary, that surface which corresponds to the mediastinum is flattened (fig. LX. 5). The basis of the lung is concave, adapted to the convexity of the diaphragm on which it rests (fig. LX. 5).
110. The air-vessel of the lungs, termed the bronchus, together with the blood-vessels and nerves, enter the organ at its flattened side, not exactly in the middle, but rather towards the upper and back part. This portion is termed the root of the lung.
111. The lungs are attached to the neck by the trachea (fig. LX. 4), the continuation of which forms the bronchus; to the spinal column by the pleura, and to the heart by the pulmonary vessels (fig. LX. 3,d): their remaining portion is free and unattached.
112. In the living body, the lungs on each side completely fill the cavity of the chest, following passively the movements of its walls, and accurately adapting themselves to its size, whether its capacity enlarge in inspiration, or diminish in expiration, so that the external surface of the lung (the pulmonary pleura) is always in immediate contact with the lining membrane of the walls of the cavity (the costal pleura); consequently, during life, there is no cavity, the chest being always completely full.
Fig. LX.a.The cut edges of the ribs, forming the lateral boundaries of thecavity of the thorax.b.The diaphragm, forming the inferior boundary of the thorax, andthe division between the thorax and the abdomen.c.The cut edges of the abdominal muscles, turned aside, exposingthe general cavity of the abdomen.1. The cut edge of the pericardiumturned aside.2. The heart.3. The great vessels in immediateconnexion with theheart.4. The trachea, or wind-pipe.5. The lungs.6. The liver.7. The stomach.8. The large intestine.9. The small intestines.10. The urinary bladder.
a.The cut edges of the ribs, forming the lateral boundaries of thecavity of the thorax.b.The diaphragm, forming the inferior boundary of the thorax, andthe division between the thorax and the abdomen.c.The cut edges of the abdominal muscles, turned aside, exposingthe general cavity of the abdomen.1. The cut edge of the pericardiumturned aside.2. The heart.3. The great vessels in immediateconnexion with theheart.4. The trachea, or wind-pipe.5. The lungs.6. The liver.7. The stomach.8. The large intestine.9. The small intestines.10. The urinary bladder.
113. The anterior surface of the pericardium, the bag which envelopes the heart, lies immediately behind the sternum, and the cartilages of the second, third, fourth, and fifth ribs, covered at its sides by the pleura, and firmly attached below to the diaphragm (fig. LX. 1).
114. Surrounded by its pericardium, within the mediastinum, the heart is placed nearly in the centre of the chest, but its direction is somewhat oblique, its apex being directly opposite to the interval between the fifth and sixth ribs on the left side (fig. LX. 2); while its basis is directed upwards, backwards, and towards the right (fig. LX. 2). That portion of its surface which is presented to view on opening the pericardium is convex (fig. LX. 2); but its opposite surface, namely, that which rests upon the part of the pericardium which is attached to the diaphragm, is flattened (fig. LX. 1). It is fixed in its situation partly by the pericardium and partly by the great vessels that go to and from it. But under the different states of expiration and inspiration, itaccompanies, in some degree, the movements of the diaphragm; and in the varied postures of the body, the heart deviates to a certain extent from the exact position here described.
115. The second division of the trunk, theabdomen, is bounded above by the diaphragm (fig. LXI. 2), below by the pelvis (fig. LXI. 3), behind and at the sides by the vertebræ and muscles of the loins (fig. LXIII.), and before by the abdominal muscles (fig. LXIII. 9).
116. The organ which forms the superior boundary of the abdomen, the diaphragm (midriff), is a circular muscle, placed transversely across the trunk, nearly at its centre (fig. LXI. 2). It forms a vaulted partition between the thorax and the abdomen (fig. LXI. 2). All around its border it is fleshy (fig. LXI. 2); towards its centre it is tendinous (fig. LXI. 2); the surface towards the abdomen is concave (fig. LXI. 2); that towards the thorax convex (fig. LXI. 2); while its middle tendinous portion ascends into the thorax as high as the fourth rib (fig. LXI. 2).
Fig. LXI.View of the diaphragm. 1. Cavity of the thorax;2. diaphragm separating the cavity of the thorax from that ofthe abdomen; 3. cavity of the pelvis.
View of the diaphragm. 1. Cavity of the thorax;2. diaphragm separating the cavity of the thorax from that ofthe abdomen; 3. cavity of the pelvis.
117. The diaphragm is perforated by several apertures, for the transmission of tubes and vessels, which pass reciprocally between the thorax and abdomen (fig. LXII.).
1. A separate aperture is formed to afford an exit from the thorax of the aorta, the common source of the arteries (fig. LXII. 2), and an entrance into the thorax of the thoracic duct, the tube that bears the digested aliment to the heart. 2. A little to the left of the former, there is another aperture, through which passes the esophagus or gullet (fig. LXII. 3), the tube that conveys the food from the mouth to the stomach. 3. On the right side, in the tendinous portion of the diaphragm, very carefully constructed, is a third aperture for the passage of the vena cava (fig. LXII. 4), the great vessel that returns the blood to the heart from the lower parts of the body.
Fig. LXII.View of the diaphragm with the tubes that pass throughit. 1. Arch of the diaphragm; 2. the trunk of the aortapassing from the chest into the abdomen; 3. the esophaguspassing from the chest through the diaphragm to the stomach;4. the vena cava, the great vein that returns theblood to the heart from the lower parts of the body, passingfrom the abdomen, into the chest, in its way to the rightside of the heart; 5. 6. muscles that arise in the interior ofthe trunk and that act upon the thigh; 5. the muscle calledpsoas; 6. the muscle called iliacus.
View of the diaphragm with the tubes that pass throughit. 1. Arch of the diaphragm; 2. the trunk of the aortapassing from the chest into the abdomen; 3. the esophaguspassing from the chest through the diaphragm to the stomach;4. the vena cava, the great vein that returns theblood to the heart from the lower parts of the body, passingfrom the abdomen, into the chest, in its way to the rightside of the heart; 5. 6. muscles that arise in the interior ofthe trunk and that act upon the thigh; 5. the muscle calledpsoas; 6. the muscle called iliacus.
118. The partition formed by the diaphragm between the thorax and abdomen, though complete, is moveable; for as the diaphragm descends in inspiration and ascends in expiration, it proportionally enlarges or diminishes the cavities between which it is placed; consequently, the actual magnitude of these cavities varies every moment, and the size of the one is always in the inverse ratio of that of the other.
119. Between the abdomen and the pelvis there is no separation; one cavity is directly continuous with the other (fig. LXI. 3); but along the inner surface of the expanded bones, which form a part of the lateral boundary of the abdomen, there is a prominent line, termed the brim of the pelvis (fig. XLV. 15), marking the point at which the abdomen is supposed to terminate and the pelvis to commence.
120. Behind and at the sides the walls of the abdomen are completed partly by the lumbar portion of the spinal column and partly by the lumbar muscles (fig. XLV. 4), and before by the abdominal muscles (fig. LXIII. 9).
121. The inner surface of the walls of the abdomen is lined throughout by a serous membrane, termed the peritoneum (fig. LXIII.). From thewalls of the abdomen, the peritoneum is reflected upon the organs contained in the cavity, and is continued over them so as to form their external coat. The peritoneum also descends between the several organs, connecting them together, and holding them firmly in their situation; and it likewise forms numerous folds, in which are embedded the vessels and nerves that supply the organs. It secretes a serous fluid, by which its own surface and that of the organs it covers is rendered moist, polished, and glistening, and by means of which the organs glide smoothly over it, and over one another in the various movements of the body, and are in constant contact without growing together. In structure, distribution, and function, the peritoneum is thus perfectly analogous to the pleura.
122. Like the thorax, the abdomen is always completely full. When the diaphragm is in action, it contracts. When the diaphragm is in the state of contraction, the abdominal and lumbar muscles are in the state of relaxation. By the contraction of the diaphragm, the organs contained in the abdomen are pushed downwards, and the anterior and lateral walls of the cavity being at this moment in a state of relaxation, they readily yield, and, consequently, the viscera are protruded forwards and at the sides. But the abdominal and lumbar muscles in their turn contract, the diaphragm relaxing; and, consequently, the viscera,forced from the front and sides of the abdomen, are pushed upwards, together with the diaphragm, into the cavity of the thorax. A firm and uniform pressure is thus at all times maintained upon the whole contents of the abdomen: there is an exact adaptation of the containing to the contained parts, and of one organ to another. No space intervenes either between the walls of the abdomen and the organs they enclose, or between one organ and another: so that the term cavity does not denote a void or empty space, but merely the extent of the boundary within which the viscera are contained.
123. The contents of the abdomen consist of the organs which belong to the apparatus of digestion, and of those which belong to the apparatus of excretion.
124. The organs which belong to the apparatus of digestion are—1. The stomach (fig. LXIII. 2) 2. The duodenum (fig. LXIII. 4). 3. The jejunum (fig. LXIII. 5). 4. The ilium (fig. LXIII. 5). The three last organs are called the small intestines, and their office is partly to carry on the digestion of the aliment commenced in the stomach, and partly to afford an extended surface for the absorption of the nutriment as it is prepared from the aliment. 5. The pancreas (fig. LXIV. 5). 6. The liver (fig. LXIV. 2). 7. The spleen (fig. LXIV. 4). The three last organs truly belong to the apparatus of digestion, and their office is to co-operate with the stomach and the small intestines in the conversion of the aliment into nutriment.
Fig. LXIII.1. Esophagus; 2. stomach; 3. liver raised, showing itsunder surface; 4. duodenum; 5. small intestines; 6. cæcum; 7. colon; 8. urinary bladder; 9. gall bladder;10. abdominal muscles divided and reflected.
1. Esophagus; 2. stomach; 3. liver raised, showing itsunder surface; 4. duodenum; 5. small intestines; 6. cæcum; 7. colon; 8. urinary bladder; 9. gall bladder;10. abdominal muscles divided and reflected.
125. The organs which belong to the apparatus of excretion are—1. The large intestines consistingof the cæcum (fig. LXIII. 6). 2. The colon (fig. LXIII. 7). 3. The rectum (fig. LXIV. 10). It is the office of these organs, which are called the large intestines, to carry out of the system that portion of the alimentary mass which is not converted into nourishment. 4. The kidneys (fig. LXIV. 6), the organs which separate in the form of the urine an excrementitious matter from the blood, in order that it may be conveyed out of the system.
Fig. LXIV.General view of the viscera of the abdomen. 1. Stomachraised; 2. under surface of liver; 3. gall bladder; 4. spleen;5. pancreas; 6. kidneys; 7. ureters; 8. urinary bladder;9. portion of the intestine called duodenum; 10. portion ofthe intestine called rectum; 11. the aorta.
General view of the viscera of the abdomen. 1. Stomachraised; 2. under surface of liver; 3. gall bladder; 4. spleen;5. pancreas; 6. kidneys; 7. ureters; 8. urinary bladder;9. portion of the intestine called duodenum; 10. portion ofthe intestine called rectum; 11. the aorta.
126. The last division of the trunk, called the pelvis (fig. LXI. 3), consists of a circle of large and firm bones, interposed between the lower portion of the trunk and the inferior extremities (fig. XLV.). The bones that compose the circle, distinct in the child, are firmly united in the adult into a single piece; but as the original separation between each remains manifest, they are always described as separate bones. They are the sacrum (fig. XLV. 5), the coccyx (fig. XXXV.), the ilium (fig. XLV. 11), the ischium (fig. XLV. 12), and the pubis (fig. XLV. 13).
127. The sacrum, placed like a wedge between the moveable portion of the spinal column and the lower extremities, forms the posterior boundary of the pelvis. The figure of this bone is triangular (fig. XLV. 5); its anterior surface is concave and smooth, for enlarging the cavity of the pelvis and sustaining the organs contained in it (fig. XLV. 5); its posterior surface is convex,irregular, and rough (fig. XXXV.), giving origin to the great muscles that form the contour of the hip, and to the strong muscles of the back and loins that raise the spine and maintain the trunk of the body erect.
128. The base or upper part of the sacrum receives the last vertebra of the loins on a large and broad surface (fig. XLV. 4), forming a moveable joint; and the degree of motion at this point is greater than it is at the higher points of the spinal column. Firmly united at its sides with the haunch bones, it admits there of no degree of motion.
129. The coccyx, so named from its resemblance to the beak of the cuckoo, when elongated by a succession of additional bones, forms the tail in quadrupeds; but in man it is turned inwards to support the parts contained in the pelvis, and to contract the lower opening of the cavity. By means of a layer of cartilage, the medium by which this bone is connected with the sacrum, it forms a moveable articulation, continuing moveable in men until the age of twenty-five, and in women until the age of forty-five; continuing moveable in women thus long, in order that by yielding to the force which tends to push it backwards during the period of labour, it may enlarge the lower aperture of the pelvis, and so facilitate the process of parturition and diminish its suffering.
130. The lateral boundaries of the pelvis areformed by the ilium, the haunch bone (fig. XLV. 11), and by the ischium, the hip bone (fig. XLV. 12). The ilium forms the lower part of the abdomen and the upper part of the pelvis (fig. XLV. 11); its broad expanded wing supports the contents of the abdomen, and gives attachment to the muscles that form the anterior portion of its walls (figs. XLV. 11, and LXIII. 9); its external convex surface sustains the powerful muscles that extend the thigh; and along its internal surface is the prominent line which marks the brim of the pelvis (fig. XLV. 15), and which divides this cavity from that of the abdomen.
131. The ischium or hip bone is the lower part of the pelvis (fig. XLV. 12); at its undermost portion is a rounded prominence called the tuberosity (fig. XLV. 12), in its natural condition covered with cartilage, upon which is superimposed a cushion of fat. It is this part on which the body is supported in a sitting posture.
132. The pubis or share bone forms the upper and fore part of the pelvis (fig. XLV. 13), and together with the two former bones, completes the large and deep socket, termed the acetabulum (fig. XLV. 14), into which is received the head of the thigh-bone (fig. XXXIV. 4). The margin of the acetabulum and the greater part of its internal surface is lined with cartilage, so that in its natural condition it is much deeper than it appears to be when the bones alone remain.
133. The lower aperture of the pelvis, which appears large when all the soft parts are removed, is not really large, for in its natural state it is filled up partly by muscles and partly by ligaments, which sustain and protect the pelvic organs, leaving only just space enough for the passage to and from those which have their opening on the external surface.
134. The cavity of the pelvis, together with all the organs contained in it, are lined by a continuation of the membrane that invests the abdomen and its contents.
135. The organs contained in the pelvis are the rectum (fig. LXIV. 9), which is merely the termination of the large intestines, the urinary bladder (fig. LXIV. 8), and the internal part of the apparatus of reproduction.
136. The large and strong bones of the pelvis not only afford lodgment and protection to the tender organs contained in its cavity, but sustain the entire weight of the body, the trunk resting on the sacrum as on a solid basis (fig. XLV. 5), and the lower extremities being supported in the sockets in which the heads of the thigh-bones play, in the varied movements of locomotion (fig. XXXIV. 4).
137. The last division of the body comprehends the superior and the inferior extremities.
138. The superior extremities consist of the shoulder, arm, fore-arm, and hand.
139. The soft parts of theSHOULDERare composed chiefly of muscles; its bones are two, the scapula or theblade bone, and the clavicle or thecollar bone(fig. LXV. 2, 4).
Fig. LXV.1. Sternum; 2. clavicle; 3. ribs; 4. anterior surface ofscapula; 5. coracoid process of scapula; 6. acromion processof scapula; 7. margin of glenoid cavity of scapula;8. body of the humerus or bone of the arm; 9. head of thehumerus.
1. Sternum; 2. clavicle; 3. ribs; 4. anterior surface ofscapula; 5. coracoid process of scapula; 6. acromion processof scapula; 7. margin of glenoid cavity of scapula;8. body of the humerus or bone of the arm; 9. head of thehumerus.
140. TheSCAPULAis placed upon the upper and back part of the thorax, and occupies the space from the second to the seventh ribs (fig. LXV. 4)
Fig. LXVI.1. Posterior surface of scapula; 2. margin of scapula;3. acromion process; 4. margin of glenoid cavity; 5. clavicle;6. body of humerus; 7. head of humerus.
1. Posterior surface of scapula; 2. margin of scapula;3. acromion process; 4. margin of glenoid cavity; 5. clavicle;6. body of humerus; 7. head of humerus.
Unlike that of any other bone of the body, it is embedded in muscles, without being attached to any bone of the trunk, excepting at a single point. From the bones of the thorax it is separated by a double layer of muscles, on which it is placed as upon a cushion, and over the smooth surface of which it glides. Originally, like the bones of the skull, it consisted of two tables of compact bone, with an intermediate layer of spongy bony substance (diploë); but, by the pressure of the muscles that act upon it, it gradually grows thinner and thinner, until, as age advances, it becomesin some parts quite transparent and as thin as a sheet of paper.
141. The figure of the scapula is that of an irregular triangle (fig. LXVI.). Its anterior surface is concave (fig. LXV. 4), corresponding to the convexity of the ribs (fig. XLV. 7); its posterior surface is very irregular (fig. LXVI. 1), being in some parts concave and in others convex, giving origin especially to two large processes (figs. LXV. 5, and LXVI. 3); one of which is termed theacromion(fig. LXVI. 3), and the other thecoracoidprocess of the scapula (fig. LXV. 5). The margins of the bone, whatever the thinness of some portions of it, are always comparatively thick and strong (fig. LXVI. 2), affording points of origin or of insertion to powerful muscles. At what is called the anterior angle of the bone there is a shallow oval depression covered with cartilage and deepened by a cartilaginous margin, called theglenoidcavity of the scapula (figs. LXV. 7, and LXVI. 4), which receives the head of the humerus or bone of the arm (figs. LXV. 9, and LXVI. 7, 6).
142. The clavicle, the second bone of the shoulder, is a long and slender bone, of the form of an italic, projecting a little forwards towards its middle, so as to give a slight convexity of outline to the top of the chest and the bottom of the neck (fig. LXV. 2). It is attached by one extremity to the sternum (fig. LXV. 2) and by the other tothe scapula (fig. LXV. 2), by moveable joints. The nature of an immoveable joint has been explained (63). In the connexion of the bones of the trunk, while the main object is to secure firmness of attachment, some degree of motion is at the same time obtained (81 et seq.): but the mode in which the several bones of the extremities are connected with each other and with the trunk, admits of so great a degree of motion, that these articulations are pre-eminently entitled to the name of moveable joints. The component parts of all moveable joints are bone, cartilage, synovial membrane, and ligament. The great character of a moveable joint is the approximation of two or more bones; yet these bony surfaces are never in actual contact, but are invariably separated from each other by cartilage. The cartilage either covers the entire extent of the articulating surface of the bones, as in the shoulder-joint, where both the head of the humerus and the cavity of the scapula that receives it are enveloped in this substance (fig. LXV. 7. 9), or a portion of it is placed between the articulating surfaces of the bones, as in the joint between the clavicle and sternum (fig. LXVII.a); which, when so placed, is termed an interarticular cartilage (fig. LXVII.a). By its smooth surface cartilage lessens friction; while by its elasticity it facilitates motion and prevents concussion. Slightly organized cartilage is provided with comparatively few blood-vessels and nerves.Had it been vascular and sensible like the skin and the muscle, the force applied in the movements of the joint would have stimulated the blood-vessels to inordinate action, and the sensibility of the nerves would have been the source of constant pain: every motion of every joint would have been productive of suffering, and have laid the foundation of disease. The facility and ease of motion obtained by the smoothness, elasticity, and comparative insensibility of cartilage are still further promoted by the fluid which lubricates it, termed synovia, secreted by a membrane called synovial, which lines the internal surface of the joint, and which bears a close resemblance to the serous (30). Synovia is a viscid fluid of the consistence of albumen (5). It is to the joint what oil is to the wheel, preventing abrasion and facilitating motion; but it is formed by the joint itself, at the moment when needed, and in the quantity required. The motion of the joint stimulates the synovial membrane to secretion, and hence the greater the degree of motion, the larger the quantity of lubricating fluid that is supplied. The several parts of the apparatus of moveable joints are retained in their proper position by ligamentous substance, which, as has been shown (96 and 97), is oftentimes so strong that it is easier to fracture the bone than to tear the ligament, and in every case the kind and extent of motion possessed by the joint are dependent mainly on theform of the articulatory surfaces of the bones and on the disposition of the ligaments.
143. In the joint formed by the clavicle and the sternum (fig. LXVII.a) an interarticular cartilage is placed between the two bones which are united, first by a strong fibrous ligament, which envelops them as in a capsule (fig. LXVII. 1); by a second ligament, which extends from the cartilage of the first rib to the clavicle (fig. LXVII. 4), by which the attachment of the clavicle to the sternum is materially strengthened; and by a third ligament which passes transversely from the head of one clavicle to that of the other (fig. LXVII. 3). The joint thus formed, though so strong and firm that the dislocation of it is exceedinglyrare, yet admits of some degree of motion in every direction, upwards, downwards, forwards, and backwards; and this articulation is the sole point by which the scapula is connected with the trunk, and consequently by which the upper extremity can act, or be acted upon, by the rest of the body.
Fig. LXVII.1. The fibrous capsule of the sternum and clavicle; 2. thesame laid open, showinga, the interarticular cartilage;3. the ligament connecting the two clavicles; 4. the ligamentjoining the clavicle to the first rib; 5. ligamentspassing down in front of the sternum.
1. The fibrous capsule of the sternum and clavicle; 2. thesame laid open, showinga, the interarticular cartilage;3. the ligament connecting the two clavicles; 4. the ligamentjoining the clavicle to the first rib; 5. ligamentspassing down in front of the sternum.
144. The scapular extremity of the clavicle (fig. LXVIII. 6) is attached to the processes of the scapula (fig. LXVIII. 4. 3) by several ligaments of great strength (fig. LXVIII. 7, 8, 9). First by very strong fasciculi which pass from the upper surface of the clavicle to the acromion of the scapula (fig. LXVIII. 6); and secondly by two ligamentswhich unite the clavicle with the coracoid process of the scapula (fig. LXVIII. 8, 9). These ligaments are so powerful that they resist a force capable of fracturing the clavicle; and they need to be thus strong, for the clavicle is a shaft which sustains the scapula, and through the scapula the whole of the upper extremity; and the main object of the joint by which these bones are united, is to afford a firm attachment of the scapula to its point of support.
Fig. LXVIII.1. The clavicle; 2. the anterior part of the scapula;3. the coracoid process; 4. the acromion process; 5. the humerus;6. ligaments binding the scapular end of the clavicleto the acromion; 7. 8. 9. ligaments passing from oneprocess of the scapula to the other; 10. the fibrous capsuleof the shoulder-joint.
1. The clavicle; 2. the anterior part of the scapula;3. the coracoid process; 4. the acromion process; 5. the humerus;6. ligaments binding the scapular end of the clavicleto the acromion; 7. 8. 9. ligaments passing from oneprocess of the scapula to the other; 10. the fibrous capsuleof the shoulder-joint.
145. The clavicle serves the following uses: it sustains the upper extremity; it connects the upper extremity with the thorax; it prevents the upper extremity from falling forwards upon the thorax; and it affords a fixed point for steadying the extremity in the performance of its various actions.
146. The glenoid cavity of the scapula (fig. LXV. 7) receives the head of the humerus, the bone of the arm (fig. LXV. 9), and the two bones being united by ligament form the shoulder-joint (fig. LXVIII.). This joint is what is termed a ball and socket joint, the peculiarities of which are two: first, beyond all others this mode of articulation admits of free and extensive motion; in the present case, there is the utmost freedom of motion in every direction, upwards, downwards, backwards, and forwards. In the second place, this mode of articulation admits of the motion of the limb without that of the body, or of the motion of the bodywithout that of the limb. When at rest, the arm may be moved in almost any direction without disturbing the position of any other part of the frame; the manifold advantages of which are obvious. On the other hand, by careful management, very considerable variations in the posture of the body may be effected without the communication of any degree of motion to the limb; an unspeakable advantage when the limb has sustained injury, or is suffering from disease.
147. It does not seem possible to construct a joint of great strength, capable, at the same time, of the degree of motion possessed by the joint of the shoulder. So shallow is the socket of the scapula, and so large the head of the humerus, that it seems as if the slightest movement must dislodge it from its cavity (fig. LXVI. 4. 7). For sustaining heavy weights or resisting a great amount of pressure, applied to it suddenly and in various directions, the arm is obviously unfitted. But this is not its office. The superior extremities are the organs of apprehension—the instruments by which the mind executes the commands of the will. They do not need the strength required by the organs that sustain the weight of the body and that perform the function of locomotion; but they do need freedom and extent of motion: to this strength may be sacrificed, and so it is; yet what can be done to combine strength with mobility is effected. Large and strong processes of bone, proceedingas has been shown (141), from the convex surface of the scapula (figs. LXV. and LXVI.), overhang, and to a considerable extent surround, the head of the arm-bone, especially resisting the force that would dislodge it from its socket and drive it upwards, inwards, and backwards (fig. LXV.), the directions in which force is most commonly applied to it. By these processes of bone the joint is greatly strengthened, especially in those directions. Moreover, a strong ligament, termed the fibrous capsule (fig. LXVIII. 10) envelops the joint. This ligament, arising from the neck of the scapula (fig. LXVIII. 10), expands itself in such a manner as completely to surround the head of the humerus (fig. LXVIII. 10); and then again contracts in order to be inserted into the neck of the bone (fig. LXVIII. 10). This ligament is strengthened by the tendons of no less than four muscles which are expanded over it, as well as by the powerful substance termed fascia which is reflected upon it from both the processes and ligaments of the scapula. In addition to all these expedients for fortifying the joint, it receives a further security in the position of the scapula, which is loose and unattached; which slides easily over the ribs upon its cushion of flesh; which thus obtains, by its facility of yielding, some compensation for its want of strength, eluding the force which it cannot resist.
148. The arm consists of numerous and powerfulmuscles, and of a single bone, the humerus, which belongs to the class of bones termed cylindrical (185).
149. The upper end of the humerus terminates in a circular head (fig. LXV. 9), which is received into the socket of the scapula (fig. LXV. 9. 7)termed, as has been stated (141), its glenoid cavity. The middle portion of the bone, or what is termed its shaft (fig. LXV. 8), diminishes considerably in magnitude, and becomes somewhat rounded (fig. LXV. 8), while its lower end again enlarges, and is spread out into a flattened surface of great extent (fig. LXIX. 1, 3, 2, 4). Of this broad flattened surface, the middle portion is grooved (fig. LXIX. 2): it is covered with cartilage; it forms the articulating surface by which the arm is connected with the fore-arm. On each side of this groove there is a projection of bone or tubercle, termed condyle (fig. LXIX. 3, 4), the inner (fig. LXIX. 3) being much larger than the outer (fig. LXIX. 4). The inner condyle gives origin to the muscles that bend, the outer to those that extend the fore-arm and the fingers (figs. LXXXIV. 1, 2, and LXXXV. 1).
Fig. LXIX.1. Lower extremity of the humerus; 2. grooved surface;3. internal condyle; 4. external condyle; 5. the upper partof the ulna; 6. the head; 7. the neck; 8. the tubercle ofthe radius.
1. Lower extremity of the humerus; 2. grooved surface;3. internal condyle; 4. external condyle; 5. the upper partof the ulna; 6. the head; 7. the neck; 8. the tubercle ofthe radius.
150. The muscles that act upon the arm arise from the back (fig. LXXII. 2), the chest (fig. LXXI. 1), the clavicle (fig. LXXI. 1), and the scapula (fig. LXXI. 3); and they move the arm with freedom and power upwards, downwards, forwards, backwards, inwards, and outwards. The chief muscle that raises the arm is the deltoid (fig. LXXI. 3), which arises partly from the clavicle and partly from the scapula (fig. LXXI. 3). It has the appearance of three muscles proceeding in different directions, the different portions being separated by slight fissures (figs. LXXI. 3, and LXXII. 3). The fibres converging unite and form a powerful muscle which covers the joint of the humerus (fig. LXXI. 3). It is implanted by a short and strong tendon into the middle of the humerus (fig. LXXI. 4). Its manifest action is to pull the arm directly upwards. Its action is assisted by the muscles that cover the back of the scapula, which are in like manner inserted into the humerus, andwhich, at the same time that they elevate the arm, support it when raised.
Fig. LXXI.View of the muscles on the fore part of the chest thatact upon the arm. 1. The muscle called the great pectoral;2. the small pectoral; 3. the deltoid; 4. the humerus.
View of the muscles on the fore part of the chest thatact upon the arm. 1. The muscle called the great pectoral;2. the small pectoral; 3. the deltoid; 4. the humerus.
151. The principal muscle that carries the arm downwards is the latissimus dorsi (fig. LXXII. 2), the broadest muscle of the body, which, after having covered all the lower part of the back and loins, terminates in a thin but strong tendon which stretches to the arm, and is implanted into the humerus (fig. LXXII. 2), near the tendon of a muscle immediately to be described,—the great pectoral. When the arm is raised by the deltoid and its assistant muscles, the latissimus dorsi carries it downwards with force, and its powerful action is increased by that of other muscles which arise from the scapula and are inserted into the arm.
152. The principal muscle that carries the arm forwards towards the chest, is the great pectoral (fig. LXXI. 1), which, arising partly from the clavicle (fig. LXXI. 1), partly from the sternum (fig. LXXI. 1), and partly from the cartilages of the second, third, fourth, fifth, and sixth ribs (fig. LXXI. 1), covers the greater part of the breast (fig. LXXI. 1). Its fibres, converging, terminate in a strong tendon, which is inserted near the tendon of the longissimus dorsi into the humerus, about four inches below its head (fig. LXXI. 1). These two muscles form the axilla or armpit, the anterior border of the axilla consisting of the pectoral muscle. Though each of these muscles has its own peculiar office, yet they often act in concert, thereby greatly increasing their power, and the result of their combined action is to carry the arm either directly downwards or to the side of the chest.
Fig. LXXII.View of the muscles seated on the back part of the trunkthat act upon the shoulder and arm. 1. The muscle calledthe trapezius; 2. the latissimus dorsi; 3. the deltoid.
View of the muscles seated on the back part of the trunkthat act upon the shoulder and arm. 1. The muscle calledthe trapezius; 2. the latissimus dorsi; 3. the deltoid.
153. Some of the muscles that elevate the arm carry it inwards, and others outwards; the muscles that carry it forwards likewise carry it inwards; while of the muscles that pull it downwards, some direct it forwards and inwards, and others backwards and outwards (151 and 152).
154. It has been already stated that the shoulder-joint is completely surrounded by the muscular fibres or the tendinous expansions of several of these powerful muscles, which have a far greater effect in maintaining the head of the humerus in its socket than the fibrous capsule of the joint; the latter being necessarily loose, in order to allow of the extended and varied motions of the arm, whereas the muscles that encompass the joint adhere closely and firmly to it. Moreover, by virtue of their vital power these muscles act with an efficiency which a mere ligamentous band is incapable of exerting; for they apportion the strength of resistance to the separating force, and react with an energy proportioned to the violence applied.
155. The bones of the fore-arm are two, the ulna and the radius (figs. LXIX. and LXXIII.). The ulna is essentially the bone of the elbow (figs. LXIX. 5, and LXXIII. 3); the radius that of thehand (fig. LXXV.). Supposing the arm to hang by the side of the body, and the palm of the hand to be turned forwards, the ulna, in apposition with the little finger, occupies the inner; and the radius, in apposition with the thumb, occupies the outer part of the fore-arm (fig. XXXIV. 3).