In relation to the deleterious properties of different gases. 136, 137 33. In relation to the dust that floats in the air. 137, 138 34. What are the properties of carbonic acid gas? 132, 138, 141 35. In what places is carbonic acid gas commonly found? 132, 138, 139 36. Describe the effects of carbonic acid gas. 132, 138, 139, 141 37. What are the general effects of breathing any impure atmosphere? 139, 140 38. What are Nature's provisions for purifying the air? 141, 142 39. What hints and directions are given on the subject of ventilation? 142, 143 40. How does the temperature of the body compare with the medium in which it lives? 143 41. How is temperature of the body regulated and sustained? 143, 144, 145 42. State what you can on the subject of spontaneous combustion. 145, 146 * * * * * {148} CHAPTER IX. THE NERVOUS SYSTEM. _Animal and Vegetative Functions--Sensation, Motion, and Volition--The Structure of the Nervous System--The White and Gray Substances--The Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System of Nerves--The Anterior and Posterior Roots--The Sympathetic System of Nerves--The Properties of Nervous Tissue--Excitability of Nervous Tissues--The Functions of the Spinal Nerves and Cord--The Direction of the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the Brain._ [Sidenote: 1. What processes are known as the vegetative functions? Why so called? What properties and functions does the plant possess? Their object?] 1. ANIMAL FUNCTIONS.--The vital processes which we have been considering, in the three previous chapters, of digestion, circulation, and respiration--belong to the class of functions known as _vegetative_ functions. That is, they are common to vegetables as well as animals; for the plant, like the animal, can originate nothing, not even the smallest particle of matter; and yet it grows, blossoms, and bears fruit, by reason of obtaining and digesting the nutriment which the air and soil provide. The plant has its circulatory fluid and channels, by which the nutriment is distributed to all its parts. It has, also, a curious apparatus in its foliage, by which it abstracts from the air those gaseous elements so necessary to its support; and thus it accomplishes vegetable respiration. These vegetative functions have their beginning and end within the organism of the plant; and their object is the preservation of the plant itself, as well as of the entire species.