Part 63

The first costal bronchus is the largest; those following it gradually become smaller. At their points of origin they adhere closely to the ribs. They are all imperforate, which is a distinguishing feature from those occupying the opposite face.

The canaliculi or tertiary tubules given off by these secondary bronchial tubes do not differ greatly in caliber in the various parts. They are given off at right angles from the pulmonary wall of each bronchus, and extend perpendicularly into the lung substance. Thus we find three kinds of conduits, the primary, the secondary, and the peripheral, or tertiary. The first are like the barbs of a feather on its shaft; and the second and parenchymatous are implanted on the pulmonary walls of the first, like the hairs of a brush on their common base. Thus instead of the branching of the bronchi being dichrotomous, as in mammals, it is piniform.

The canaliculi, or finer tubules, communicate with one another. The inner microscopic appearance of the canaliculi indicate that they are divided into areola, which gives them a cellular aspect. These tertiary bronchi open on a dense labyrinth of blood capillaries (Fig. 61, C). At this point the ciliated epithelial cells give way to simple squamous epithelium.

Thus we find three kinds of bronchi, or their ramifications, as follows: the primary, the secondary, and the tertiary.

=The Air-sacs= (Fig. 61, A).—The air-sacs are bladder-like structures consisting of a delicate cellulo-serous membrane, an extension from the bronchial tubes, in some places strengthened by an external envelope of elastic fibrous tissue. Long thin blood-vessels are distributed in the substance of these walls. They are branches from vessels of the general circulation and not extensions from those of the lungs. No lymphatics have been found in the air-sacs.

FIG. 61, A.—Diagram of air-sacs and their location. 1, The proximal end of the humerus. 2, The proximal end of the right clavicle. 3, The cervical air cell. 4, The right coracoid bone. 5, The anterior thoracic air cell. 6, The right side of the sternum. 7, The right side of the liver. 8, The peritoneum. 9, The right abdominal air cell. 10, The coccyx. 11, The proximal end of the right femur. 12, The right supero-posterior air-sac. 13, The right infero-posterior air-sac. 14, The right lung. 15, The axillary extension of the air-sac. 16, The obturator foramen. 17. The pelvis. ]

These sacs do not communicate with each other and normally they are not fully inflated. In some locations they extend into the bones and are in communication with the extensions of the bronchial tubes. In fact, by some anatomists they have been called “bladder-like, extra-pulmonary expansions of the bronchial tubes, free from cartilage.” The air-sacs make the bird’s body lighter, thus making long-continued flight possible. They are best developed in those birds which fly most. There are four pairs of cells and one single cell from which all other expansions and extensions are made. These sacs are as follows: a single anterior thoracic, and, in pairs, cervical, anterior diaphragmatic, posterior diaphragmatic, and abdominal.

The Anterior Thoracic Air-sac.—The anterior thoracic air-sac is located above the clavicles and the interclavicular space, in the cavity of the thorax. It is related superiorly with the trachea and the esophagus; laterally with the lungs and the cervical air-sacs; inferiorly with the sternum, the clavicle, and the interclavicular aponeurosis; posteriorly with the heart and the anterior diaphragmatic reservoir; and anteriorly with the integuments of the neck. It contains the inferior larynx and the two primary bronchi, and large vascular trunks from which are given off vessels supplying the neck and the wings.

Three prolongations, called subpectoral, subscapular, and middle, or humeral, arise from the lateral walls of this air-sac. These prolongations cross the walls of the thorax and pass around the articulation of the shoulder.

The subpectoral prolongation extends from the thoracic reservoir by an orifice situated behind the coracoid, and passes beneath the tendon of the great pectoral muscle. When the pectoralis major contracts, this contraction dilates the subjacent cell and draws into it a greater quantity of air.

The subscapular and the humeral prolongations communicate with the thoracic air cell by a common opening situated behind the small adductor muscle of the humerus. The subscapular prolongation, after leaving this point, spreads under the scapular and the subscapular muscle, which it separates from the ribs and corresponding intercostal muscles, and extends in a longitudinal direction.

The humeral prolongation, smaller than the subscapular, occupies the axilla, and is in shape triangular. It has from its summit into an infundibular fossa, an extension which enters the canal of the humerus. The walls of this cell form the lining of the air space in the humerus.

The thoracic air-sac thus possesses numerous membranous folds which divide its cavity. The contiguous structures which it overlies, as the trachea, the esophagus, the muscles of the inferior larynx, as well as the arteries and veins, make its outer walls irregular. This orifice is dilated during inspiration, by the contraction of the two first fasciculi of the diaphragm.

The Cervical Air-sacs.—The two cervical air-sacs are located just above the thoracic air-sac at the inferior part of the neck and in front of the lungs. They are cone-shaped with the base directed forward and the apex backward. They are related superiorly with the cervical muscles, and inferiorly with the thoracic air-sac from which they are separated by the trachea, the esophagus, the pneumogastric nerve, and the jugular veins. The walls touch each other internally, and form a median septum which includes in its substance the two common carotid arteries. Externally they are related to the origin of the cervical nerves, to each of which they contribute a small sheath. They surround the vertebral artery, and are connected with the subcutaneous muscles and the skin. The summits communicate with the anterior diaphragmatic bronchus. Prolongations extend from their bases which conduct the air into all the vertebræ of the neck and the back, into all the vertebral ribs, and into the spinal canal. Parallel with and adjacent to the vertebral arteries, and lodged in the canals excavated in the transverse processes of the cervical vertebræ, are two cervical prolongations, one on each side, which extend to the cranium from the base of the cervical reservoirs. From their sides, at the last six cervical vertebræ, are six extensions in the form of diverticuli, which, lying against each other, pass from each side into the muscles of the neck. They are surrounded by a thin fibrous envelope, a continuation of the mucous lining of the sac, and apparently form a canal in the inferior part of this region. These prolongations are better developed in palmipedes than in chickens. On the internal side of these prolongations, one or more foramina penetrate the vertebral segment, which allow the extensions of the prolongations into the spinal canal. Chauveau states that “as the medullary tissue is replaced by air in the bones of birds, so is the subarachnoid fluid replaced by air around the spinal cord.”

The prolongations extending from the cervical air-sacs, having entered the thorax, terminate by passing into the first dorsal vertebra. After permeating every part of this vertebra, it escapes by a lateral opening and forms a small sac located between the first two ribs, near the origin of the first dorsal nerve. From this sac an extension is given off, which enters the second vertebral segment at the antero-lateral part; from this point it passes back, forming a new air-sac between the second and third ribs. It now passes in the same manner into the third vertebra and extends through the third intercostal sac, and so on till the last dorsal vertebra has been served. At the same time that these sacs receive the air from the vertebræ preceding them, and transmit it to those which follow, they communicate it to all the vertebral ribs. The aerial currents which leave the cervical air-sacs do not communicate with those of the cranium. Experiments show that the cranial bones have apparently no communication with the respiratory apparatus.

The Anterior Diaphragmatic Air-sac.—The two anterior diaphragmatic or supero-posterior air-sacs are related with the lungs anteriorly, and with the abdominal viscera posteriorly. Anteriorly also is the thoracic air-sac, posteriorly are the posterior diaphragmatic air-sacs, and laterally the ribs and the intercostal muscles and internally is the esophagus. The lungs communicate with these air-sacs through circular openings from the great posterior diaphragmatic bronchus and frequently by a second opening from this same tube. These are the only sacs which receive air from the lungs through two openings.

The Posterior Diaphragmatic Air-sac.—The two posterior diaphragmatic, or infero-posterior air-sacs are oval in shape and located between the thoracic and the abdominal cavity. They are related anteriorly with the anterior diaphragmatic air-sacs. These two sacs form a vertical transverse partition. The posterior diaphragmatic air-sacs are related posteriorly with the abdominal air-sacs from which they are separated by the diaphragm. They are related below with the lateral parts of the sternum and the sternal ribs, and externally with the ribs and the intercostal muscles. These air-sacs communicate with the lungs through openings located in the middle part of the external border of the lung, into the extremity of voluminous bronchial tubes which follow the direction of the largest air tubes.

The Abdominal Air-sacs.—The two abdominal air-sacs located on each side of the abdominal cavity, when inflated with air, form enormous bladder-like structures. They are related laterally with the abdominal wall and internally with the abdominal viscera. The anterior extremities are in communication with the mesobronchi and are somewhat inflected to pass under the fibrous arches extending from the spine to the pelvis. Anteriorly these sacs adjoin the diaphragm, the testes in the male, and ovary in the female, and to the parietes of the abdomen and those of the pelvis. Below and in front, they rest on a fibrous septum, which in all birds divides the abdominal cavity into two smaller cavities: one anterior, representing the abdomen and containing the liver; the other posterior, representing the pelvis and containing the gizzard and the intestines. The anterior portion overlies the posterior part of the lobes of the liver, the proventriculus, the spleen, and the gizzard. The kidneys are located above these air-sacs. Dorsal to the sacs is also a part of the intestines and in the female the oviduct. The abdominal air-sacs are attached by a ligament-like structure in their medial, their anterior, and their lateral margin. The posterior, the dorsal, and the ventral margin are free. Mesially this attachment is to the mesentery, connecting the left cæcum to the dorsal margin of the gizzard, and also to the mesentery of the proventriculus. The anterior attachment is to the body wall and extends in front of the end of the ovary and the adrenal glands. At the antero-lateral part of the body cavity the attachment extends in a widening band along the lateral side of the ovary and of the oviduct, as far back as the caudal margin of the sac. The lateral attachment is related to the kidney, the dorsal ligament of the oviduct, and the abdominal wall.

Each of these abdominal sacs has three extensions: one suprarenal and two femoral.

The suprarenal extension leaves the principal sac at the postero-external part of the kidney, extends upward, and forward, and expands over the surface of the kidney. At the internal border of the kidney, this prolongation extends between the transverse processes of the sacral vertebræ, reaches a height of the first dorsal vertebra, forms a triangular canal located above the sacrum in the sacral channel, and is separated from its fellow by a series of corresponding spinous processes.

The two femoral extensions, an anterior, small, and a posterior, large, extend from the abdominal air-sac at the cotyloid cavity, leave the pelvis through the bony passage occupied by the crural vessels, extend around the coxo-femoral articulation, and terminate in a blind extremity. In some birds, particularly in birds of prey and ostriches, there are prolongations extending into the femur, entering through a foramen at the anterior part of the great trochanter.

Summary of Bones Supplied by Each Air-sac.—The thoracic air-sac communicates on each side of the thorax with twelve bones, including the four sternal ribs. It supplies air to the clavicles, which are perforated at both their extremities, and to the coracoids, which are perforated just below their scapular extremity. The sternum is supplied through two series of openings, the middle ones that conduct air into the sternal ridge and the lateral ones, eight in number and very small, correspond to the intercostal spaces. The sternal ribs are penetrated by small foramina at their inferior extremities. From the subscapular extension the scapulæ receive air through one or two foramina at their anterior extremity. The humeral prolongation supplies the humerus through a foramen located at the upper edge of the humeral fossa, at the infero-internal part of the articular head.

The cervical air-sac furnishes air to all the cervical vertebræ, to all the dorsal vertebræ, and to all the vertebral ribs. The anterior parts of the vertebræ of the neck are supplied with air through the passage accommodating the vertebral artery. The posterior parts of the vertebræ are supplied by extensions from the interspinal canal. The first extensions obtain entrance to the anterior segments by one or more openings of the inner wall of the intertransverse canals; the median extensions penetrate the posterior segments by two openings, a right and a left, situated on the inner wall of those segments. The first dorsal vertebra is supplied with air in the same manner, by the middle and the lateral canals of the neck. This air, after passing through the first vertebra, leaves by a lateral exit to enter a small air-sac. From this it passes into the superior part of the second vertebra, escapes from this through its lower portion, to be received into a lateral sac, and so on to the last dorsal vertebra. These sacs also supply the vertebral ribs with air, which enters them by very small openings located on their spinal extremities.

The diaphragmatic air-sacs do not have communications with the bones.

The abdominal air-sacs communicate with the sacrum, the coccygeal vertebræ, the iliac bones, and the femurs. The air passing through the sacrum, the coccyx, and the ilium comes directly from the suprarenal extensions; the air which fills the femoral cavity comes from the femoral extensions.

In some birds these air spaces are more greatly developed than in others. The bones that are always aerated in all birds are the cervical and the dorsal vertebræ, the sternum, and the humeri. Those aerated in some kinds only are the furculum, the scapulæ, the vertebral and the sternal ribs, the sacrum, the coccyx, and the femurs. Those that are never aerated are the bones of the forearm, the hand, the leg, and the foot.

The service of air to the bones in most parts of the body by the air-sacs, as just shown, is in special cases otherwise rendered. The Eustachian tubes furnish air to the bones of the cranium and to the upper jaw; while the lower jaw receives air from the pneumatic foramen situated upon each ramus behind the tympanic articulation, and from an air cell which surrounds the joint.

The cavities of the embryonic bones, which afterward become pneumatic, are filled with marrow. Selenka states that the invasion of the bones by the air is a late development, and that in the humerus this invasion occurs after the twenty-second day in the life of the chick.