6 INTRODUCTORY LECTURE, 



another organ in the same body, by considering them in relation to 

 the general nature and powers of the entire animal^, together with 

 its relations to other animals, and to the sphere of its existence, that 

 we are chiefly enabled to elucidate the uses of the several super- 

 additions which are met with in following out the series of com- 

 plexities of a single organ. 



Comparative Anatomy fulfils only a part of its services to Physiology, 

 if studied exclusively in relation to the varieties of a given organ in 

 different animals : the combinations of all the constituent organs in 

 one animal must likewise be studied ; and these combinations with 

 the principles governing them, or the correlations of organs, must 

 be traced and compared in all their varieties throughout the animal 

 kingdom. 



It is in this point of view that I now propose to bring before you 

 the leading facts of Comparative Anatomy, to discuss and demonstrate 

 the organs as they are combined in the individual animal, and, com- 

 mencing with the lowest organised species in which the combination 

 is of the simplest kind, to trace it to its highest state of complexity 

 and perfection, through the typical species of the successively ascend- 

 ing primary groups and classes of the animal kingdom. In short, as 

 my previous courses of Hunterian lectures, agreeably with the 

 arrangement of the Hunterian Collection, have treated of Com- 

 parative Anatomy according to the organs, in the ascending order, so, 

 in the present course, Comparative Anatomy will be considered ac- 

 cording to the class of animals, and also in the ascending scale. 



Many examples suggest themselves of the advantage of this mode 

 of studying the organisation of animals for the purpose of acquiring 

 just conceptions of the uses of the organs. In tracing, for example, 

 the progressive complication of the heart, we first find the simple 

 dorsal vessel; it is next concentrated into a ventricle, and to this 

 single cavity an auricle is afterwards appended : then the auricle 

 becomes divided ; afterwards there are two ventricles : there are 

 instances even in the animal kingdom where there are three ventricles 

 and ten ventricles. Now, the two-cavitied, dicoelous, or bipartite 

 heart is met with in the snail and in the fish ; but the physiology of 

 such conformation of the organ can only be explained by its connec- 

 tions with other organs, and by the general structure and habits of 

 the animal. 



First, then, as to the connections of the bipartite heart. In the 

 snail it is so placed, in reference to the breathing organ, that it 

 receives the aerated blood from that organ and propels it to the 

 system: it is an organ for the circulation of arterial blood; in other 

 words, a systemic heart. The bipartite structure of the central organ 



