APA
Portis S. A., . (1953). Diseases of the digestive system. Philadelphia: Lea & Febiger.
Chicago
Portis Sidney A, . 1953. Diseases of the digestive system. Philadelphia: Lea & Febiger.
Harvard
Portis S. A., . (1953). Diseases of the digestive system. Philadelphia: Lea & Febiger.
MLA
Portis Sidney A, . Diseases of the digestive system. Philadelphia: Lea & Febiger. 1953.