After this plea for the employment of comparative anatomy to elucidate human anatomy, Harvey proceeds to deal in a most logical manner with the various difficulties in following the course taken by the blood in passing from the vena cava to the arteries, or from the right to the left side of the heart. He begins with fish, in which the heart consists of a single ventricle, for there are no lungs. He then discusses the relationship of the parts in the embryo, and arrives at the conclusion that "in embryos, whilst the lungs are in a state of inaction, performing no function, subject to no movement any more than if they had not been present, Nature uses the two ventricles of the heart as if they formed but one for the transmission of the blood." He therefore concludes that the condition of the embryos of those animals which have lungs, whilst these organs are yet in abeyance or not employed, is the same as that of the animals which have no lungs. From this he wishes it to be understood that the blood passes by obvious and open passages from the vena cava into the aorta through the cavities of the ventricles. A statement which was in direct opposition to the generally received tradition of the time that the blood passed from the right into the left ventricle by concealed pores in the septum which separates the two cavities in the heart. Thus far Harvey's teaching has been excellent, but now, leaving the highway of fact, he plunges into theory and is at once involved in error. He proceeds, "And now the discussion is brought to this point, that they who inquire into the ways by which the blood reaches the left ventricle of the heart and pulmonary veins from the vena cava will pursue the wisest course if they seek by dissection to discover why, in the larger and more perfect animals of mature age, Nature has rather chosen to make the blood percolate the parenchyma of the lungs, than as in other instances chosen a direct and obvious course--for I assume no other path or mode of transit can be entertained. It must be because the larger and more perfect animals are warmer, and when adult their heat greater, ignited I might say, and requiring to be damped or mitigated, that the blood is sent through the lungs, in order that it may be tempered by the air that is inspired, and prevented from boiling up and so becoming extinguished or something else of the sort. But to determine these matters and explain them satisfactorily were to enter upon a speculation in regard to the office of the lungs and the ends for which they exist. Upon such a subject, as well as upon what pertains to respiration, to the necessity and use of the air, &c., as also to the variety and diversity of organs that exist in the bodies of animals in connection with these matters, although I have made a vast number of observations, I shall not speak till I can more conveniently set them forth in a treatise apart."