HUMAN STRUCTURE AND LIFE- ACTIVITIES 459 



ness of the body-wall depends upon the amount of fat ; hence very 

 lean bacon is relatively thin. Notice that the skin is fastened 

 firmly to the muscles and fat ; this is due to fibers of connective 

 tissue. In some parts of the body of mammals the skin is not 

 fastened down so closely as in bacon ; e.g., note the loose skin on 

 the back of your own hand, or on the backs of young puppies. 



373. Relative Positions of Internal Organs in Man. 



Study diagrams, charts, and a manikin, if available ; and note 

 the following positions of the largest organs, which are in the 

 same relative position as in the frog. (1) The body-cavity 

 is ventral to the backbone. (2) The alimentary canal ex- 

 tends through the body-cavity from anterior to posterior. 

 (3) The heart is ventral to the alimentary canal (esophagus 

 part). (4) The liver lies ventral to the alimentary canal 

 (stomach and intestine part). (5) The kidneys lie dorsal to 

 the alimentary canal and in the posterior part of the body- 

 cavity (abdominal part). (6) The brain and spinal cord lie 

 in the cavities formed by the bony skull and the backbone. 



374. Life-activities of the Human Body. The character- 

 istics of living things already studied in connection with 

 animals and plants apply to the human body, for it is a 

 living mechanism which performs the functions necessary 

 for life. 



The life-activities are located in the cells; and these, 

 as in the case of the frog, are in the tissues (epithelial, 

 connective, muscular, bony or osseous, cartilaginous, and 

 nervous) . 



Probably the two activities which most attract our atten- 

 tion are those concerned with food and breathing. Moreover, 

 these are the basis of all the other processes occurring in the 

 human body. For these reasons we shall specially consider 

 the taking of foods and oxygen into the human body, and 

 later we shall trace the changes of foods and oxygen in the 

 body. This will lead us on to consider all the essential life- 

 activities. 



