GENERAL STRUCTURE. 15 
cell activity is of two kinds, gaseous and liquid. Both 
diffuse through the capillary walls into the blood. The 
Fic. 10. PHOTOMICROGRAPH OF FIBERS OF VOLUNTARY MUSCLE. X 100. 
Note the finer threads of connective tissue. 
former, which is carbon dioxid, is carried to the heart and 
thence by the pulmonary arteries to the lungs, where it 
passes into the terminal branches of the trachea, and finally 
by expiration reaches the exterior. The liquid excretion is 
transported by means of the veins and arteries to the 
kidneys, where it diffuses through the walls of the capil- 
laries into the minute tubules opening into the ureters lead- 
ing to the bladder. It must be remembered that the 
