10 



of the blood, said Mr. Dowker, this interesting sight has been 

 a favourite study by the microscopist. Indeed, the frog plate 

 was considered an indispensable part of the apparatus of the 

 older niioroscopists. This was a contrivance by which a 

 frog cnuld be placed under the microscope, so as to exhibit 

 the circulation of the blood in the web of the foot. 

 The young salmon, presents to us facilities for study- 

 ing this interesting phenomena, in a far greater degree 

 than in the Frog, Newt, or any other animal with which 

 we are acquainted. Before pointing out the peculi- 

 arities in the young salmon, to render the subject 

 plain to you all, I propose to exhibit by means of a dia- 

 gram, the course of the blood in the human subject. I 

 will be seen from this diagram, which represents a section 

 of the human heart, with its accompanying large blood ves- 

 sels, that the heart is first divided into two separate cavities, a 

 right, and a left, and these again subdivided by a series of 

 valves into upper and lower cavities ; thus we have a 

 right left auricle and right left ventricle. The office of 

 the auricles is to receive the blood, and force it into the 

 ventricles ; the ventricles receive the blood from the 

 auricles, and force it through the lungs and through the 

 body. The vena cava receives the blood from the 

 system, and conveys it to the right auricle, from which it 

 descends to the right ventricle, which forces it through the pul- 

 monary artery into the right and left lungs, where it is ex- 

 posed to the influence of the air in minute vessels — 

 thence it is collected by the pulmonary veins and conveyed 

 to the left auricle, thence to the left ventricle, which con- 

 tracting, forces the blood by the great aorta through the 

 entire system by means by the arteries. After passing the 

 capilaries, the return current is collected in the veins to 

 go through a similar course. It will thus be seen that the 

 heart in the human subject and also in all mammalia is 

 double, and it was pointed out by Dr. Murie, that a pe- 

 culiarity of some of the Sirenia, was the presence of a 

 heart, in which the two halves were separated. The cir- 

 culation of the blood in fishes, differs in this impor- 

 tant peculiarity fi'ora that of mammalia, that the heart is 



