INTRODUCTION. xix 



oil does in the interior of" a joint: in the same manner as that 

 serves to prevent friction between bones, the bursa:, mitcoscc, 

 which likewise contain synovia, obviate similar inconvenience 

 arising between tendons, or between tendons and bones. 



From the organs of support and locomotion, we proceed to 

 those concerned in the circulation of the blood over the animal 

 machine. These consist of the heart, the arteries, and the veins. 

 The heart in its situation in the middle of the cavity of the thorax, 

 operates after the manner of a forcing pump, throwing blood 

 into the trunks of the arteries, through whose manifold ramifi- 

 cations it is conducted over every part of the body, to be after- 

 wards conveyed back again by the veins. In this manner are 

 executed the impoitant functions of //M^r?7?ow, groioth, and secre- 

 tion. Blood is to the animal what sap is to the vegetable : it 

 nourishes the body ; supplies new materials for its growth and 

 repair; and, at the same time, becomes the vehicle for the re- 

 moval of the old and worn-out matters; besides furnishing the 

 various secretions, such as bile, urine, semen, perspiration, Stc, 

 which, by a process alike wonderful and mysterious, are elabo- 

 rated in organs constructed especially for the purpose, through 

 the extreme ramifications of the arterial system. Above all, blood 

 is the medium through which the spirit of life itself is elimi- 

 nated and diffused over the body : every part owing it vitality 

 and susceptibility of action to a contiimal fresh supply of this 

 'fluid. 



These important purposes served by the blood, necessarily 

 tending to a consumption as well of its material quantity as of its 

 vital principle, it became necessary that means should be provided 

 in the animal economy for the replenishment of both these losses. 

 Accordingly, we find one system of parts formed for the supply of 

 fresh blood ; another for the purpose of the vivification of the 

 fluid. This latter function is performed by the RESPrRATORv 

 SYSTEM. The lungs, constituting that system, are two bulky 

 bodies, in structure bearing much resemblance to a honeycomb, 

 through whose cells a large proportion of the blood is conducted 

 by the pulmonary vessels at every round of the circulation, in 

 order that it may be exposed to atmospheric air, and from its in- 

 fluence derive those vital properties that render it fit for the sup- 



