84 THE INDIVIDUAL ORGANISM 



The spleen. This is an organ closely associated with and important 

 for the proper functioning of the vascular system. It is an ovoid body 

 about 5 inches in length and 6 ounces in weight, located in the upper left 

 portion of the abdominal cavity. The functions of the spleen are still not 

 fully understood but are known to include the following: 



1. Destruction of old red corpuscles by two kinds of phagocytic cells 

 that abound in the splenic tissues. Most of the iron-containing substances 

 thus liberated are carried by the white cells to the liver for storage. 



2. The filtering out from the blood of solid particles, including cell 

 debris, foreign protein masses, and disease-producing organisms. These 

 particles are "eaten" by the phagocytes. The spleen serves as the great 

 blood filter, and this may be its most important function. 



3. Production of nongranular white cells in the lymphoid tissue. In 

 embryonic life and in certain types of anemia the spleen also produces 

 granular white cells and red corpuscles. 



4. Serving as a reservoir of red blood corpuscles. When there is a 

 deficiency of hemoglobin or of oxygen in the blood, the spleen contracts, 

 driving some of the corpuscles it contains into the general circulation. 

 This contraction is at least in part a response to the hormone adrenalin, 

 which is discussed in the chapter dealing with the endocrine glands. 



The heart. This is the powerful double pump that forces the blood 

 to circulate through the blood vessels. It is a muscular organ situated 

 nearly in the center of the lower part of the thoracic cavity. It is enclosed 

 in a tough membranous sac, the pericardium, which contains the peri- 

 cardial fluid that bathes the heart and protects it from friction with the 

 surrounding organs. 



The heart has four chambers. The upper two are the relatively thin- 

 walled auricles or atria, which receive blood from the veins. The two lower 

 chambers, the ventricles, have heavy muscular walls that, by contracting, 

 propel the blood throughout the body. The auricle (atrium) and ventricle 

 on the same side of the heart are connected by a valve that permits blood 

 to pass from auricle to ventricle; but the chambers of the right side are 

 completely separated from those of the left by strong, impervious parti- 

 tions. The heart may therefore be thought of as composed of two sepa- 

 rate pumps, often designated the right heart and the left heart. 



The walls of the heart, except for thin outer and inner, epithelial linings 

 (the inner lining called endothelium) and some connective tissue, are 

 composed of a special type of contractile tissue known as cardiac or heart 

 muscle. The heart is a modified blood vessel, and its muscle tissue has the 

 same origin as the smooth muscle of artery and vein walls ; but its fibers 

 are nevertheless cross-striated, though imperfectly so, and in this respect 

 suggest the structure of skeletal muscle. Cardiac muscle also resembles 



