28 INTRODUCTION. 



The air is introduced into these cells in the following manner : — 

 The chest and the helly are divided from each other by a very strong 

 muscular expansion called the diaphragm^ or midriff, or skirt. In its 

 natural state it is of an arched form, and bulges considerably into the 

 chest. When it is excited to action, it contracts ; it becomes straighter ; 

 the bulging into the chest is diminished, and the cavity of the chest 

 proportionally increased ; and if, at the same time, the muscles which 

 are between each rib, and which have the power to elevate or depress 

 the ribs, likewise act and raise them, the cavity becomes yet more 

 enlarged ; and the consequence of this necessarily is, that there would, 

 if possible, be a vacuum between the lungs and the walls of the chest. 

 To prevent this, or, more properly speaking, forced by the inequality 

 of atmospheric pressure thus produced, air rushes into the nose, passes 

 down the windpipe, inflates and fills up the lungs, and is thus Isrought 

 into contact with the blood. After a short time the diaphragm and 

 the muscles of the ribs cease to act, and the former begins to bulge 

 again into the chest, and the latter to fall, and the cavity of the chest 

 is contracted, and the lungs are squeezed into their former bulk, and 

 the air which had entered is pressed out again. 



A most important process, however, has been performed during 

 this entrance and expulsion of the air. Both the air and the blood 

 have been changed : the air has become poisonous, and the blood has 

 become capable of supporting life. A great quantity of what used to 

 be termed pure air, oxygen^ is taken from that which was inhaled ; a 

 portion of it unites with the poison of the blood — the carbon — and 

 forms carbonic acid gas — fixed air — and which is expelled when 

 the air is returned in the act of expiration ; while another portion of 

 it enters into the composition of the blood, and either remains there 

 unchanged, or becomes combined with some of the multifarious sub- 

 stances that make up the blood. The air has taken carbon from the 

 blood, and communicated oxygen to it. 



This change, both of the air and the blood, is not so marked in the 

 ox as in most other animals. He does not seem to have so much 

 poison to get rid of, nor needs he to steal so much vital air from the 

 atmosphere, and therefore the cow-house, however close, is seldom 

 offensive. The breath of the cow is even pleasant, and consumptive 

 persons have fancied that they have derived benefit from inhaling it. 



The inside of the chest, and of the diaphragm, and also the whole 

 external surface of the lungs, are covered by a smooth membrane 

 called the pleura, which secretes a serous fluid, which in its natural 

 quantity preserves the surfaces moist, and prevents friction, but in 

 undue and unhealthy quantity constitutes dropsy of the chest. 



The Organs of Digestion. — The food received by all animals 

 affords, by the process of digestion, a nutritious fluid of a milky co- 

 lour, called chyle ; which is absorbed into the system, and soon 

 enters the circulating mass of blood, and becomes itself converted 

 into blood : thereby repairing the waste that this fluid suffers in 

 nourishing the body, and also supplying the materials for all the 



