CHAPTER IV 

 RESPIRATION 



Section i. 



The Lungs. 



The Chest Wall. — The chest is formed of a moderately rigid case 

 furnished by the ribs and a flexible wall provided by the dia- 

 phragm. The contents of the chest are vascular and respiratory, 

 and the provision made for their protection is not quite the same 

 in each case. The vascular contents are mainly situated in the 

 anterior portion, for here the heart is found with its bloodvessels, 

 and relatively speaking only a small amount of lung. The 

 horizontal spine of all animals throws upon the ribs a function 

 not required in the biped. It would exceed the space at our 

 disposal to deal with the peculiarities of the chest wall in all 

 animals. That of the horse may be selected as a type, more 

 particularly on account of its practical importance. 



It is usual to speak of the ribs as true and false, but this con- 

 veys nothing as to their function. The true or sternal ribs form 

 a case for the vascular organs, and help to support the horizontal 

 spine. To enable this function to be performed they are shorter, 

 stouter, and straighter than the false, or, as we would prefer to 

 call them, respiratory ribs. Further, they are all inserted into 

 the sternum either directly, as in the first pair, or indirectly by 

 means of cartilage, as with the remaining seven pairs. The 

 chest is very narrow between the two first ribs, but rapidly 

 increases in width, as may be seen in the following measurements 

 taken at the widest place between each pair in a riding-horse of 



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