FIG. 431. Ovum fourteen days old, 432. Ovum and embryo fifteen days old, after Maygrier, PAGE 533 533 433. Ovum and embryo twenty-one days old, after Maygrier, 533 434. Foetus at forty-five days, 534 435. Foetus at two months, 534 436. Corpora Wolffiana, with kidney and testes, from embryo of birds, 437. Foetus at three months, in its membranes, 534 535 438. Full period of utero-gestation, . 539 439. The extremity of a villus magnified 200 diameters, after Weber, 440. Transverse section of the uterus and placenta, after J. Reid, 547 548 441. Connexion between the maternal and foetal vessels, after J. Reid, 446. Section of thymus gland at the eighth month, after Sir Astley Cooper, 447. Circulatory organs of the foetus, after Wilson, 557 559 448. Descent of the testicle, after Curling, . 562 449, 450. Diagrams illustrating the descent of the testis, 563 451. Schemes of sections of the lower jaw of the foetus at different periods, to show the stages of developement of the sac of a temporary incisor and of the succeeding permanent tooth from the mucous membrane 454. Vertical section of an adult bicuspis, cut from without inwards; greatly magnified, after Retzius, 600 455, 456. View of an incisor and of a molar tooth, given by a longitudinal 461. Side view of upper and lower jaw, showing the teeth in their sockets. The outer plate of the alveolar processes has been taken off, 465. Curves indicating the developement of the height and weight of male 472. Curves indicating the viability or existibility of male and female at dif ferent ages, after Quetelet, 716 HUMAN PHYSIOLOGY. BOOK II. CHAPTER III. RESPIRATION. THE consideration of the function of absorption has shown how the different products of nutritive absorption reach the venous blood. By simple admixture with this fluid they do not become converted into a substance, capable of supplying the losses sustained by the frame from the different excretions. Nothing is better established than the fact, that no being, and no part of any being, can continue its functions unless supplied with blood, that has become arterial by exposure to air. It is in the lungs, that the absorbed matters undergo their final conversion into that fluid,-by a function, which has been termed hæmatosis, the great object of that which we have now to investigateRESPIRATION. This conversion is occasioned by the venous blood of the pulmonary vessels coming in contact with air in the air-cells of the lungs, during which the blood gives to the air some of its constituents, and, in return, the air parts with its elements to the blood. Fig. 262. Anterior View of Thorax. 6. 1. Superior piece of sternum. 2. Middle piece. 3. Inferior piece, or ensiform cartilage. 4. First dorsal vertebra. 5. Last dorsal vertebra. against transverse process of first dorsal verte First rib. 7. Its head. 8. Its neck, resting bra. 9. Its tuberosity. 10. Seventh or last true 11. Costal cartilages of true ribs. 12. Two last false ribs-floating ribs. 13. The groove along lower border of rib for lodgment of intercostal vessels and nerve. To comprehend this mysterious process, we must be acquainted with the pulmonary apparatus, as well as with the properties of atmospheric air, and the mode in which the contact between it and the blood is effected. VOL. II.-2 1. ANATOMY OF THE RESPIRATORY ORGANS. The thorax or chest contains the lungs,-the great agents of respiration. It is of a conical shape, the apex of the cone being formed by the neck, and the base by a muscle, which has already been referred to more than once,—the diaphragm. The osseous framework, Fig. 262, is formed, posteriorly, of twelve dorsal vertebræ; anteriorly, of the sternum, originally composed of eight or nine pieces; and laterally, of twelve ribs on each side, passing from the vertebræ to, or towards, the sternum. Of these, the seven uppermost extend the whole distance from the spine to the breast-bone, and are called true or sternal, and at times, vertebro-sternal ribs. They become larger as they descend, and are situate more obliquely in regard to the spine. The other five, called false or asternal, do not proceed as far as the sternum; the cartilages of three of them join that of the seventh true rib, whilst the two lowest have no union with those above them, and are, therefore, called floating ribs. These false ribs become shorter and shorter as we descend; so that the seventh true rib may be regarded as the common base of two cones, formed by the true and false ribs respectively. The different bones constituting the thorax are so articulated as to admit of motion, and thus of dilatation and contraction of the cavity. The motion of the vertebræ on each other has been described under another head. It is not materially concerned in the respiratory movements. The articulation of the ribs with the spine and sternum demands attention. They are articulated with the spine in two places, -at the capitulum or head, and at the tubercle. In the former of these, the extremity of the ribs, encrusted with cartilage, is received into a depression, similarly encrusted, at the side of the spine. One half of this depression is in the body of the upper vertebra; the other half in the one beneath it; and, consequently, partly in the intervertebral fibro-cartilage between the two. The joint is rendered secure by various ligaments; but it can move readily up and down on the spine. In the first, eleventh, and twelfth ribs, the articulations are with single vertebræ respectively. In the second articulation, the tubercle of the rib, also encrusted with cartilage, is received into a cavity in the transverse process of each corresponding vertebra; and the joint is rendered strong by three distinct ligaments. In the eleventh and twelfth ribs, this articulation is wanting. The articulation of the ribs with the sternum is effected by an intermediate cartilage, which becomes gradually longer, from the first to the tenth ribs, as seen in Fig. 262. The end of the cartilage is received into a cavity at the side of the sternum; and the junction is strengthened by an anterior and a posterior ligament. This articulation does not admit of much motion; but the existence of a synovial membrane shows, that it is destined for some. The cavity of the thorax is completed by muscles. In the intervals between the ribs are two planes, whose fibres pass in inverse directions, and cross each other. These are the intercostals. The diaphragm forms the septum between the thorax and abdomen. Above, the cavity is open; and through the opening numerous vessels and nerves enter. The muscles, concerned in the respiratory function, are numerous. The most important of them is the diaphragm. It is attached, by its circumference, around the base of the chest; but its centre rises into the thorax; and, during its state of relaxation, forms an arch, the middle of which is opposite the inferior extremity of the sternum. It is tendinous in its centre, and is attached by two fasciculi, called pillars, to the spine,-to the bodies of the first two lumbar vertebræ. It has three apertures; one before for the passage of the vena cava inferior; and two behind, between the pillars, for the passage of the oesophagus and aorta. The other great muscles of respiration are the serratus posticus inferior, serra Fig. 263. the blood and air, in Anterior View of the Thoracic Viscera in situ, as shown by the the small space occupied by the lungs. The admirable arrangement adopted consists in this, -that each of the minute vessels, in which the pulmonary artery terminates and the pulmonary veins originate, removal of the Anterior Parietes of the Thorax. 1. Superior lobe of right lung. 2. Its middle lobe. 3. Its in ferior lobe. 4, 4. Lobular fissures. 5, 5. Internal layer of costal pleura forming the right side of the anterior mediastinum. 6, 6. Right diaphragmatic portion of pleura costalis. 7,7. Right pleura costalis on the ribs. 8. Superior lobe of left lung. 9. Its inferior which forms the left side of the anterior mediastinum. 12. Left lobe. 10, 10. Interlobular fissures. 11. Portion of pleura costalis diaphragmatic portion of pleura costalis. 13. Left pleura costalis. 14, 14. The middle space between the pleuræ, known as the anterior mediastinum. 15. Pericardium. 16. Fibrous partition over which the pleuræ are reflected. 17. Trachea. 18 Thyroid gland. 19. Anterior portion of thyroid cartilage. 20. Primitive carotid artery. 21. Subclavian vein. 22. Internal jugular vein. 23. Brachio-cephalic vein. 24. Abdominal aorta. 25. Xiphoid cartilage. 1 Précis, &c., ii. 307. Posterior View of the Thoracic Viscera, showing their relative positions by the removal of the Posterior Portion of the Parietes of the Thorax. 1, 2. Upper and lower lobes of right lung. 3. Interlobalar fis sides of posterior mediastinum. 5. Twelfth rib and lesser dia is surrounded on every side by air. The lungs are two organs of considerable size, situate in the lateral parts of the chest, and subdivided into lobes and lobules, the shape and number of which cannot be readily determined. They are termed right and left, respectively, according to the side of the cavity of the chest which they occupy. The former consists of three lobes; the latter of two. Each of these exactly fills the corresponding cavity of the pleura; and they are separated from each other by a duplicature of the pleura-(the serous membrane that lines the chest, and is reflected over the lungs) and . by the heart. The colour of the lungs is generally of a marble sures. 4. Internal portion of pleura costalis, forming one of the phragm. 6. Reflection of the pleura over the greater muscle of the blue; and the exterior diaphragm on the right side. 7, 7. Right pleura costalis adhering is furrowed by figures to the ribs. 8. 9 The two lobes of the left lung. 10, 10. Interlobu lar fissures. 11, 11. Left pleura, forming the parietes of the poste- of hexagonal shape. side. 14, 14. Left pleura costalis on the parietes of the chest. 15. The appearance is not, Trachea. 16. Larynx. 17. Opening of the larynx and the epiglot- however, the same at tic cartilage in situ. 18. Root and top of the tongue. 19, 19. Right all ages, and under all rior mediastinum. 12, 13. Its reflections over the diaphragm on this and left bronchia. 20. The heart enclosed in pericardium. 21. Upper portion of diaphragm on which it rests. 22. Section of sophagus. 23. Section of aorta. 24. Arteria innominata. 25. Primi- circumstances. In intive carotid arteries. 26. Subclavian arteries. urtera jugu fancy, they are of a pale red; in youth, of lar veins. 28. Second cervical vertebra. 29. Fourth lumbar. a darker colour; and in old age, of a livid blue. The elements that compose them are ;-the ramifications of the trachea; those of the pulmonary artery and pulmonary veins, besides the organic elements, that appertain to every living structure,-arteries, veins, lymphatics, nerves, and areolar tissue. The ramifications of the windpipe form the cavity of the organ of respiration. The trachea is continuous with the larynx, from which it receives the external air conveyed to it by the mouth and nose. It passes down to the thorax, at the anterior part of the neck, and bifurcates opposite the second dorsal vertebra, forming two large canals called bronchi or bronchia. One of these goes to each lung; and, after numerous subdi |