296 MORPHOLOGICAL STUDY OF TRACHEAL AND BRONCHIAL CARTILAGES. 



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 which 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 given off, however, there are present cartilages of various 

 types which I shall describe in detail, for it seems to me that they have an impor- 

 tant relation to the mechanical support of the bronchi, the direction and freedom 

 of their movements, and to certain densities which are often seen in roentgenograms 

 of the lung. 



In figure 20 a bronchus of the third order is seen arising from the ventral sur- 

 face of the bronchus passing to the right cardiac lobe, just at the point where it has 

 a slight curve towards the mid-line. The general direction of the bronchus is ventro- 

 mesial. Three cartilages are grouped around this 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 some- 

 what 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 Homer, 

 which is situated on the mesial side of a bronchus of the second order, which 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 appears 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 which extends behind the small dorsal 

 branch already mentioned. 



During inspiration the bronchi elongate and the angle which 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 



