MULTICELLULAR ANIMALS 245 



* 



into the intestine. The question as to how such gland cells extract 

 certain materials from the blood, form them into secretions and 

 expel the secretions into the collecting duct is not wholly answered. 

 In some glands, for example in the tubules of the kidney (p. 189), 

 the secretion passes from a region of lower osmotic pressure within 

 the tubule into the blood, which has a higher osmotic pressure. Like- 

 wise the gland cells of the salivary gland secrete water from the 

 blood to form saliva. Now the blood has a higher osmotic pressure 

 than that of the saliva; so in forming the gland product water is 

 withdrawn against osmotic pressure. It is necessary to assume that 

 in such cases work is done by the cells of the gland wall in accom- 

 plishing the expulsion of products against osmotic pressure. 



Suggested Readings 



Parker, G. H.: The Elementary Nervous System. J, B. Lippincott Com- 

 pany, 19 19. 



Child, C. M.: Physiological Foundations of Behavior. Henry Holt and 

 Company, 1924. 



Hogben, L. T.: Comparative Physiology of Internal Secretions. Cam- 

 bridge University Press, 1927, 



Sharpey-Schafer, E.: The Endocrine Organs. Longmans, Green and 

 Company, 1925. 



Gerard, R. W.: Nerve Conduction in Relation to Nerve Structure. Quar- 

 terly Review of Biology, Vol. 6, 1931. 



Howell, W. H.: Textboo\ of Physiology. W. B. Saunders Company, 

 1930. 



