.THE LUNGS. 



409 



of the lungs. The smaller bronchi divide, for the most part, in 

 a regular, dichotomous manner, as shown on the right-hand 

 side of Fig. 163. The branching at first affects the hypo- 

 blastic lining alone, but about the thirteenth or fourteenth day 

 (Fig. 163) the mesoblastic wall becomes divided by external 



Fig. 163. — A transverse section across the thorax of a, Eabbit Embryo of the 

 sixteenth day. x 15. 



A) dorsal aorta. CH, iiotochord. CP, pericardial cavity. CE, pleural cavity. 

 !FW, iiem-al arch of vertebra. LB, bronchus. LGr, luug. i3"S, spinal cord. WY, 

 sympathetic uen'e cord. EA, right auricle. RB, left auricle. ED, iuter-aiiricular 

 septum. RI, rili. RK", capitulum of rib. EO, tubercle of rib. EV, right ventricle. 

 EY, left ventricle. ST, ventral end of rib, from which the sternum is formed. TO, 



ihagus. VD, left anterior vena cava. VI, posterior vena cava. 



grooves or clefts, which mark out the boundaries of the main 

 lobes of the lungs. The trachea, or median part of the air 

 passage, is at first very short ; but, as the neck elongates, and 

 the lungs get carried back into the thorax, the trachea rapidly 

 increases in length, and by the eighteenth day (cf. Fig. 151) 



