OF NATURAL HISTORY. 15 



The bones of the pelvis, and of the four extremities, are joined to 

 the back-bones by articulations and membranes. By the fame con- 

 trivance, the cranium is fixed to the upper end of the back-bones. 

 Into different proceffes and portions of all thefe bones, a great num- 

 ber of mufcles, or bundles of flefhy fibres, are inferted. Thefe 

 mufcles are the inftruments which give rife to all the varieties of 

 animal motion. The bones of the head, or cranium, contain the 

 brain and cerebellum, a prolongation of which runs through the 

 whole extent of the canal in the back-bone, and is known by the 

 term fpinal marroiv. From the brain and fpinal marrow proceed 

 all the nerves, or inftruments of fenfation. Thefe nerves, the ra- 

 mifications of which are infinitely various and minute, are diftri- 

 buted upon the heart, lungs, blood-veffels, bowels, and mufcles, till 

 they terminate on the fkin, or external covering of the body. The 

 heart is the fountain, or general receptacle of the blood. The con- 

 traction of the heart propels the blood through the arteries, which 

 are likewife diftributed, by numerous and complicated ramifications, 

 over every part of the body, and terminate in the veins, which 

 again colleft the whole arterial blood into one cavity, and reconvey 

 it to the heart. This circulatory procefs goes on during life. 



Befide the organs already mentioned, there are others, termed 

 fecretory, becaufe they feparate peculiar fluids from the general mafs 

 of circulating blood. The ftomach and inteftines are furnifhed 

 with a vaft number of fmall tubes, called laBeal du£isy which fe-^- 

 parate and abforb the nutritious parts of the aliment, and rejedt all 

 the groffer and ufelefs particles. Thefe duds, after innumerable 

 communications with each other, unite into one large tube, diftin- 

 guiftied by the name of the thoracic dtici^ which is the general re- 

 fervoir of the chyle, or fecreted liquor. This chyle, which is a 

 mild fluid, pafl"es from the thoracic dudt to the fubclavian vein ; 

 and by this vein it is conveyed to the heart, where it mingles with 



the 



