CHORDATA 



341 



secretions are called hormones or internal secretions. The 

 spleen, the thyroid, the thymus, adrenal bodies, and pituitary body, 

 are the chief ductless glands. 



FIG. 170. 



FIG. 171. 



c. 



frrec.- 



FIG. 170. Diagram of the heart and branchial arches in a Reptile. Position and lettering as in 

 preceding figures, l.v., left ventricle; r.v., right ventricle. 



Questions on the figure. Compare this with figures 166-169 and make a note 

 of the differences. How much communication is there between the two sides of 

 the heart? What tends to insure that the purest blood in the heart shall go the to 

 head? That the least pure goes to the lungs? 



FIG. 171. Diagram of the heart and the branchial arches in Mammals. A dotted outline of 

 the arches of the Fish is drawn for ready comparison. The auricles are represented in a posterior 

 position, as in the preceding figures. 



Questions on the figure. What changes in the heart are shown in this as conv 

 pared with former figures? In the systemic branchial arch? Remember that the 

 heart is not represented in its normal position; the auricles are really at the anterior 

 of the heart (see Fig. 166). Compare this condition with table, page 343. What 

 are the grounds for believing that the auricles are, morphologically, the posterior 

 part of the heart? 



The spleen is a dark body on the mesentery near the begin- 

 ning of the small intestine. In its cavities old red blood cor- 



