254 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(2) The Lobes of the Lungs. — -In the embryonic con- 

 dition (Fig. 206) it is seen that the right and left lung 

 buds are nearly symmetrical. Aeby supposed the upper lobe 

 of the right lung to be absent in the left, but that is not 

 so. Each bronchus gives off three primary buds. All three 

 remain separate on the right side ; on the left the upper and 

 middle primary buds arise together (Fig. 206). Hence the upper 

 lobe of the left lung represents the upper and middle lobes of the 

 right. In the sheep the upper right lobe springs from the trachea. 

 The bronchus of the upper right lobe (the reason for it is not clear) 

 lies above its artery — that is to say, it is eparterial. The other 

 bronchi are hyparterial. 



(3) The Diameters of the Thorax. — The peculiar branching 

 of the bronchi in man and upright primates is due to the 

 shape of their lungs, which in turn is due to the shape of 

 the thorax. In quadrupedal animals such as the horse or dog, in 

 which the chest rests and is supported between the fore limbs, the 

 thorax has its greatest diameter in the dorso-ventral direction 

 (Fig. 208). In upright animals (man, anthropoids, and also in 



vertebra 



V—B. 



x -^®^ x broad sternum 

 narrow sternum 



Fig. 20S.— Diagrammatic Section of the Thorax of a Quadrupedal JIammal (A), con- 

 trasted with a corresponding section in Man (£). 



some water-living mammals, such as seals, etc.) the transverse 

 diameter becomes the greater. At birth the diameters of the 

 child's thorax are nearly equal. The thorax is flattened by the 

 spine becoming invaginated within it ; the thorax thus comes to 

 lie within the axis of gravity of the upright body. 



