A STUDY OF LIVING SUBSTANCE 21 



to the head end of any animal. A corresponding term pos- 

 te'ri-or (Latin post = after) refers to the opposite end of the 

 body. We can include, for instance, under the term anterior 

 appendages the wings of a bird, the front feet of a horse, 

 and the arms of man, since all these appendages are located 

 toward the head end of the trunk. 



The descriptive term dor'sal (Latin dor 1 sum = back), when 

 applied to vertebrates, 'always designates the region of the 

 body in which the backbone is found. If this term is used 

 in describing the structure of invertebrates, it refers to the 

 upper surface of the animal. Ven'tral (Latin ven'ter = 

 belly), on the other hand, designates the under or, in man, 

 the front surface. Both in man and in the horse the mouth 

 opening is on the ventral surface of the body, even though 

 its position apparently differs so much in the two animals. 



Organs. 1 When we study the body more closely, espe- 

 cially its interior, we find, in various regions, parts that 

 carry on special kinds of work. Within the chest cavity in 

 the upper or anterior part of the trunk is the heart, which 

 forces the blood through the body. Here, also, are the lungs, 

 which take in oxygen and give it to the blood, and which 

 remove some of the waste matters from the body. Below 

 the heart and lungs, or in other words, posterior to the trans- 

 verse muscular partition, called the di'a-phragm, are the 

 stomach and the intestines, the liver and the pancreas, all 

 of which help to change our food into liquid form. Here, 

 too, are the kidneys and the spleen. All these and other 

 parts of the body, like the brain, the spinal cord, and 

 the hands, are called organs. An organ is a part of a liv- 

 ing body that has some special work to do : this special work 

 is called its function. Our hands are organs, because with 



1 On the next page is a* figure of the internal organs of a rabbit 

 (Fig. 4). In this figure most of the organs enumerated above can 

 be identified ; the form and position of these organs in the human 

 body, however, are somewhat different, as one sees after studying the 

 figures on pp. 76, 99, 130, and 255. 



