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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera of the Sauropsida
 
 
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The Larynx in Sauropsida

 
     
 

All the Sauropsida possess a larynx, a trachea, and one or two lungs. The bronchi do not divide dichotomously, as they do in Mammalia.

In Chelonia and Crocodilia the larynx consists of a circular cartilage apparently corresponding with both the thyroid and the cricoid of the higher Vertebrata; and of the arytenoid cartilages, articulated with its anterior and dorsal edge.

The Lacertilia have, for the most part, a similar larynx, but the circular cartilage is often interrupted by round, or elongated, membranous fontanelles. In the Chameleons, the mucous membrane of the larynx between the circular cartilage and the first ring of the trachea protrudes in the form of an air-sac.

In the Amphisboenoidea, and in the Ophidia, the skeleton of the larynx consists of two lateral longitudinal bands of cartilage, united by from four to sixteen transverse bands. In other words, the structure which answers to the circular cartilage is greatly elongated, and has many transversely-elongated fontanelles. There is a single arytenoid cartilage, which is sometimes represented by a process of the anterior dorsal margin of the circular cartilage. An epiglottis is rarely present.

In Birds there are distinct thyroid, cricoid, and arytenoid cartilages, which may be more or less completely ossified. Sometimes an epiglottis is added.

The voice of Birds, however, is not formed in the larynx, but in the syrinx, or lower larynx, which may be developed in three positions: 1. At the bottom of the trachea, from tlie trachea alone. 2. At the junction of the trachea and bronchi, and out of both. 3. In the bronchi alone. The syrinx may be altogether absent, as in the Ratitae and the Cathartidae, or American vultures.

The commonest form of syrinx is the second mentioned above, or the broncho-tracheal syrinx. It is to be met with in all our common song-birds, but is also completely developed in many birds, such as the crows, which have no song. In its commonest condition this form of syrinx presents the following characters: The hindermost rings of the trachea coalesce, and form a peculiarly-shaped chamber, the tympanum. Immediately beyond this, the bronchi diverge, and from their posterior wall, where one bronchus passes into the other, a vertical fold of the lining membrane rises, in the middle line, toward the tympanum, and forms a vertical septum between the anterior apertures of the two bronchi. The anterior edge of this septum is a free and thin membrana semilunaris, but in its interior a cartilaginous or osseous frame is developed, and becomes united with the tympanum. The base of the frame is broad, and sends out two cornua, one along the ventral, and the other along the dorsal, edge of the inner wall of the bronchus of its side; which, in this part of its extent, is membranous and elastic, and receives the name of the membrana tympaniformis interna.

The bronchial "rings" opposite this are necessarily in complete internally, and have the form of arches embracing the outer moiety of the bronchus. The second and third of these bronchial arcs are freely movable, and elastic tissue, accumulated upon their inner surfaces, gives rise to a fold of the mucous membrane, which forms the outer boundary of a cleft, bounded, on the inner side, by the membrana semilunaris. The air forced through these two clefts from the lungs sets their elastic margins vibrating, and thus gives rise to a musical note, the character of which is chiefly determined by the tension of the elastic margins and the length of the tracheal column of air. The muscles, by the contraction of which these two factors of the voice are modified, are extrinsic and intrinsic. The former are possessed by birds in general, and are usually two pair, passing from the trachea to the furcula and to the sternum. Some birds possessing a bronoho-tracheal syrinx such as has been described, as the Alectromorphae, Chenomorphae and Dysporomorphae, have no intrinsic muscles. Most others have one pair, attached, one on each side, to the rings of the trachea above, and to the tympanum, or the proximal bronchial arcs, below. The majority of the Coracomorphae, have five or six pairs of intrinsic syringeal muscles, which pass from the trachna and its tympanum to the movable bronchial ares. The Parrots have no septum, and only three pairs of intrinsic muscles.

The tracheal syrinx occurs only in some American Coracomophae. The hinder end of the trachea is flattened, and six or seven of its rings above the last are interrupted at the sides, and held together by a longitudinal ligamentous band. These rings are excessively delicate, so that this part of the trachea is in great part membranous.

The bronchial syrinx occurs only in Steatornis, and Crotophaga.

In the genus Cinyxis, among the Chelonia, and in some species of Crocodilus (C acutus, e. g.), the trachea is bent upon itself. Similar flexures attain an extraordinary development in many birds, and may lie outside the thorax under the integument (Tetrao urogallus, some species of Crax and Penelope); in the cavity of the thorax (some Spoonbills); on the exterior of the sternum (some Swans and Cranes); or even in a sort of cup formed by the median process of the furcula (the Guinea-fowl). In the Emeu some of the rings of the trachea are incomplete in front, and bound the aperture of an air-sac which lies in front of the trachea. Some birds (Aptenodytes Procellaria) have the trachea divided by a longitudinal septum, as in Sphargis among the Chelonia. The tracheal tympanum is greatly enlarged in Cephalopterus, and in many Ducks, Geese, and Divers; and in these aquatic birds the enlargement is more marked in the males, and is usually asymmetrical, the left side being generally the larger.

In the Ophidia, the bronchus opens at once into the lung; and the latter is an elongated sae, the walls of which are produced into numerous septa, which render the cavity highly cellular near the bronchus, while, at the opposite end, they become smooth and but little vascular. In this latter region the lung may receive its blood from the systemic and not from the pulmonary circulation. The lungs are always unequal in size, and the left is usually the smaller. Very frequently, especially among the poisonous snakes, one lung is rudimentary or altogether absent; and the posterior portion of the trachea may take on the structure of a lung.

The lungs of Lizards much resemble those of the Ophidia, and they are elongated and unequal in size in the snake-like Lacertilia. In the ordinary lizards they are more rounded and the trachea and bronchi are shorter. In manv Chamaeleons, and in some Geckos, the posterior half of each lung is produced into narrow diverticula, which lie among the abdominal viscera, and foreshadow the air-sacs of birds.

In the Crocodilia each bronchus traverses its lung, and at first retains, but soon loses, its cartilaginous rings. Lateral apertures in the walls of the bronchus lead into sacculated pouches, each of which resembles the lung of an ordinary Lacertilian.

The Chelonia have similar lungs; but while, in the foregoing groups, the two lungs are free and invested on all sides by the peritonaeum, in this they are fixed against the inner periosteum of the carapace, and are covered by peritonaeum on their ventral face only. This resemblance to the arrangement of the lungs in birds is increased by the presence of a muscular diaphragm, the fibres of which spread over the ventral faces of the lungs.

In Aves the lungs are firmly fixed on each side of the vertebral column, the dorsal surface of each lung being moulded to the superjacent vertebrae and ribs. The muscular fibres of the diaphragm arise from the ribs outside the margins of the lungs, and from the vertebral column, and end in an aponeurosis upon the ventral surface of the lungs.

Each bronchus enters its lung nearer the centre than the anterior edge; and, immediately losing its cartilaginous or bony rings, dilates, and then traverses the lung, gradually narrowing, to the posterior edge of that viscus, where it terminates by opening into the posterior air-sac, which generally lies in the abdomen. From the inner side of the bronchus, canals are given off, one near its distal end, and others near its entrance into the lung, which pass directly to the ventral surface of the lung, and there open into other air-sacs. Of these there are four. Two, the anterior and the posterior thoracic, lie on the ventral face of the lung in the thorax. The other two are situated in front of its anterior end, and are extrathoracic. The external and superior is the cervical, the internal and inferior, the interclavicular. This last unites into one cavity with its fellow of the opposite lung. Thus there are altogether nine air-sacs; two posterior or abdominal, four thoracic, two cervical, and one interclavicular. Other large canals given off from the bronchus do not end in air-sacs, but those which pass from the inner side of the bronchus run along the ventral surface, and those on the outer side, along the dorsal surface, of the lung. Here they give off, at right angles, series of secondary canals, and these similarly emit still smaller tertiary canals; and thus the whole substance of the lung becomes interpenetrated by tubuli, the walls of the finest of which are minutely sacculated. The different systems of tubuli are placed in communication by perforations in their avails.

 
     
 
 
     



     
 
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