158 HIGH SCHOOL ZOOLOGY. 



cavity, in front of which is the heai*t with its separate serous 

 sac, the pericarcUum ; the lungs, when present, project into the 

 cceloin, and are covered with the same serous coat as the in- 

 testines, but, in the niammals, the ca'lom is sub-divided by a 

 muscular partition (the diaphragm), into a thoiacic cavity con- 

 taining the lungs and the heart, and an abdominal cavity con- 

 taining the other viscei'a. That part of tlie cavity wliich 

 belongs to the lungs is now called the pleural cavity, the heart 

 with its pericardium remaining independent between the two 

 I>leur8e. The diaphragm acts chiefly as a muscle of respiration, 

 serving by its construction to enlarge the thoracic cavity, and 

 thus to facilitate the changes of the air in the lungs. Certain 

 structures like the Oisophagus, the aorta, and various nerves, 

 must pierce the diaphragm so as to icach the abdominal ca\ity ; 

 they do so after traversing the mediastinum, or space between 

 the periccirdium, the pleurae and the vertebral column. The 

 intervention of the diaphragm thvis forms a sharper boundary 

 between the oesophagus and stomach than we have met with 

 in the lower forms. In no mammal has the stomach the 

 simplicity met with in the Ichthyopsida or Reptiles. If it be 

 considered as a dilatation of the intestinal tube, it is always a 

 one-sided dilatation, so that there is a short curvatxu-e (the un- 

 dilated side of the tube), and a greater curvature (the dilated 

 side). Its form in the Cat is quite simple, but in herbivorous 

 animals there is generally a much more complex stomach, and 

 in these also the intestine is i-elatively much longer. The chief 

 difierence in the mammal's intestine as compared with lower 

 forms, is the proportionately much greater length of the lai'ge 

 intestine. 



15. As regards the respiratory system, apart from the differ- 

 ences referred to above, the most important is the more min- 

 utely " cellular " character of the lungs, the bronchi dividing 

 up dichotomously into smaller tubes which eventually end in 

 the alveoli or au-.cells. There are thus no membranous por- 



