ORGANS OF CIRCULATION 285 



laxed. When it contracts it flattens, thus increasing the size of the 

 pleural cavity and drawing air in through the trachea. The abdom- 

 inal muscles also have their efi'ect. Expiration is caused in part by 

 the action of the intercostal and abdominal muscles, in part by the 

 elastic tissue and smooth muscles in the lungs themselves. 



ACCESSORY RESPIRATORY STRUCTURES 



Allusion has already been made to the pharyngeal and dermal 

 respiration of the amphibia (p. 277). There are several fishes in 

 which the hinder part of the alimentary tract is also respiratory. 

 Thus in Cohitis water is drawn in and expelled from the anus, and the 

 posterior half of the digestive canal is richly vascular and is the seat 

 of considerable respiration. 



Before hatching or birth the lungs of the amniotes are unable to 

 function, while a certain amount of oxygen is necessary for the 

 development and the carbon dioxide formed must be carried away. 

 This respiratory function is assumed by the allantois. The allan- 

 tois (fig. 308) is a ventral diverticulum from the hinder part of the 

 alimentary canal, which during fcetal or embryonic life, acquires a 

 relatively enormous development. It extends beyond the body 

 limits and in reptiles and birds comes into close relations with the 

 porous egg shell, while in the mammals it plays an important part in 

 the formation of the placenta. In all these the allantois is extremely 

 vascular, developing a rich network of blood-vessels close to the shell 

 (sauropsida and monotremes) or to the walls of the maternal uterus 

 (mammals), which serves for the rather limited exchange of gases 

 necessary for the young. After free life begins the allantois is either 

 absorbed (sauropsida) or is lost with the rest of the placenta (mam- 

 mals), only the basal part persisting as the urinary bladder, described 

 in connexion with the urogenital system. 



ORGANS OF CIRCULATION 



The functions of the circulation are two-fold: to carry food and 

 oxygen to the tissues and organs of the body and to remove the waste 

 from them. In addition it has been made probable that every ac- 

 tivity of the body results in the formation of peculiar substances — 

 activators — which have fixed and definite eflfects upon the various 

 organs. These activators pass into the blood and form the stimulus 

 which may cause other organs or cells, remote from the place where 



