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The posterior facial vein may terminate entirely in the common facial vein, or in the external or the internal jugular vein. It may be very small, and occasionally it is absent.
Variations of the cranial blood sinuses are not numerous. One transverse sinus may be absent or very small, when, as a rule, that of the opposite side is enlarged. The inferior sagittal, the occipital, or the spheno-parietal sinuses may be absent, and there may be an additional petrosquamous tributary to the transverse sinus. The petro-squamous sinus, when present, is the remains of a sinus which crossed the temporal bone, passed through the post-condyloid foramen and terminated in the lateral cerebral vein. In the human adult, in rare cases, it pierces the skull behind the condyle of the mandible, and terminates in the posterior facial vein. This is the normal arrangement in some mammals.
THE VEINS OF THE SUPERIOR EXTREMITY.
The superficial veins of the forearm are extremely variable; any of them may be absent, but most commonly it is the median or the cephalic vein which is wanting. The median cephalic and the cephalic veins may be small or absent, and, on the other hand, the cephalic vein may be larger than usual. Moreover the cephalic vein may end in the external jugular vein, its original termination; or it may be connected with the external jugular vein by an anastomosing channel which sometimes passes over the clavicle and sometimes through that bone.
The basilic vein is sometimes larger and sometimes smaller than usual, and it may pierce the fascia of the arm at a more proximal or at a more distal level than usual.
The venæ comites of the arteries of the upper extremity generally terminate at the lower border of the subscapularis, where they join the axillary vein, but they may end above or below the position of their usual termination.
The subclavian vein 'sometimes passes behind instead of in front of the scalenus anterior muscle, and it has been seen passing between the clavicle and the subclavius muscle.
THE INFERIOR VENA CAVA.
The lower part of the inferior vena cava is sometimes absent, in which case the common iliac veins ascend, one on the right and the other on the left of the aorta, to the level of the second lumbar vertebra, where the left common iliac vein receives the left renal vein, and then crosses in front of or behind the aorta to fuse with the corresponding vein of the right side; in such cases, therefore, the inferior vena cava commences at the level of the second lumbar vertebra, and it represents only the upper and last-formed part of the ordinary vessel; the common iliac veins, each of which receives the lumbar veins of its own side, are exceptionally long, and they may or may not be united at the pelvic brim by a small transverse anastomosing channel. Cases of this kind are sometimes described as partial doubling of the inferior vena cava.
Occasionally the inferior vena cava does not terminate in the right atrium, but is continuous with the vena azygos, which is much enlarged, all the inferior caval blood being then carried to the superior vena cava. In such cases the hepatic veins open directly into the right atrium without communicating with the inferior vena cava.
The lower part of the inferior vena cava sometimes lies to the left instead of to the right of the aorta; this condition is associated with a long right common iliac vein, which crosses obliquely from right to left to join the shorter left common iliac vein. After receiving the left renal vein the misplaced inferior vena cava crosses in front of the aorta, reaching the right side at the level of the second or first lumbar vertebra. In other cases, however, the left inferior vena cava continues upwards through the left crus of the diaphragm, usurping the place of a greater or smaller part of the hemiazygos vein; having entered the thorax, it may cross to the opposite side and terminate in the vena azygos, or it may continue upwards on the same side, and after arching over the root of the left lung, descend behind the left atrium to terminate in the right atrium in the situation of the coronary sinus. In this group of cases also the hepatic veins open separately into the right atrium.
The inferior vena cava may lie ventral instead of dorsal to the right internal spermatic artery, in which case the lower part of the vessel has been derived from the subcardinal vein instead of from the posterior cardinal vein. (Johnston, Journ. of Anat. and Phys. xlvii. 1913.)
The tributaries of the inferior vena cava are also subject to variation. Additional renal, spermatic, ovarian, or suprarenal veins may be present. Two or three lumbar veins of one or both sides may unite into a common trunk which terminates in the inferior vena cava, and the hepatic veins may open separately, or after fusing into a common trunk, into the right atrium near the opening of the inferior vena cava.
No explanation of the variations of the inferior vena cava and its tributaries is necessary, beyond the statement that they are due to persistence of portions of the cardinal and subcardinal veins which usually disappear, and to the persistence of transverse anastomoses and tributaries which usually atrophy, or to modifications of those which ordinarily take part in the formation of the inferior vena caval system.
The left common iliac vein is short and the right long when the inferior vena cava lies on the left side. The common iliac veins may be absent, the hypogastric veins uniting to form the commencement of the inferior vena cava, into which the external iliac veins open as lateral tributaries.
THE VEINS OF THE INFERIOR EXTREMITY.
The great saphenous vein is not subject to much variation, but the small saphenous vein may terminate by joining the great saphenous, or, after piercing the deep fascia in the distal part of the thigh, it may ascend and join the inferior gluteal vein or one of the tributaries of the profunda vein.
The venæ comites are generally described as terminating in the lower extremity, at the distal part of the popliteal fossa, but they may ascend as far as the femoral trigone; as a matter of fact, one or more small additional veins usually accompany the popliteal and femoral arteries, although as a rule there is only one large popliteal and one large femoral vein.
In a few cases the popliteal vein does not pierce the distal part of the adductor magnus, but ascends behind that muscle and becomes continuous with the profunda vein, the femoral artery being unaccompanied by any large vein during its passage through the adductor canal.
ABNORMALITIES OF THE LYMPH VESSELS.
Variations of the glands and smaller vessels of the lymphatic system are so common that they can hardly be regarded as abnormalities; variations of the larger vessels, however, are comparatively rare. This is especially the case with respect to the two terminal trunks, the thoracic duct and the right lymph duct, the abnormalities of which are interesting and important.
When the arch of the aorta is on the right side instead of on the left side, the thoracic duct terminates usually in the right innominate vein, in which case it receives the tributaries which usually open into the right lymph duct, whilst the corresponding area on the left side is drained by lymph vessels terminating in a left lymph duct which opens into the commencement of the left innominate vein. A similar arrangement of the terminal lymph trunks sometimes occurs even when the arch of the aorta is in its normal position on the left side. In either case the thoracic duct may commence in the usual way, and after reaching the level of the fifth thoracic vertebra continue upwards on the right side, instead of crossing to the left side of the vertebral column; more rarely it commences on the left side and crosses over to the right at a higher level In one case in which the thoracic duct opened into the right innominate vein, instead of the left, no trace of a lymph duct was discovered on the left side.
Occasionally the thoracic duct commences and terminates in the usual manner, but crosses the vertebral column immediately after its origin and ascends on the left side.
Not uncommonly there is no distinct cisterna chyli, in which case the terminal lymph vessels of the abdomen merely unite to form a larger vessel which does not present any obvious dilatation, and from which the thoracic duct is continued. The terminal lymph trunk may open into the internal jugular vein, previous to its junction with the subclavian, instead of into the commencement of the innominate vein.
Occasionally the thoracic duct is double, either in the whole or in part of its extent, and sometimes it breaks up into a plexus of vessels which may reunite into a single trunk in the upper part of the thorax. Both the thoracic duct and the right lymph duct terminating, divide into branches which, though sometimes reuniting on each side into a single trunk, not infrequently open separately into the great veins at the root of the neck.
As a rule the thoracic duct joins the commencement of the left innominate vein, but it may end in the internal jugular, vertebral, or subclavian veins of the left side; whilst very rarely, it opens into the vena azygos.
1 (see p. 995). It is stated by H. Downey (Anat. Record, 1915) that there are no endothelial cells covering the trabecule of lymph glands. He asserts that the cells described as endothelial are connected with the fibrils of the reticulum.
2 (see p. 1025). More recent evidence throws doubt on this statement; it appears probable that blood and blood-vessels may be formed in situ in the embryonic region.
THE RESPIRATORY SYSTEM.
THE ORGANS OF RESPIRATION AND VOICE.
By the late D. J. CUNNINGHAM, F.R.S.,
Revised by RICHARD J. A. BERRY, F.R.C.S.,
THE organs of respiration are the larynx and trachea, which, together, constitute a median air-passage; the two bronchi or branches into which the inferior end of the trachea divides; and the two lungs to which the bronchi conduct the air. In connexion with the lungs there are also the pleural membranes-two serous sacs which line the portions of the thoracic cavity which contain the lungs, and at the same time give a thin coating to those organs.
The larynx opens above into the inferior or caudal part of the pharynx, and the air which passes in and out from the air-passages likewise traverses the pharynx, the nasal cavity, and also the oral cavity if the mouth be open. This connexion between the digestive and respiratory systems is explained by the fact that the respiratory apparatus is secondarily developed, as an outgrowth, from the ventral aspect of the primitive fore-gut of the embryo. mammals the superior or cranial aperture of the larynx opens into the part of the pharynx which lies immediately dorsal, or posterior, to the nasal cavities. In man, however, the superior opening of the larynx is placed below, that is inferior or caudal to, the communication between the mouth and pharynx, and both nasal and oral breathing may be carried on with very nearly equal ease.
The larynx or organ of voice is the upper part of the air-passage, specially modified for the production of voice. Above it opens into the pharynx, whilst below its cavity becomes continuous with the lumen of the trachea or windpipe.
Position and Relations of the Larynx. In the natural position of the neck, and whilst the organ is at rest, the larynx is placed on the ventral side of the bodies of the fourth, fifth, and sixth cervical vertebræ. Its highest point, represented by the tip of the epiglottis, extends to the inferior border of the body of the third cervical vertebra, whilst its lowest limit (the inferior border of the cricoid cartilage) usually corresponds to the inferior border of the body of the sixth cervical vertebra. From the vertebral column the larynx is separated, not only by the prevertebral muscles and the prevertebral fascia, but also by the dorsal wall of the pharynx-indeed the dorsal surface of the larynx forms the inferior part of the ventral or anterior wall of the pharynx, and is covered by the lining mucous membrane of that section of the alimentary canal.
The larynx lies below the hyoid bone and the tongue, and in the interval between the great vessels of the neck. It forms a more or less marked projection on the ventral side of the neck, and, in the median plane, it approaches very closely to the surface, being merely covered by skin and the two layers of fascia. Laterally
it is more deeply placed. There, it is overlapped by the sterno-cleido-mastoid muscle and is covered by the two strata of thin ribbon-like muscles which are attached to the thyreoid cartilage and the hyoid bone; and it is hidden, to some extent, by the upper prolongations of the lateral lobes of the thyreoid gland.
The position of the larnyx is influenced by movements of the head and neck. Thus it is elevated or raised when the head moves dorsally, and depressed when the chin is carried downwards towards the chest. Again, if the finger is placed upon it during deglutition, it will be noted that the larynx moves to a very considerable extent. The pharyngeal muscles attached to it, and more especially the stylo-pharyngeal muscles, are chiefly responsible for bringing about these movements. During singing, changes in the position of the larynx may also be noted, a high note being accompanied by a slight upward movement, and a low note by a similarly slight downward movement of the
The position of the larynx is not the same at all periods of development and growth. In the foetus, shortly before birth, it lies much nearer the head, and its inferior border corresponds to the inferior border of the fourth cervical vertebra. Its permanent position is not reached until the period of puberty is attained (Symington). This downward movement of the larynx has been stated to be due to the rapid and striking growth of the facial part of the skull (Symington). It is very doubtful, however, if the facial growth has any influence in this direction. In the anthropoid ape, in which the face forms a much greater part of the skull than in man, and in which, in the transition from the infantile to the adult condition, the facial growth is even more striking than it is in man, the larynx occupies a relatively higher position in the neck. In the early stages of growth all the thoracic viscera undergo a gradual subsidence and the larynx follows them. Indeed, it cannot do otherwise, seeing that the bifurcation of the trachea between infancy and puberty moves downwards towards the caudal end of the body more than the depth of one thoracic vertebra.
General Construction of the Larynx.-The wall of the larynx is constructed upon a somewhat complicated plan. There is a framework composed of several cartilages. These are connected together, at certain points, by distinct joints and also by elastic membranes. Two elastic cords, which stretch in a ventro-dorsal direction from the ventral to the dorsal wall of the larynx, form the groundwork of the vocal folds (O.T. true vocal cords). Numerous muscles also are present. These operate upon the cartilages of the larynx, and thereby not only bring about changes in the relative position of the vocal folds, but also produce different degrees of tension of these folds. The cavity of the larynx is lined with mucous membrane, under which, in certain localities, are collected masses of mucous glands.
Three single cartilages and three pairs of cartilages enter into the construction of the laryngeal wall. They are named as follows:
Single cartilages Cricoid.
Arytenoids. Corniculate cartilages (Santorini).
Cartilago Thyreoidea. The thyreoid cartilage, the largest of the laryngeal cartilages, is formed of two quadrilateral plates termed the laminæ, which meet ventrally at an angle, and become fused along the median plane. Dorsally the laminæ diverge from each other, and enclose a wide angular space which is open dorsally. The ventral borders of the lamina are fused only in their inferior parts Above they are separated by a deep, narrow V-shaped median notch, called the incisura thyreoidea or thyreoid notch. In the adult male the angle formed by the meeting of the ventral borders of the two laminæ, especially in its upper part, is very projecting, and with the margins of the thyreoid notch, which lies above, constitutes a marked subcutaneous prominence in the neck, which receives the name of the prominentia laryngea (O.T. Adam's Apple).
The angle which is formed by the meeting of the two lamine of the thyreoid cartilage varies, to some extent, in different individuals of the same sex, and shows marked differences in the two sexes and at different periods of life. In the adult male the average angle is said to be 90°; in the adult female it is 120°; whilst in the infant the laminæ meet in the form of a gentle curve, convex ventrally.
Inferior cornu of thyreoid cartilage
head, with a slight dorso-medial in- Pomum Adami
As it pro
FIG. 845.-VENTRAL ASPECT OF THE CARTILAGES
The superior border of each lamina is, for the most part, slightly convex, and, ventrally, it dips suddenly to become continuous with the margin of the thyreoid notch. Dorsally, where it joins the superior cornu, it exhibits a shallow notch or concavity. The inferior border is almost straight, but it is marked off by a
Superior cornu of hyoid bone Superior tubercle on the ala of
projection, termed the inferior thyreoid tubercle, into a short dorsal part, which shows a shallow concavity close to the inferior cornu and a longer part which lies ventral to the tubercle, and is also concave, but to a less degree.
The lateral surface of each lamina is divided into two unequal areas by the linea obliqua. This line begins above at the superior thyreoid tubercle, a prominence situated immediately below the superior border, and a short distance ventral to the root of the superior cornu. From the tubercle the oblique line proceeds forwards and downwards to end in the inferior tubercle on the inferior border of the lamina. The area which lies dorsal to the oblique line is much smaller than that which lies on its ventral side. It is covered by the inferior constrictor Cricoid cartilage muscle of the pharynx. The larger ventral area is for the most part covered by the thyreohyoid muscle. To the oblique line are attached the sterno-thyreoid, thyreo-hyoid, and inferior constrictor muscles. The medial surface of the lamina of the thyreoid cartilage is smooth and slightly concave.
Inferior cornu of
FIG. 846.-PROFILE VIEW OF THE CARTILAGES
AND LIGAMENTS OF THE LARYNX.
Cartilago Cricoidea.-The cricoid cartilage is shaped like a signet-ring. Dorsally there is a broad, thick plate, somewhat quadrilateral in form, termed the lamina; whilst ventrally and laterally the circumference of the ring is completed by a curved band, called the arch. The