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. 1G3. The branching at first affects the hypo- 

 blastic lining alone, but about the thirteenth or fourteenth day 

 (Fig, 1G3) the mesoblastic wall becomes divided by external 



RO 



NY 



TO 



Rl 



RY 



KV 



FlG. 163. A transverse section across the thorax of a Rabbit Embryo of the 

 sixteenth day. x 15. 



A, dorsal aorta. CH, notoolionl. CP, pericanlial cavity. CR, pleural cavity. 

 FN. neural arch of vertebra. LB, bronclius. LGr, luntr. NS. spinal cord. NY, 

 sympathetic nerve cord. RA. right auricle. KB, left auricle. RD. inter-auricular 

 >r|itum. HI. rib. RN. capituluiu of rib. RO. tubercle of rib. RV. right ventricle. 

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

 oesophagus. 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) 



