72 INTRODUCTION. 



of generation. The cellular lungs and the heart are contained 

 in a cavity formed by the ribs, and separated from the abdo- 

 men by the diaphragm, where the surface is free. The circu- 

 lation is double, and respiration aerial and simple. There is a 

 larynx at the upper extremity of the trachea, which opens 

 into the posterior fauces, this communication depending on a 

 moveable, fleshy veil, called the veil of the palate, or the ve- 

 lum pendulum palati. 



What principally distinguishes the organization of the 

 mammalia, is their generation. It is essentially viviparous, 

 i. e. the membranous egg descends and fixes itself in the ute- 

 rus after conception, which requires a coitus, by which the 

 sperm of the male is thrown into the organs of the female. 

 They have all, like other vertebrate vivipari, at least, in the 

 beginning, an umbilical or intestinal vesicle ; they have also, 

 like the lung'd ovipari, an allantoid; but besides this, they 

 have other envelopes, the superior of which, the chorion, 

 fixes itself to the parieties of the uterus by one or more 

 plexuses of vessels, called placentas, that establish a communi- 

 cation between it and the mother, by which it receives nou- 

 rishment, and probably oxygen. When the foetus has acquir- 

 ed the necessary developement, it is expelled with the rup- 

 tured membranes. The mammae, secretory glands, produce 

 milk, for the support of the young, as long as they need it. 



It is to this kind of organization, presenting, however, cer- 

 tain variations, that man belongs. 



62. The mammalia have some organs peculiar to them, 

 such as the hairs of the skin, and the mammae; otherwise they 

 only differ from the other vertebrata, in the greater deve- 

 lopement of certain organs, as the ear for instance, the brain, 

 &c., or by different combinations of the organs of circulation, 

 respiration and motion. 



The blood of the mammalia, differs from that of the ovipari 

 in the form of its coloured particles ; in the former, they are 

 circular, or rather lenticular; whereas in the latter, they are 

 ovals, or flattened ovoids. 



The hairs of the mammalia are not essentially different 

 from the other horny appendages of the skin: like all organs 



