CIRCULATION. 129 



near to general rules, such as in the general structure of the more per- 

 fect animals down to fish ; but beyond them it is not clear where the 

 heart may be placed in an unknown animal, and it is even differently 

 situated in the same genus ; for example, the situation of the heart in 

 the shell-snail [Helix] x is not the same with that of the black snail or 

 slug \_Limax~] 2 . 



The external form [of the heart] varies in different animals, and that 

 difference most commonly corresponds with the shape of the part in 

 which it lies. In the human, for example, it is flattened on the anterior 

 surface, answering to the flat breast 3 ; [it is] more so in the seal, otter, &c; 

 but this does not always take place in flat chests. In general it is a 

 cone 4 more or less flattened on one or two sides, but principally on one ; 

 this flatness generally constitutes the great difference in shape in any 

 one class ; the shape of each class differing from each other according to 

 the different purposes of each, or number of parts of which it is composed. 

 A heart is essentially simple in its construction ; the use of each part 

 is perfectly understood and of course [that of] the whole. It is a 

 muscle or muscles making the parietes of cavities which have no fixed 

 point of action, excepting an imaginary one, viz. the centre of the cavity, 

 to which the whole body of the muscle moves in its action, by which 

 means the cavities are lessened. 



The heart is, in general, divisible into a number of cavities, the 

 greatest number consisting of four, the fewest consisting of one only. 

 The first [or most complex] division constitutes [causes] a distinct and 

 double motion of the blood ; the second, a mixed motion ; the third, a 

 single circulation, but attended with a very singular circumstance in its 

 passage, viz. in gills ; and the fourth, not a circidation, but an undulation. 

 Of the First Division of Hearts. — Here the body called heart is formed 

 of two distinct hearts, each having its auricle and ventricle, with distinct 

 veins opening into the auricle, and each ventricle its artery passing out. 

 Although the above division is true, from whence it might be con- 

 jectured that there was no communication between the two circulations, 

 yet nature has connected them by means of the viscus, viz. the veins 

 corresponding with the arteries of the right side, shift sides and go to 

 the left, and vice versa of the left side. The valves are similar in all. 



This division comprehends the most perfect animals, which have a 

 double circulation, one through the lungs, the other through the whole 

 body, and for that purpose are furnished with a double heart. The 

 two auricles and two ventricles make up the four cavities by which 

 these animals are distinguished, whence they may be called Tetracoilia. 



1 [Hunt. Prep. No. 882.] 2 [lb. No. 883.] 



3 [lb. No. 929.] * [lb. No. 928.] 



