Encyclopaedia of the medical sciences ... forming a portion of the Encyclopaedia Metropolitana.

Date:
1847
Catalogue details

Licence: Public Domain Mark

Credit: Encyclopaedia of the medical sciences ... forming a portion of the Encyclopaedia Metropolitana. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    116/590 (page 482)
    SECTION VII. ORGANS OF RESPIRATION. Anatomy. Under the above head are to be included the air- ^^/^-^ tube, consisting of the larynx, trachea and bronchi, and the lungs : but as a particular description of the larynx has been already given,* a brief recapitulation of its anatomy is all that will be necessary in the present section. The form, development, and organiza- tion of this division of the air-tube characterize it as designed to fulfil the two-fold function of an organ of respiration and voice. It consists of an expanded car- tilage (the thyroid), the two plates of which are united at an angle in front, but widely separated behind : below this is an annular cartilage (the cricoid), which is broad behind and narrow anteriorly. This again is sur- mounted by a pair of moveable cartilages (the aryte- noid), which are bound by strong elastic ligaments (chordae vocales) to the posterior angular portion of the thyroid cartilage. Strong elastic membranes fur- ther connect the thyroid and cricoid cartilages in front, and the latter to the first ring of the trachea; and a further development of similar tissue unites the thyroid cartilage and os hyoides, which in its turn is attached by round ligaments to the styloid processes of the tem- poral bones. The epiglottis surmounts the glottis or upper orifice of the larynx, and is connected partly by fibrous tissue, but principally by reflections of the mucous membrane, to the notch in the upper edge of the thyroid cartilage, to the os hyoides and base of the tongue, and to the arytenoid cartilages. An appropriate muscular apparatus performs the various motions of elevation and depression of the larynx, as well as of con- traction and dilatation of the rima glottidis, and modi- fication of the tension of the chordae vocales. The ante- rior and lateral parts of the larynx are embraced by a body of a soft and spongy character, and reddish-brown colour, known under the title of Thyroid gland ; it is convex in front, and consists of two oval lobes, con- nected anteriorly by a transverse band, which lie on the lower part of the larynx and upper rings of the tra- chea ; posteriorly and laterally it is in connexion with the carotid sheath and its contents. This organ varies much in dimensions: it has no duct, and its title to be called a gland is more than questionable : its function is not understood. The Trachea extends from the larynx to the bronchi, and consists of a series of fibro- cartilages ( about eighteen or twenty in number), which, though annular in form, are incomplete, each consti- tuting about three-fourths of a circle, the posterior fourth being occupied by a fibrous membrane, which is continued around, and in the interval between, the tra- cheal rings. The fibro-cartilages are flattened, and vary in their diameter ; but the calibre of the tube itself is the same throughout. The outer membrane is studded with glands, which open by small ducts upon the surface * See ' Muscular System,' p. 414. 482 of the mucous membrane: this latter is continuous Anatomy, with the lining membrane of the mouth, and extends ^^v*■-/ through all the ramifications of the bronchi. The fol- lowing are the relations of the trachea to surrounding parts: it is partially covered in the neck by the thyroid body, the sterno-hyoid and thyroid muscles, and the inferior thyroid veins: in the chest it lies posterior to the arch of the aorta, the arteria innominata and left vena innominata : its membranous portion rests upon the oesophagus; and it has on either side of it the carotid sheath and its contents. Opposite the second or third dorsal vertebra, the trachea divides into the right and left bronchus. Of these the right is larger and shorter, and its course is more horizontal than that of the left: the former is related to the curve of the vena azygos, and the latter passes obliquely downwards, and to the left side through the arch of the aorta to the left lung. Each bronchial tube divides, and the lower division of that destined for the right lung gives off a branch to its middle lobe. This binary arrangement is continued through five or six subdivisions, until ultimately the tubes diminish to capillary dimensions, and terminate in the air-vesicles. The bronchi and their earlier ramifications are similarly constituted to the trachea. The Lungs occupy the lateral divisions of the thorax, and correspond in form to the containing ca- vity ; that is, they are conical, with their base below and apex above. The varying dimensions of the thorax being constantly accompanied by proportionate disten- sion of the lungs, it necessarily follows that these organs at all times accurately fill the chest: they are surrounded by a serous membrane to be presently de- scribed, and are separated by the heart, which occupies the middle mediastinum. The colour of the Lungs varies at different periods of life, and under different circumstances : in the adult they are of a bluish-grey tint, and mottled with darker patches, which latter are more apparent later in life : in the young the tint is of a brighter and more pink character. These organs are not symmetrical, the points of difference being attri- butable to the position of the liver on the right side, and the inclination of the heart to the left. Externally the Lungs are convex, and internally slightly con- cave, where they correspond to the pericardium: their anterior margin is sharp, and notched opposite the apex of the heart on the left side; but their posterior margin is obtuse and rounded, corresponding to the furrow on each side of the spine: their base is slightly concave, and rests on the convexity of the diaphragm; and their summit is obtuse but narrow, and rises a little above the level of the first rib. Each Lung is divided by a deep fissure into two lobes: it descends from behind the summit of the organ obliquely downwards and forwards, and ends in front of the base; by which