Talk:Book - Contributions to Embryology Carnegie Institution No.38

From Embryology


By William Snow Miller,

Professor of Anatomy, University of Wiscojusin.

With two plates and eleven text-figures.

Horner, writing in 1839, said that "at the orifice of each branch of the bronchia there is a semi-kmar cartilage, forming rather more than one-half of its circumference and having its concave edge upwards. The whole arrangement resembles somewhat the pasteboard of an eared bonnet, and is evidently to keep the orifice open." Passing over his reference to the feminine fashions of his day, we find in the above quotation the earUest description of the form of the cartilages found at the place where bronchi divide; moreover, a reason is assigned for the particular form of the cartilage.

In 1840 Jonas Iving published a short article "On the forms of the cartilages which keep open the principal divisions of the bronchial tubes." In a footnote he frankl}' gives the priority of description to Professor Horner and intimates that his paper was inspired by a demonstration Professor Horner made in Philadelphia to Mr. T. Wilkinson King "some months previous to the last edition of his work." Mr. King must have, on his return to London, imparted the information thus acquired to his namesake, who at once made it the subject of a special study. Iving found, however, that not all the cartilages present at the place where a bronchus divides had the characteristic saddle shape described by Horner but, "that a considerable number of varieties will be met wdth." His paper is illustrated ^vith several small and unsatisfactory drawings of the cartilages.

An extended search of the literature fails to bring to Ught any other investigation of the form of the tracheal or bronchial cartilages ; in no place has the author been able to find the cartilages illustrated in plastic form, and such illustrations as are found show the cartilages as irregular plates and are evidentlj' schematic.

Henle's illustrations are poor, but his description is fairly good. He describes the cartilages as having the plates or strips sometimes with short prolongations, arranged, as a rule, transverse to the long axis of the bronchi; thej' vaay also have a longitudinal or obUque direction —

" Je tiefer hinab, um so mehr reduciren sie sich und um so weiter riicken sie aus einander, bis sie endlich nur noch als platte Ringe oder HalbrLnge um die Miindungen der Seitenzweige und als Stiitzen der die beiden Aeste einer gabligen Theilung trennenden Scheidewand vorkommen."

The description of Waters is inexact and his illustrations still more so. He states that the cartliages —

" In the largest bronchial tubes are elongated transversely and placed more as they are in the bronchi, but in the secondary' and subsequent divisions they are placed

very irregularly, and are elongated longitudinally In the larger tubes, wherever

a branch arises, a cartilage is always placed, and in the largest vessels the cartilage forms two processes below, one of which belongs to each vessel of the di^'ision. The free margin of the cartilage, placed at the orifice of an air-tube, is always concave and sharp, and is surmounted by a band of yellow elastic tissue. At the points of origin of the smaller air-tubes, the cartilages exist as thin semilunar pieces, with sharp concave margin looking upwards; these becoming smaller, at length disappear."

Heller and v. Schrotter studied the cartilages wliich enter into the formation of the carina tracheae and, although shown by fiat drawings, they indicate better than anj' previous illustrations the bizarre forms which the tracheal cartilages often assume.

In a study of the carina trachea? of the domestic cat, made by the author of this paper, a similar method of illustration was used, but while it served the purpose it did not in the end prove satisfactory.

Schafer, describing the cartilages within the lungs, says:

[They] "no longer appear as imperfect rings running onlj^ upon the front and lateral surfaces of the air tubes, but are disposed over all sides of the tubes in the form of irregular-shaped plates and incomplete rings of various sizes. These are most developed at the points of division of the bronchia, where they form a sharp concave ridge projecting inwards into the tube."

Further quotations are unnecessary, for all the descrij^tions of the bronchial cartilages seem to have taken their coloring from the original descriptions given by Horner and by Iving. No one, however, has followed out the plastic representation of the cartilages first attempted by King.

In longitudinal sections of the trachea or of the bronchi a considerable number of so-called plates are seen (fig. 10) and it was a desire to see in plastic form the shape and arrangement of these plates that led to the present study.

Cuvier was the first to point out that the musculature of the trachea in some animals is inserted on the outer surface of the cartilages, in others on the inner surface of the cartilages. This apparently docs not influence the shape of the cartilage but it led to the selection, for this study, of the tracheal and bronchial cartilages of man and of the guinea-pig (Cavia cohnya), both of which have the muscle attached on the inner surface of the cartilages. The study was begun on

Fig. 1. — Outline of a reconstruction of the lower portion of the trachea of a week-old child. Only a part of the uppermost cartilage enters into the reconstruction. The lines drawn across the reconstruction show the j>lanc of the sections indicated. Note the angle at the bifurcation, the greater diameter of right bronchus, and that the carina is at left of midline. X6. the trachea and bronchi of a week-old child (fig. 1), but was later transferred to the guinea-pig, the latter having a smaller trachea and lung.

It must be understood from the first that animals of different species do not have the same form of cartilage in similar positions and that no two individuals of the same species have exactly the same form and arrangement of the cartilages. This point is well brought out in the studies of the cartilages of the carina trachea above mentioned. The last eight cartilages of the human trachea and the last five cartilages of the trachea of the guinea-pig were modeled. In each case it was found that only about one-half of the first of the cartilages entered into the model.

The outline of a transverse section of the trachea and of the cartilage in each of the models does not have the elongated horse-shoe shape of the figure given by Kolliker, or the flattened horse-shoe shape of the figure given by Sobotta; it is more nearly circular (figs. 2 and 5) . As the carina is approached, the posterior membranous portion of the trachea flattens and eventually the section has an elongated oval outline; or it may even have, as is the case with the human trachea, a dumb-bell shape (fig. 3) before it divides into the right and left bronchus.

Fig. 2. —Outline of the section along the line A, in figure 1. Note circular outline of the section. The numbers 2, 3. 4 indicate the cartilages through wliich the plane of the section passes. X6.

Fig. 3. — Outline of the section along the line B in figure 1. Note the outline of the lumen of the trachea; it has notonly adumb-bell shape, but there is also a marked anterior swing of what is to be the right bronchus. 1 B, section of the first right bronchial cartilage; C, C, sections of the carinal cartilage. XO.


Trachea. — The cartilages of the human trachea present marked irregularities (fig. 1). The first of the cartilages is incomplete and must therefore be left out of the account; the two following cartilages are regular in their formation; the next five are either fused cartilages or are bifurcated, and have one or more irregular openings, evidently due to incomplete development, for they give passage to neither blood-vessels, nerves, nor gland ducts. The carinal cartilage is interesting, being formed by the fusion of tracheal and left bronchial cartilages (fig. 4) ; both elements present bifurcations and irregularities of outhne.

Right bronchial cartilages. — The first bronchial cartilage on the right side consists of two elements wliich fuse as they arch around the mesial side of the bronchus. The next cartilage is a typical crescent. Inmaediately below this cartilage the eparterial bronchus is given off, and the cartilage placed at this point has a complicated structure, part of it belonging to the eparterial bronchus and part of it to the main stem bronchus. The portion belonging to the eparterial bronchus has a prolongation which is associated with a small bronchus, arising from the eparterial bronchus, which is not shown in the outUne. Three openings are present in the main portion of the cartilage wliich arc probably due to incomplete develojiment. From the description given by Horner and by Iving, a cartilage should be present in the angle formed by the eparterial bronchus and the main stem bronchus, having more or less of a saddle shape with the concave surface uppermost; but, as is the casein the guinea-pig, quite another type of cartilage is present.

Left bronchial cartilages. — On the left side the cartilages were not followed out completely, but, as far as they were followed, they formed a single fused cartilage made up of several elements, with numerous openings which, hke those mentioned above, did not give passage to either blood-vessels, nerves, or gland ducts.

Fig. 4.

The study by Heller and v. Schrotter, taken in connection with the study made by Miller, seems to indicate that the cartilages of the trachea and of the right and left bronchus in man are subject to a greater number of fusions and bizzare forms than is the case in the lower mammals; a more extended study of the lower forms might, however, prove tliis conclusion to be erroneous.


The cartilages of the trachea and bronchi will be described first as they appear in a ventral view (fig. 12), and then as they appear in a dorsal view (fig. 13). The one shows what may be termed the bodies of the cartilages, the other, the ends of the cartilages.

Trachea. — In contradistinction to the human trachea, that of the guincu-pig presents no marked irregularities. The last tracheal cartilage has the triangular prolongation downward that is described as characteristic of tliis cartilage, but it does not enter into the carina tracheae, the carinal cartilage being the first cartilage of the left bronchus.

Right bronchial cartilages.— The first bronchial cartilage on the right side forms a smaller segment of a circle than any of the other bronchial cartilages. It ends abruptly on the ventral side of the bronchus near the midline and has two small openings near its mesial end. The second right bronchial cartilage forms nearly a complete circle (fig. 14). The eparterial bronchus is given off just below this cartilage and the third right bronchial cartilage, belonging mainly to the right bronchus, will be considered in connection with its cartilages.

Left bronchial cartilages.— The cartilages of the left bronchus, seven in number, are quite regular in their formation and are placed sUghtly oblique to the long axis of the bronchus. The first cartilage, as already stated, enters the carina tracheae

and continues for some distance in that structure (fig. 6). The fifth, sixth, and seventh cartilages do not encircle the bronchus as extensively as do the second, third, and fourth cartilages.

Turning now to a dorsal view of the tracheal and bronchial cartilages (fig. 13), it will be seen that onlj^ the left tip of the first tracheal cartilage modeled is visible, and that the mesial end of the first bronchial cartilage on the left side is hidden from view by the expanded end of the second right bronchial cartilage. Of the remaining cartilages the left extremity of the last tracheal cartilage and the mesial extremity of the second right bronchial cartilage attract attention. The former of these is broadly expanded and has a well-marked posterior prolongation (fig. 13); the latter is expanded into an oval plate which fits into the angle between the right and left bronchus. Near the center of this expanded ext remit}' is an oblong foramen (fig. 14) which gives passage to a small blood-vessel. It will also be noted that where the extremity of a cartilage is expanded the extremity of one of the adjoining cartilages is, as a rule, reduced in size — for example, the tracheal cartilages Nos. 2 and 4, and the bronchial cartilages Nos. 1 and 3 on the left side.

Considering the dorsal view of the trachea and bronchi as a whole, it will be noted that the interval between the ends of the cartilages in the trachea, which corresponds to the membranous portion, occupies the mid-hne; as this is followed into either the right or the left bronchus it gradually rotates 90° and comes to occupy a mesial portion.

The angle which the bronclii form with the trachea dififers from that which they form with the human trachea, in that it is practically the same for each bronchus; on the other hand, in man the right bronchus continues in nearly the same direction as the trachea, but the left bronchus has a more oblique direction. As a control for this point a number of celloidin corrosions were made and all showed the same mode of division. Narath's figure of a celloidin corrosion of the trachea and bronchi of Cavia {Coelogenys) paca also shows the same characteristic. In still another point the bronchi differs from the human bronchi; in man the right bronchus exceeds the left in diameter, while in the guinea pig the two have practically the same diameter.

On the mesial surface of the left bronchus, just before the bronchus going to the left superior lobe is given off, there are present, in the interval between the ends of the main cartilages, two cartilaginous plates which are independent of any connection with the main cartilages. Such plates have been named by Luschka "intercalated cartilages." The more anterior of these two cartilages is long and narrow and is placed obUquel}^ in the membranous intercartilaginous space; the other cartilage, placed transverse to the long axis of the bronchus, is broad on the dorsal side of the bronchus, tapers as it arches around the mesial surface of the bronchus, and ends on its ventral surface just behind the seventh broncliial cartilage.

Fig. 8.

right bronchus opposite its third cartilage and quickly divides into five branches, which are distributed to the superior lobe of the right lung. The main stem of the eparterial bronchus, at the place where it leaves the right bronchus, is partially surrounded by two large cartilages. The one on the anterior (cephaUc) surface consists of two elements, one of which belongs to the right bronchus, the other to the eparterial bronchus. In a ventral view of the bronchus (fig. 12) the bronchial element appears to be an independent cartilage, but when seen from the dorsal side (figs. 13 and 15) the fusion of the two elements becomes at once apparent.

The bronchial element differs from any of the preceding bronchial cartilages in that it encircles the stem bronchus sjnrally. It is irregular in outUne and the lateral end tapers to a blunt point where it overli(>s th(> ch>ft mesial end. From the posterior surface of the lateral end there is a prolongation outward which fuses with a mesial prolongation from the dorsal surface of the element belonging to the eparterial bronchus.

This element is in the form of an elongated cap which incloses nearly three-fourths of the circumference of the bronchus and extends from the point where it leaves the right bronchus to the point where the apical branch arises. On the ventral side of the cartilage three deep notches and a single opening are seen; none of them gives passage to either blood-vessels, nerves, or glands. On the dorsal side is a deep notch which ends in a crescentic baj^ that gives passage to blood-vessels; the lateral margin is irregular, and the entire dorsal surface slopes toward the bronchial element with which it eventually fuses.

The second of the large cartilages associated with the eparterial bronchus belongs essentially to the main stem bronchus. In a ventral view (fig. 12) it appears as a band, placed transversely across the long axis of the stem bronchus, with a prolongation springing from its lateral margin which bears a slight resemblance to the bow of a ship and supports the eparterial bronchus in a manner somewhat similar to that by wliich the bowsprit of a ship is supported by the bow. In a dorsal view (figs. 13 and 16) this resemblance is not as pronounced as in the ventral view; this is due to a sUght dorsal swing of the eparterial bronchus.

Fig. 10. The dorsal view also shows the close approximation of the two extremities of the cartilage, a foramen for the passage of a small branch of the bronchial artery and a small hook-Une process arising from the posterior border of the lateral end of the cartilage.

The relation of these two cartilages to the eparterial bronchus is best shown in a view taken dorso-laterally (fig. 17). Here the capping of the eparterial bronchus by the first cartilage and the bracing of the bronchus by the second cartilage are clearly brought out. The relation of the second cartilage to the eparterial bronchus is also shown in the outline of a section taken through the anterior portion of the cartilage (fig. 7). This section also shows the position of the sections of the cartilages belonging to the left bronchus.

A longitudinal section (fig. 8) of the greater part of the eparterial bronchus before it breaks up into its principal branches shows the position of the various cartilages found along its course. The most interesting cartilage is the one that nearly surrounds the first of the small branches which arises from the dorsal side of the bronchus. It is shown in figure 8 in section, and in figure 13 it is shown reconstructed. This cartilage belongs primarily to the first dorsal branch; it also is the main cartilage associated with the second dorsal branch. The first branch takes a dorso-mesial direction, while the second branch takes a shghtly dorsolateral direction. An elongated oval opening is seen in the portion of the cartilage situated between the first and second branches. Inadvertently the apical bronchus was not carried out far enough in the drawings of the reconstruction to show clearly that both the mesial and the lateral portions of this cartilage are also associated with the origin of this branch from the eparterial bronchus. This cartilage demonstrates a relationsliip lietween a cartilage and bronchi which has already been noted and which will frequently recur; namely, a cartilage may be associated with two or more bronchi.

Only a single cartilage is found between the eparterial bronchus and the bronchus going to the lobus medius that is exclusively a cartilage of the stem bronchus. This cartilage is deficient dorso-laterally, the interval between its two extremities being occujiied by two small and one large, bizarre shaped, intercalated cartilages. Both extremities of this cartilage are broad; the dorsal extremitj' is perforated by a foramen for the passage of a branch of the bronchial artery.

The description of the cartilages has now reached the point where the bronchus going to the right lobus medius arises from the right stem bronchus, and the bronchus going to the left lobus superior arises from the left stem bronchus. These two bronchi arise nearly opposite each other from their respective stem bronchi. Figure 10 shows the outline of a section taken through the right and left stem bronchi, the greater part of the bronchus going to the right lobus medius, the bronchus going to the left lobus superior, and the position of the cartilages. In the outUne drawing the cartilages appear as "plates;" that they are not plates but portions of well-formed cartilages is shown in figure 12 and 13, in which they are represented plastic.

It is not necessary to describe each cartilage in detail; a careful study of the ventral and dorsal views (figs. 12 and 13) will enable the reader to follow them individually without difficulty. It will be noticed that on both the upper (anterior) and lower (posterior) wall of each bronchus there are small cartilages which extend only a short distance around the circumference of the bronchus. These might, not inaptly, be called "plates," though the name introduced by Luschka, "intercalated cartilages," best describes them.

One cartilage belonging to the bronchus going to the left lobus superior deserves special consideration, for its form is unlike anj^ other cartilage the author has studied. The first branch given off from the bronchus supplying the left lobus superior is the apical bronchus. This does not arise from the cephaUc (upper) surface of the bronchus, but its origin is sUghtly rotated to the dorsal side of the bronchus. The cartilage in (juestion is associated with this bronchus and can be easily followed in the ventral and dorsal views of the reconstruction (figs. 12 and 13), but is best seen in figure 19, which is a ventro-mesial view.

For convenience of description the cartilage may be said to begin near the posterior border of the bronchus as a slender bar which passes obhquely along the ventral surface of the bronchus. Arriving at the origin of the apical bronchus it divides into two broad arms which pass one on either side of the apical bronchus. The mesial arm arches over the main bronchus, then rotates so that its broadest surface is turned towards the main bronchus, and, passing along the corresponding side of the apical bronchus, terminates in a pointed extremity. The lateral arm is the broader of the two, and after arching over the main bronchus and rotating in a manner similar to the mesial arm it passes along the lateral border of the apical bronchus to the posterior border of the main bronchus; it then widens out and, arching around the posterior surface of the main bronchus, continues along its ventral surface to terminate in a pointed extremity lateral to the bar of origin. The point of termination extends beyond the point of origin; the main bronchus is, therefore, surrounded by a cartilage which makes more than a complete spiral turn. This is the best example of a spiral bronchial cartilage which the author has found in any of the animals usually studied in the laboratory.

Spiral cartilages are frequently found in the Cetacean. Owen gives an illustration of a bronchial cartilage from the dugong, which makes four comjilete turns, but the cartilage is shown of uniform width and regular in its formation, while that above described is irregular in its formation and width and is associated with more than one bronchus.

On the mesial and dorsal side of the main stem bronchus, opposite to the origin of the bronchus going to the right lobus medius and the bronchus going to the left lobus superior, there are a number of intercalated cartilages which vary in size and shape. These are more numerous on the right side, especially in an area where there appears to be a deficiency in the development of the cartilages.

In the ventral view (fig. 12) the cartilages supj^orting these two bronchi are not well shown, but, as was the case with the eparterial bronchus, the dorsal view (fig. 13) shows clearly that in each instance the bronchus is supported by the same type of cartilage as the eparterial bronchus. That this is due to the vnde angle at which these bronchi leave their main stem bronchi seems quite probable, for in no other instance do we meet with this type of cartilaginous support, the angle at which the remaining bronchi are given off being much more acute.

The supporting cartilage of the bronchus going to the lobus medius of the right lung is made up of two broad elements which have a very irregular outline and are perforated by two small openings, which do not give passage to either blood-vessels or nerves. The more anterior of these elements partially encircles the main stem bronchus, but the second element is confined to the dorso-lateral region of the stem bronchus. The supporting cartilage of the bronchus going to the lobus superior of the left lung consists of a single element. Immediateh' posterior to it, on the dorsal side, is a second cartilage of the same general shape, while on the anterior surface there is a long narrow cartilage which bears the same relation to the supporting cartilage as the second cartilage on the dorsal side. These two cartilages must be considered as forming a secondary series of supports for the bronchus going to the left lobus anterior. As this lobe is of considerable size and is attached to the main stem by a single bronchus, it undoubtedly requires a strong supporting apparatus, which is supplied by these three cartilages, and they occupy the same relationship to that bronchus as the two fused elements which support the bronchus going to the right lobus medius.

In the guinea-pig there are two cardiac (sometimes termed infracardiac) lobes, one for each lung; the right is the larger of the two. The bronchi passing to these lobes arise directly posterior to the cartilages just described. In the angle between the bronchus passing to either cardiac lobe and its main stem bronchus there is no distinct cartilage present; but capping each bronchus there is an irregular, horse-shoe-shaped cartilage, the lateral arm of wliich in each instance curves somewhat toward the mid-line and thus comes to occupy the position of a supporting cartilage.

We now find that the cartilages are no longer in the form of crescents arranged more or less parallel to one another; they are irregularly curved pieces scattered over every part of the circumference of the bronchi, with here and there small, variously shaped bits of cartilage interposed between the larger pieces. At the place where branches are gi\'en off, however, there are jiresent cartilages of various types which I shall describe in detail, for it seems to me that they have an important relation to the mechanical support of the bronchi, the direction and freedom of their movements, and to certain densities wliich are often seen in roentgenograms of the lung.

In figure 20 a bronchus of the third order is seen arising from the ventral surface of the bronchus passing to the right cardiac lobe, just at the point where it has a slight curve towards the mid-Une. The general direction of the bronchus is ventromesial. Three cartilages are grouped around tliis bronchus; first, a curved, tri- angular plate situated dorsal to the bronchus; second, a narrow, curved cartilage which arches around the anterior surface of the bronchus and is provided with a short spur which extends along the axis of the latter; the third cartilage is a somewhat wider plate which has a spur that arches around the posterior and lateral surface of the bronchus. From the arrangement of these three cartilages it appears that the bronchus possesses the greatest freedom of movement in a lateral direction.

In the case of the second ventral branch the opposite condition exists, for the bronchus is surrounded by a horseshoe-shaped cartilage, the arms of which pass dorsally on either side, permitting the greatest freedom of movement in a mesial direction. The more anterior of these arms extends in a curved direction along the dorsal side of the main bronchus and ends in a broad, flattened extremity which takes part in supporting a small dorsal branch of the main bronchus (fig. 13).

We now come to a typical saddle-shaped cartilage, as described by Horner, which is situated on the mesial side of a bronchus of the second order, wliich leaves the stem bronchus at nearly a right angle. The bronchus is directed posteriorly and the cartilage fits into the angle formed by the two bronchi. When viewed from the ventral .side (fig. 13) the cartilage; a])pears to be associated with only two bronchi, but when seen from the dorsal side (fig. 21) it is found to be associated with a third bronchus by means of a long mesial process wluch extends behind the small dorsal branch already mentioned.

During inspiration the bronchi elongate and the angle wliich they make with the main stem bronchus becomes wider; while in expiration the bronchi shorten and the angle becomes more acute. Under these circumstances cartilages of this type have a tendency to be drawn more snugly into the angle during inspiration and in expiration the curved processes extending along each bronchus serve to sup- port them in an efficient manner and to prevent their collapse.

In figure 22 we have two saddle-shaped cartilages placed, one on the ventral side, the other on the dorsal side of a small bronchus which is directed anteriorly. In figure 6 only the anterior cartilage can be seen, l)ut in figure 22 the anterior and mesial side of each cartilage is shown. Each cartilage has two pairs of processes, the posterior of which fits over the main bronchus, while the anterior extends along the smaller bronchus. Between each anterior pair of processes there is a deep concavity which allows the bronchus to have a dorsal and a ventral movement but does not permit as much freedom of movement in a lateral direction. This type of cartilaginous support is intermediate between the simple saddle-shaped cartilage and the ring-like cartilage of the succeeding type.

The first dorsal branch of the main stem bronchus going to the right lobus inferior (fig. 13) is surrounded by an exceedingly interesting cartilage. Its shape is best seen in figure 23, which shows a portion of the main bronchus and a portion of the dorsal branch with the cartilage in situ. The cartilage is quite irregular in

Fig. 11. — A typical saddle-shaped cartilage situated at the first division of the main stem bronchus in the left lobus inferior. A shows the cartilage in situ when seen from the dorsal side; B when seen from the ventral side; C shows the cartilage alone. X30.

shape and its two ends are placed, one sUghtly above the other, directly behind the dorsal branch. These ends do not fuse with each other, neither are they in direct contact. The opening through which the dorsal branch passes is ample and permits free movement of the bronchus in any direction. Another example of this type of cartilage can be seen surrounding the first dorsal branch of the left stem bronchus (fig. 13). With sUght modifications I have found this tj^pe constantly associated with these bronchi.

In roentgenograms of the human lung a circular density is not infrequently noted where a branch is given off from one of the larger bronchi. This is quite often seen in what may be termed the middle zone (third) of the lung and is usually explained as being due to a bronchus which opens directly towards or away from the observer. That this is occasioned by the bronchus alone is not borne out by my own experience; for I have found it due to either a circular cartilage (figs. 23 and 2-4) or to two saddle-shaped cartilages (fig. 22). In either instance the bronchus must be viewed in the manner described above in order to produce the circular density. Bronchial cartilages, even in normal lungs, often show a sUght calcification, and when this is present it serves to intensify the density. A modification of the circular cartilage is seen in figure 24, where a spur from one of the ends of the cartilage is associated with a second and smaller bronchus. The cartilage itself is shown in figure 25.

In the lobus inferior of each huig the common type of cartilage in the angle between the main stem bronchus and its larger branches is the saddle-shajjed car- tilage (fig. 11); branches of the third and fourth order may have associated with them any one of the other types of cartilage.

When the smaller bronchioles are reached and the last trace of cartilage appears, it is not in the form of a thin plate, but as a short rod-shaped cartilage placed, as a rule, lengthwise along the bronchiole. This has been found to be the case, not only in the animals usually studied in the laboratory, but also in man.

In sections, a piece of cartilage is frequently found superimposed over one or more pieces of cartilage; this is due to the contraction of the trachea, or the bronchus, or to the plane of the section. Often pieces of different cartilages appear in a transverse section of the trachea or of a bronchus (fig. 2), giving the appearance of a series of plates arranged about its lumen ; the preceding study, however, shows that they are integral parts of cartilage which have a definite form.

From the anatomical standpoint the act of respiration seems to be even more complex than the usual description. This morphological study of the cartilages shows thai; they constitute a mechanism for maintaining the normal position or capacity of the lung, and that a critical study of this mechanism (that is, the stress and strain to which it is subjected) is desirable. Associated with this is the relation of the bronchial musculature to the cartilages, especially at the point where bronchi divide. A study of the musculature at the carina tracheae has convinced the author that it is by no means easily accom])lished, though quite necessary for a complete understanding of the bronchial architecture.


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, 1913. The trachealis muscle: its arrangement at the carina trachea and its probable influence on the lodgment of the foreign bodies in the right bronchus and lung. Anatom. Record, vol. 7.

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Scn/iFER, E. A., 1912. Text-book of microscopic anatomy, Quain's Anatomy, vol. 2, Pt. 1.

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Fig. 12.— Ventral view of a reconstruction from serial sections of the last five cartilages of the trachea and of the right and left bronchi of the guinea-pig as far as the begi nnin g of the first lateral branch of the main stem bronchus within each lobus inferior. Only a portion of the first cartilage entering into the reconstruction is shown. Note the angle at the bifurcation and the uniform size of the right and left bronchi. 1-5, tracheal cartilages. 1 R. B., first right bronchial cartilage. 1 L. B., first left bronchial cartilage. The lines drawn across the reconstruction show the plane along which tHe indicated figure is taken. The position of the cartilages shown in indiridual drawings is also indicated. X6.

Fio.13. — Dorsal view of the preceding reconstruction. The plane along which the indicated figures are taken is shown; also the position of the cartilages which are shown in individual drawings. X6.

Fig. 14. — Ventro-mesial view of the second right bronchial cartilage. The expanded dorsal end of the mesial arm can be seen; also a portion of the oblong foramen mentioned in the text. Xlo. Fig. 15. — Dorsal view of the fused elements that form the anterior cartilage which caps the eparterial bronchus. Xlo. Fig. 16. — Dorsal view of the second cartilage associated with the origin of the eparterial bronchus. Xlo.

Fig. 17. — Dorso-lateral view of the two cartilages which are placed about the origin of the eparterial bronchus from the right bronchus. X 15. Fig. 18. — Dorso-mesial view of the first cartilage on the bronchus pa.ssing to the left lobus anterior. X15. Fig. 19. — Ventro-mesial view of the cartilage described in detail in the text. Xlo. Fig. 20. — Small bronchus surrounded at its point of origin by three special-shaped cartilages. Xlo.

Fig. 21. — Dorsal view of a small typical saddle-shaped cartilage shown in sUu. Its relation to the two principal bronchi is clearly shown, also the prolongation by which it comes into relation with a third bronchus. The bronchi are shown as though transparent. Xlo. Fig. 22. — Top view of a sm 11 bronchus surrounded by two saddle-shaped cartilages. X15. Fig. 23. — A small bronchus surrounded by a ring-shaped cartilage which is quite irregular in its formation. X15. Fig. 24. — A ring-shaped cartilage which surrounds a small bronchus and by means of a spur partially surrounds a second and smaller bronchus. X15. Fig. 25. — The cartilage of figure 24 shown independent of its bronchi viewed from above and slightly from ventral side. Xlo.