THE POPULAR EDUCATOR. 



different cavities of the heart, is arranged in folds, which, with 

 the addition of some muscular fibres and fibrous tissue, compose 

 tlio valves which protect these openings. From the inner 

 surface of the walls of the heart jut out irregular masses of 

 muscular fibre, many of which stand out like columns or pillars, 

 and are thence called " columnas carneae" (fleshy columns). 

 From the free extremities of some of these, cords run to the 

 under- surf ace of the valves, and act as check-strings, preventing 

 them being forced too far backwards by the pressure of the 

 blood. Into the right auricle, as has been said, the used blood is 

 emptied by two largo veins named respectively the superior and 

 inferior cava; it is rather larger than the left auricle, and is 

 situated at the base of the right ventricle, forming a part of the 

 anterior surface of the heart ; it communicates with its ventricle 

 by a large oval aperture, which is protected on the ventricular 

 side by one of the valves before-mentioned, called, on account of 

 its being composed of three segments or cusps, the tricuspid 

 valve. The spot whore the inferior cava enters is also protected 

 by a valve. The right ventricle is triangular in form, and 

 forms the greater part of the anterior surface of the heart ; it 

 has opening into it the pulmonary artery, which is guarded by a 

 set of valves, called from their shape the semi-lunar. The left 

 auricle is thicker but rather smaller than the right ; it receives 

 the pulmonary veins, which are four in number, and communi- 

 cates, by an opening similar to the one on the right side, with the 

 left ventricle. The left ventricle is conical in shape, is longer, 

 and its walls are much thicker tban those of the right ; it forms 

 the posterior surface, and in consequence of its greater length 

 the apex of the heart : the opening into it from the auricle is 

 guarded by the mitral valve, which consists of two segments, 

 but is larger, thicker, and stronger than the one on the opposite 

 side ; from the upper and back part of it springs the great main 

 trunk, the aorta, the large vessel which conveys the blood for 

 th-3 use of the whole body. The heart, in common with all other 

 organs that have important duties to fulfil, has special vessels 

 to supply it with nutritive blood ; these are called the coronary 

 arteries, and are nearly the first branches given off from the 

 aorta. It also is abundantly supplied with nerves, which it de- 

 rives mainly, though not entirely, from the sympathetic system. 

 Such, then, being the engine by which the blood is put 

 in motion, we must now consider tha channels through which it 

 travels. The blood-vessels are divided into three Bets : the 

 arteries, which are the vessels carrying the blood from the heart; 

 the veins, which return the blood to the heart ; and an inter- 

 mediate set, the capillaries, which form the connection between 

 the arteries and the veins. The arteries are cylindrical tubular 

 vessels, owing their name to an ancient opinion, which was 

 based on the fact that they are found empty after death, and 

 were consequently supposed to contain air (aer, air ; terein, to 

 contain). They have three coats ; the external, called also the 

 elastic, is a membrane of great strength and elasticity ; the 

 middle or muscular coat is thick, composed of several layers of 

 muscular and clastic tissue, which form a firm, solid, but rather 

 brittle membrane ; the internal, or serous, is a thin, nearly 

 transparent membrane, moistened, as its name implies, by a 

 serous fluid. The effect of these differences in the constitution 

 of the coats of arteries is manifested when an artery is cut off 

 by a ligature as in surgical operations, or torn through as in an 

 injury. In either case the external coat shows the purpose for 

 which it is made of so great strength and elasticity. In the 

 case of the ligature, the internal and middle coats are at once 

 cut through, but the external coat bears the pressure, and thus 

 enables a permanent healing of the wound and closure of the 

 vessels to take place before it separates. Also, when a limb is 

 torn off, the artery, in common with the other structures, is 

 forcibly lacerated, the inner and middle coats give way at once 

 and retract, the external coat is pulled out to a fine point before 

 it divides, and thus shuts off the channel of the vessel, and pre- 

 vents that profuse bleeding which without this provision would 

 otherwise take place. 



PNEUMATICS. VIII. 



TENTILATION WIND ITS PRESSURE AND EFFECTS TRADE 

 WINDS MONSOONS LAND AND SEA BEEEZES SIMOOMS, ETC. 



A. SIMPLE experiment, which may be easily tried, affords a good 

 illustration of the principles on which ventilation is arranged in 



Fig. 20. 



mines or buildings. Procure a straight glass chimney (Fig. 20),, 

 such as is used with a moderator lamp, and place it over a short 

 piece of lighted candle ; in a few minutes the 

 flame will flicker and then be extinguished. 

 The oxygen of the air contained in the 

 chimney has been consumed, and its place 

 occupied by the poisonous carbonic acid gas 

 thereby produced ; and as there is no way 

 of allowing the foul air to escape and fresh 

 to enter, the candle goes out. Now cut a 

 piece of cardboard of the shape represented 

 at A, such that it will pass down the chim- 

 ney nearly to the flame, but will be pre- 

 vented by the enlarged top from falling 

 lower, and, having relit the candle, place the 

 chimney over it as before, but with the card 

 in it. The candle will now continue to burn, 

 and we shall find that the flame is drawn 

 a little to one side. The partition has di- 

 vided the chimney into two compartments, 

 up one of which the smoke and carbonic 

 acid pass, while fresh air comes down the 

 other and maintains a constant supply. The 

 heat of the flame rarefies the air over it, and 

 thus creates an upward current in the one compartment, just as- 

 the furnace does at the bottom of a mine shaft. 



If, instead of inserting the piece of card, we raise the chimney 

 a little from the table by a few small pieces of wood, the candle- 

 will continue to burn ; an inlet is now provided for the fresh, 

 air, and the whole of the chimney is occupied by the up-current. 

 The essential points are to provide an exit for the foul air and 

 an entry for fresh air. 



In a building the air, heated by the lights and by being 

 breathed, ascends. This will usually suffice to ventilate the 

 place if a proper way of escape be made for the foul air. and 

 the importance of doing this cannot well be overrated. Care., 

 however, is required in regulating the entry of fresh air so as to- 

 avoid draughts. 



Frequently, however, the gas is not lighted, and the heat of 

 the breath is insufficient to ventilate the building in this way, 

 and then other means must bo adopted. One of the simplest 

 and best of these is to carry a metal shaft from the top of the 

 room or building, and in the upper part of this to place a num- 

 ber of gas-burners. These, when lighted, create a powerful 

 current of air up the shaft, and thus the foul air is removed 

 from the place. 



Gas-burners have also been invented of such a construction as 

 to remove the impure air by means of a pipe opening over them, 

 and, at the same time, by the draught thus created to remove 

 the foul air from the upper part of the room. 



The usual defect in ventilation is that this foul air is allowed 

 to accumulate near the ceiling. If we stand on steps so as to- 

 have our head near the top of a room with several people and 

 lighted gas burners in it, we shall be fully convinced of this fact. 



As a knowledge of the fact that heated air always ascends is 

 of great importance to us in understanding the theory of the 

 wind, we will mention another simple illustration of it. Take a. 

 circular piece of card, about six or eight inches in diameter, and, 

 beginning at the exterior, cut it spirally inwards so as to form 

 a strip about f of an inch wide. Stick a pin through the centre, 

 and by this suspend it over a lighted gas-burner. The upward 

 current of heated air will cause it to turn rapidly round, and it 

 will somewhat resemble a serpent. The old smoke-jack used to- 

 act in a similar way. 



If we open the door of a warm room a very little way, and 

 hold a candle in the opening so as to observe the direction of 

 the current by that of the flame, we shall see the mode in which 

 the air is changed. When the candle is held close to the floor, a 

 strong inward current of air will be perceived. Being cold, it 

 sinks to the floor and there enters. As we raise the candle the 

 current becomes weaker, till, about half-way up, it is quite 

 imperceptible. Higher still the current is reversed, and we shall 

 find the flame driven outwards, showing that the heated air is. 

 escaping near the upper part of the room. 



This experiment is a very instructive one, as it explains to us 

 the principle on which rooms must be warmed, and generally the 

 way in which gases become heated. If we place one end of a. 

 bar of iron in the fire, the heat will gradually pass along it until 



