Integrative Systems 155 



are present— e.g., poison-glands of amphibians, sweat-glands of mam- 

 mals. Attached to the follicle which contains the "root" of a hair are 

 small nonstriated muscles which cause the hair to "stand on end" (Fig. 

 443). The iris of the eye contains muscles which regulate the size of the 

 aperture of the pupil, and certain internal muscles of the eyeball 

 (ciliary muscles: Fig. 175) effect accommodation of the eye for vision at 

 varying distances. All of these intrinsic muscles of the eye are smooth. 

 It may seem, therefore, that a distinction between body-wall and vis- 

 cera based on the nature of their respective muscles is mainly quanti- 

 tative and not of great significance. But a little consideration of the 

 functions of the several structures in question reveals some highly valid 

 distinctions. 



Consistent with the fact that the great bulk of the body- wall is con- 

 stituted of muscles and the skeletal structures necessary for their opera- 

 tion, the chief function of the body-wall is movement — changes in 

 relative position of parts of the body, or locomotion of the body as a 

 whole. The energy expended in muscular contraction is derived from 

 products of digestion of food in the inner tube. The oxygen nec- 

 essary for the chemical liberation of this energy is obtained from the 

 external medium by the visceral respiratory organs. Waste products, 

 some of them toxic, must be drained away from the muscles for elimi- 

 nation by the kidneys. These necessities are all supplied by visceral 

 organs, but only via the bloodstream. Therefore, the blood-vessels that 

 enter the body-wall and all their ramifications throughout the body- 

 wall must be regarded as being, both anatomically and functionally 

 (but not in their embryonic origin), extensions of the coelomic visceral 

 systems making available the necessary visceral services to every part 

 of the body-wall. 



Whereas muscular contraction is the dominant somatic function, it 

 is secondary or lacking in most visceral organs. The essential process 

 of digestion is the chemical action of secreted substances. Muscular 

 movements in the walls of the digestive tube are accessory. Secretion 

 of specific substances and the absorption or diffusion of substances 

 through cell-membranes (as in the organs of respiration and excretion) 

 are the primary and characteristic functions of most visceral organs. 

 Therefore, in so far as glandular organs are present in the body-wall 

 (with a few exceptions, they are derived only from the skin), they are 

 structures whose functions are of the visceral type and not of the domi- 

 nant somatic type. Sweat-glands of mammals are important thermo- 

 regulators (Fig. 160). Muscular contraction is a source of heat. In 

 regulating the disposition of a by-product of muscular contraction, the 

 sweat-glands perform a function somewhat analogous to that of an ex- 

 cretory organ. 



