84 Animal Morphology. 



Here the proper muscular structure, which constitutes 

 the contractile membrane, suffers great displacement, owing 

 to the office it has to fulfil, in giving the slight initiatory 

 start to respiration, by confining in the air cells the inspired 

 air for one or two seconds, and then freeing it, which 

 is afterwards carried on by the striped muscles of respi- 

 ration belonging to the bony walls of the chest, or the 

 respiratory muscles of Sir C. Bell. 



By confining the air for an instant in the air sac, not only 

 is the eliminative or destructive process aided by external 

 motion ceasing, but, increase of heat expanding gas, when 

 it relaxes again, gives an impetus to the exit of effete 

 matter by elasticity. 



Thus, under considerable displacement, we retain in the 

 lung membrane the triple division of parts, and, as far 

 as muscle is concerned, in a very limited and restricted 

 degree. 



It is not improbable that we may find the nasal mucous 

 membrane, with the palate regions, a distinct membrane, 

 with the levators of the palate as its special muscles ; and 

 that the rings of the trachea and larger bronchi are nothing 

 else than very highly-differentiated and highly-modified 

 serous membrane, fulfilling a purely mechanical function, and 

 so far displaced and modified as to aid another membrane 

 in the due fulfilment of its function ; as the teeth are the 

 millstones for the stomach, and the systemic arteries and 

 veins are the food-distributors to the chylo-lymphatic 

 system ; the one being dependent upon the other for the due 

 fulfilment of their functions. 



The Genito-urinary organs are singular in their diffe- 

 rentiations, and require in some measure a wider con- 

 sideration than might at first appear, yet in itself simple and 

 singular. 



The oneness of mucous membrane which pertains to the 



