376 



ELEMENTARY ZOOLOGY 



raise and lower. These movements, together with the 

 contraction of the diaphragm, cause the expansion and 

 contraction of the thoracic cavity 

 whereby the lungs are regularly filled 

 and emptied. Note that the abdomen 

 is covered by a double layer of mus- 

 cular tissue, the outer part made up 

 of the external oblique muscles, the 

 inner by the internal oblique muscles. 

 Examine the heart. How many 

 auricles has it ? The ventricles in 

 the mouse, as in the bird, are entirely 

 separated, forming two complete 

 compartments, a right and a left 

 ventricle. The blood flowing from 

 the veins of the body is collected in 

 the right auricle, thence it passes into 

 the right ventricle, whence it is con- 

 veyed to the lungs ; returning it flows 

 through the left auricle into the left 

 ventricle, whence it is forced through 

 the arteries of the body. For a 

 study of the circulatory system in 

 mammals (fig. 148), a rat or a rabbit 

 should be injected by the teacher and 

 an advanced text-book, as Parker's 

 1 ' Zootomy ' ' or Marshall and Hurst's 

 ' used as a guide. A sheep's heart 

 is very good to cut open for a class demonstration. 



Make a drawing of the organs observed thus far in the 

 dissection. 



The kidneys in the mouse are situated in the dorsal 

 region next to the backbone. They consist of two bean- 

 shaped smooth glands. From them a pair of ducts, the 

 ureters, can be traced down to a median thin-wallecl 



FIG. 148. Diagram of 

 the circulation of the 

 blood in a mammal; a, 

 auricles ; /, lung ; Iv, 

 liver; /, portal vein 

 bringing blood from the 

 intestine; v, ventricles; 

 the arrows show the di- 

 rection of the current; 

 the shaded vessels 

 carry venous blood, the 

 others arterial blood. 

 (From Kingsley.) 



" Practical Zoology 



