Retinal image, 210 51. Different shapes of the globe of the eye, 212 52. Function of accommodation, 214 53. Diagram of the ear, 218 54. Section of the ear, 221 55. Section of larynx and trachea, 229 56. View of vocal cords by the laryngoscope, 232 57. Different positions of vocal cords, 232 58. Simple microscope, 238 59. Compound microscope, 239 60. Household microscope, 240 61. Popular microscope, 241 {11} INTRODUCTION. ------o------ The Human Body is the abode of an immortal spirit, and is the most complete and perfect specimen of the Creator's handiwork. To examine its structure, to ascertain the uses and modes of action of its various parts, how to protect it from injury, and maintain it in a healthy condition, is the design of this work. The departments of knowledge which are concerned in these investigations, are the science of Human Physiology and the art of Hygiene. PHYSIOLOGY treats of the vital actions and uses of the various parts of living bodies, whether vegetable or animal. Every living thing, therefore, has a Physiology. We have a _Vegetable_ Physiology, which relates to plants; and an _Animal_ Physiology, relating to the animal kingdom. The latter is also divided into _Comparative_ Physiology, which treats of the inferior races of animals, and _Human_ Physiology, which teaches the uses of the various parts of the human body. HYGIENE, or the art of preserving health, is the practical use of Physiology. It teaches us how to cultivate our bodily and mental powers, so as to increase our strength and to fit us for a higher enjoyment of life. It also shows us how to prevent some of the accidents which may befall the body, and to avoid disease. It is proper that we should {12} understand the construction and powers of our bodies; but it is our duty, as rational beings, to know the laws by which health and strength may be maintained and disease warded off. There are various means by which we gain important information respecting the Physiology of man. Plants aid us in understanding the minute structure of the human body, its circulation, and absorption. From inferior animals we learn much in respect to the workings of the different _organs_, as we call those parts of the system which have a particular duty to perform. In one of them, as in the foot of the frog, we can study the circulation of the blood; in another, we can study the action of the brain.