14 THE DOG IN HEALTH. 



uted throughout the length of this digestive tube, and in 

 part through special vessels with appropriate cells covering 

 them, which act as minute porters {villi). 



The impure blood is carried periodically to an exten- 

 sive surface (lungs), usually much folded, and there ex- 

 posed in the hair-like tubes referred to before, and thus 

 parts with its excess of carbon dioxide and takes up fresh 

 oxygen. But all the functions described do not go on in 

 a fixed and invariable manner, but are modified somewhat 

 according to circumstances. The forcing-pump of the 

 circulatory system does not always beat equally fast ; the 

 smaller blood-vessels are not always of the same size, but 

 admit more or less blood to an organ according to its 

 needs. 



This is all accomplished in obedience to the commands 

 carried from the brain and spinal cord along the nerves. 

 All movements of the limbs and other parts are executed 

 in obedience to its behests ; and, in order that these may 

 be in accordance with the best interests of each particular 

 organ and the whole animal, the nervous centers, which 

 may be compared to the chief officers of, say, a telegraph 

 or railway system, are in constant receipt of information 

 by messages carried onward along the nerves. The com- 

 mand issuing is always related to the information arriving. 



All those parts commonly known as sense-organs — the 

 eye, ear, nose, tongue, and the entire surface of the body 

 — are faithful reporters of facts. They put the inner and 

 outer worlds in communication, and without them all 

 higher life at least must cease, for the organism, like a 

 train directed by a conductor that disregards the danger- 

 signals, must work its own destruction. Without going 



