IN VERTEBRATES 



If we recall the metabolic requirements of individual cells, we can im- 

 mediately grasp the fact that the problems of maintenance and growth in 

 many-celled animals must be complex. Living cells must be bathed with a 

 continuous supply of essential nutrients and oxygen and be kept clean, so to 

 speak, through a continuous removal of waste products of their living. Anal- 

 ogies can be drawn between the needs of a community of cells and those of a 

 community of human beings. Let us, instead, undertake a scientific analysis 

 of the way in which vertebrates, like ourselves, manage to live. We shall do 

 this by first looking at the structure or morphology of these animals with special 

 reference to the parts related to maintenance problems. Most of you are dis- 

 secting a vertebrate animal, such as the frog, in the laboratory; the brief 

 discussion of vertebrate structure here is only intended to highlight the infor- 

 mation you already have. Major emphasis will be on the kinds of individual 

 tissues that exist in a vertebrate and especially on the way in which the tissues 

 and organs function. 



We shall see that cells are specialized to form tissues and that special 

 arrangements of these form the organs of the various anatomical divisions or 

 systems of the body. The organ systems to be considered in this chapter 

 are: (1) the digestive system, (2) the circulatory system, (3) the respiratory 

 system, and (4) the excretory system. 



Organ Systems Related to Metabolism 



The Digestive System. The digestive system is made up of the digestive 

 tract, which is a tube, and the attached digestive glands, the liver, the pancreas, 

 and, in many vertebrates, the salivary glands. The digestive tract is, in effect, 

 a tunnel through which food passes as it is digested or chemically changed 

 for use by the cells of the body. Through the lining of certain parts of the 



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