■XIII MAMMALIA 441 



opening into the mouth, the secretion of which serves to keep its Hning 

 moist as well as to play a preliminary part in the process of digestion. 



It is characteristic of the mammal that the forwardly projecting 

 portions of the splanchnocoele, containing the lungs and lying one on 

 each side of the pericardiac chamber with the heart, become separated 

 off as pleural cavities. Between the two pleural cavities and the inter- 

 vening pericardiac cavity in front and the main peritoneal cavity behind 

 there is interposed a characteristic transverse partition — the diaphragm. 

 This is strongly convex on its headward side and concave on its tailward 

 side : it is highly muscular— the fibres being arranged radially and passing 

 at their inner ends into a sheet of tendon forming the central part of the 

 diaphragm. The result of this structural arrangement is that contraction 

 of the muscle-fibres of the diaphragm serves to flatten it, thus increasing 

 the space upon its headward side and drawing air into the elastic lungs 

 to occupy that space. Thus we have in the mammal the diaphrag- 

 matic method of inspiring air into the lungs in addition to the costal 

 method. 



The kidney of the mammal is a compact metanephros, so characteristic 

 in form as to have given rise to the common adjective " kidney-shaped." 

 The ureters commonly open into the stump of the allantois which persists 

 as a urinary bladder. Both ovaries and testes commonly become dis- 

 placed in a tailward direction from their original position and the testes 

 in most mammals pass, either temporarily during the breeding season or 

 permanently, into a thin-walled pocket termed the scrotum. The urino- 

 genital sinus is in the male prolonged as the urethra through the penis, 

 but in the female the diminutive equivalent of this organ is, except in a 

 few cases (e.g. rodents, mole), not traversed by the urethra. 



The Miillerian duct of the mammal is differentiated into three portions, 

 known respectively as Fallopian tube or oviduct in the restricted sense, 

 uterus, and vagina. It is also usually the case in the mammal that the 

 two Miillerian ducts show a tendency to undergo fusion in the mesial 

 plane. In the Metatheria this approximation (Opossum) or actual 

 fusion (other Marsupials) takes place at the level of the inner end of 

 the vagina, but in the Eutheria the fusion involves the outer ends of 

 the Miillerian ducts, extending inwards for a distance that differs in 

 different mammals. Thus in the Rabbit the two vaginae are completely 

 fused to form a single channel while the two uteri remain quite distinct ; 

 in the majority of mammals the two uteri also undergo fusion for a 

 part of their length so as to form a " bicornuate uterus " ; while in 

 Man they fuse so completely as to form a pjrriform uterus with the 

 Fallopian tube opening into it on each side. 



