PHYSIOLOGY. 457 



It is supplied, in conformity with its origin, by nerves 

 from the upper cervical vertebrae, since prior to this the 

 abdomen extended as far as the head, and the branchiae 

 were appended to it as lateral strips of integument as 

 in Fishes. 



2775. As the thoracic cavity is drawn by the dia- 

 phragm's descent towards the abdomen, so by the pec- 

 toral muscles is it raised upwards to the head. The 

 latter movements are what is animal in the respiratory 

 process, for they lift the thorax towards the head. Pec- 

 toral muscles and diaphragm stand opposite to each 

 other, like limbs and trunk ; the ribs are the limbs, the 

 diaphragm the upper abdominal tunic or covering. 

 Through this antagonism what is limbed, as belonging 

 to the thoracic cavity, is drawn upwards, what is abdo- 

 minal in its character, downwards ; the result is expan- 

 sion, and through this pumping in of the air. 



2776. The air is in part voluntarily swallowed like 

 the food, as e. g. by the movement of the ribs, in part 

 involuntarily by the diaphragm. The diaphragm may 

 be termed the heart of the abdominal cavity. 



2777. In the lower organized animals, where merely 

 branchiae are present, the animal motion encroaches but 

 little upon the act of respiration. In Molluscs and Snails, 

 the oxygen is almost always on the branchiae, as is the 

 case also in Insects. In Fishes the water is still taken in 

 like the food through the mouth, and driven out or ex- 

 pelled by the pharyngeal muscles from between the 

 branchiae. In them the air is in both respects swallowed. 

 In many Reptiles the air is indeed drawn in through the 

 nostrils, but conveyed into the lungs by a true act of 

 deglutition. These kinds of functions are necessary, be- 

 cause as yet the whole trunk is abdomen, and the tho- 

 racic cavity has not as yet separated from it ; hence the 

 diaphragm is wanting. 



2778. The respiratory originates therefore from the 

 digestive organ; it is freely developed only from the 

 abdomen, and is at length entirely liberated from the 

 latter as a self-siibstai)tial cavity, e. g. first of all in the 



