OF NATURAL HISTORt. 27 



the general mafs of circulating blood. The flomach and in- 

 teftines are furniflied with a vaft number of fmall tubes, call- 

 ed laBeal dutlsy which feparate and abforb the nutritious parts 

 of the aliment, and reject all the groffer and ufelefs particles. 

 Thefe duels, after innumerable communications with each 

 other, unite into one large tube, diftinguifhed by the name 

 oit\iQ thoracic duEl^ which is the general refervoir of the chyle, 

 or fecreted liquor. This chyle, which is a mild fluid, palTes 

 from the thoracic duct to the fiibclavian vein ; and by this 

 vein it is conveyed to the hea:rt, where it mingles with the 

 blood, and is circulated through the body, for the nourifh- 

 ment of its different parts. It is of no moment, for our pre- 

 fent purpofe, to be more particular, efpecially as this fubjeft 

 will be afterwards more fully handled. I fhall therefore juft 

 mention, that there are particular organs or glands for fecre- 

 ting various fluids, which are necefTary to the exiftence of 

 the larger animals, as the kidneys for the fecretion of urine ; 

 the liver for the fecretion of gall ; the flomach for the fecre- 

 tion of the gaftric juices ; the falivary glands for the fecre» 

 tion of faliva, &c. 



From this flietch of the ftructure of man and of quadru- 

 peds, very little attention is neceflary to perceive, that na- 

 ture purfues a fimilar plan in the formation of birds and filhes. 



In that numerous clafs of animals diftinguifhed by the 

 name of mfeElSy there is a great variety of form and flrudlure. 

 In many of thefe, nature feems to depart from her general 

 mode of operation. But, upon a more accurate examination, 

 this feeming departure will appear to be only an extenfion of 

 that univerfal plan which flie obferves in the formation of 

 all animated beings. Some infe6ls, the lobfter, and all the 

 cruftaceous and fhell animals, have their bones on the out- 

 fide of their bodies. To thefe bones the mufcles and other 

 inftruments of motion are attached. Many fpecies have no 

 bones *, but their bodies coniift of a fuccefiion of rings inca- 



