GENERAL INTERNAL ORGANIZATION 



41 



ting the body exactly, but fastened only along certain lines of 

 attachment, or septa (Fig. 19). The spaces partitioned off by 

 these septa contain a colorless fluid, the lymph, and are known as 

 subcutaneous lymph sinuses. They are part of a system of cavities 

 and vessels extending throughout the body of the vertebrate ani- 

 mal (c/. Fig. 20) and draining into the circulatory system. 



General Internal Organization 



Flesh and Skeleton. — It is a famihar fact that beneath the 

 skin of a vertebrate animal lies the so-called " flesh," composed 



mainly of muscles, which are 

 attached to a supporting frame- 

 work, the skeleton. This rela- 

 tionship is illustrated by a trans- 

 verse section of the human arm 

 (Fig. 21). So familiar is this 

 type of organization, as found in 

 well-known animals, that it is 



Fig. 20. — Superficial and some 

 deeper lymphatics of the hand. 



(From Hough and Sedgwick, "Human 

 Mechanism," copyright, 1918, by Ginn 

 and Co., reprinted by permission.) 



Fig. 21. — Transverse section of upper 

 arm. 



6.1). , blood vessel; h, humerus; m, muscle; 

 n, nerve; s, skin. 



not always reahzed that such an internal or e?ic?oskeleton, with 

 its overlying muscles, is far less common in the animal kingdom 

 than are exoskeletons hke the external coverings of a locust or a 

 crayfish or the shell of a snail. As a matter of fact, only three of 

 the great animal groups, the Chordata, the Porifera, and the 

 Echinodermata (p. 241), possess an endoskeleton that is charac- 



