STRUCTURE OF THE TRACHEA. 



891 



The first or highest cartilage, which is connected by the fibrous membrane 

 with the lower margin of the cricoid cartilage, is broader than the rest, and 

 is often divided at one end. Sometimes it coalesces in a greater or less 

 extent with the cricoid or with the succeeding cartilage. The lowest 

 cartilage, placed at the bifurcation of the trachea iuto the bronchi, is peculiar 

 in shape ; its lower border being prolonged downwards, and at the same time 

 bent backwards so as to form a curved projection between the two bronchi. 

 The cartilage next above this is slightly widened in the middle line. Some- 

 times the extremities of two adjacent cartilages are united together, and not 

 uufrequently a cartilage is divided at one end into two short branches, the 

 opposite end of that next it being likewise bifurcated so as to maintain the 

 parallelism of the entire series. The use of these cartilaginous hoops is to 

 keep the trachea open, a condition essential for the free passage of air into 

 the lungs. 



Muscular fibres. Between the fibrous and the mucous membrane at the 

 posterior flattened part of the trachea, there is found a continuous pale 

 reddish layer, consisting of unstriped muscular fibres which pass across, not 

 only between the posterior extremities of the cartilages, but opposite the 

 intervals between the rings also, and have the power of diminishing the area 

 of the tube by approximating the ends of the cartilages. Those which are 

 placed opposite the cartilages are attached to the ends of the rings, and 

 encroach also for a short distance upon the adjacent part of their inner 

 surface. 



Outside the transverse fibres are some small fasciculi having a longitudinal 

 direction. These arise, by minute tendons of elastic tissue, in part from the 

 inner surface of the end of the tracheal rings, and in part from the external 

 fibrous membrane. 



Fig. 625. 



Fig. 625. CILIATED EPITHELIUM 

 OP THE RESPIRATORY Mucoos 

 MEMBRANE. 



A, vertical section of the epithe- 

 lial lining of the human windpipe 

 (from Kolliker). ^y 9 a, b, sub- 

 jacent membrane ; c, lowest or 

 spheroidal cells ; d, middle or oval 

 cells ; e, superficial elongated and 

 ciliated cells. 



B, separate columnar and ciliated 

 epithelial cells from the human 

 nasal membrane, ^ 



Mastic fibres. Situated im- 

 mediately beneath the tracheal 

 mucous membrane, and ad- 

 hering intimately to it, are 

 numerous longitudinal fibres 

 of yellow elastic tissue. They 

 are found all round the tube, 

 internal to the cartilages and 

 the muscular layer, but are 

 much more abundant along the posterior membranous part, where they are 

 principally collected into distinct longitudinal bundles, which produce visible 

 elevations or flutings of the mucous membrane. These bundles are par- 



B 



