HUMAN PHYSIOLOGY. 



diet, exercise, temperature, humidity of the air, 

 &c. 



The mechanical means by which these changes 

 are effected are partly to be sought for in the 

 elasticity of the lungs themselves, and partly in 

 the mobility of the sides and bottom of the cavity 

 of the chest, which is thus enabled to alter its size 

 with each respiratory act. The sides of the chest 

 are mainly formed by the ribs, which are so 

 attached to the spine as to be freely movable 

 upwards and downwards by two sets of inter- 

 costal muscles. Again, the diaphragm, the great 

 muscular partition between the chest and the 

 belly, is always convex to the former ; but when 

 it contracts, the convexity is lessened, and con- 

 sequently the capacity of the chest is increased. 



A knowledge of the respiratory process explains 

 the .great benefit to be derived from efficient 

 ventilation. Ten thousand parts of ordinary 

 atmospheric air contain from 2 to 4 parts of 

 carbonic acid. If this gas be present to the 

 extent of \\ to 3 parts in 1000, headache and 

 giddiness are felt; and if it be increased to 20 

 parts in 1000, death will in all likelihood be the 

 result. To secure a proper degree of dilution of 

 carbonic acid in a small room, so as to render the 

 air fit for respiration, about 2000 cubic feet of 

 fresh air should be introduced every hour. 



II. THE NOURISHMENT OF THE TISSUES. 



The various tissues of the body, such as muscle, 

 bone, nerve, or brain, are nourished by the blood. 

 But as this fluid is almost the same in chemical 

 composition in different parts of the body, and as 

 the tissues differ much in this respect from each 

 other, we must adopt the theory that each tissue 

 has an elective power in itself, whereby it selects 

 from the blood exactly the material it requires for 

 its growth. To secure healthy nutrition, we must 

 have an adequate supply of blood. If any part 

 of the body is not supplied with abundance of 

 blood, its actions are enfeebled ; and if the supply 

 be cut off altogether, it soon weakens and dies. 

 The blood must also be healthy in quality. If 

 affected by disease of any organ, so that certain 

 injurious materials are not eliminated, the general 

 nutrition of the body speedily suffers. To secure 

 proper nutrition, a part must, thirdly, be subject 

 to the influence of the nervous system. Disease 

 of the spinal cord causes paralysis of the lower 

 limbs, and the muscles become soft, flabby, and 

 diminish in r.ize. Section of a nerve supplying a 

 part is often followed by destruction of the part 

 by ulceration. Finally, the part itself must be 

 in a healthy condition to secure proper growth. 

 Any tissue which has acquired any peculiarity of 

 structure by previous disease, retains this pecu- 

 liarity for many years ; but in course of time the 

 tissue tends to revert to its original condition. 

 This explains such phenomena as the perpetua- 

 tion of cicatrices, and the influence of the vaccine 

 virus. In the latter case, the virus stamps a 

 peculiar quality on the blood and tissues, which 

 modifies any subsequent attack of small-pox. 

 Growth is dependent essentially on the supply 

 of material to the tissues by the blood, and on 

 the amount of waste of tissue. If the supply 

 exceeds the waste, as in childhood, the body in- 

 creases in weight and power ; if the supply and 

 waste are equal, the body may remain in a station- 



ary condition for many years, as in middle life 

 and if the supply is much less than the waste, the 

 body loses weight and strength, as in old age. 



12. SECRETION. 



Secretion is that function by means of which 

 certain fluids are separated from the blood for 

 further service in the economy. Various of these 

 secretions, such as the saliva, gastric juice, pan- 

 creatic juice, &c. have been already described, 

 but here we may briefly refer to the process of 

 secretion generally. However complicated the 

 structure of the various secreting glands may be, 

 it is found, on minute examination, to consist of a 

 delicate membrane, called a basement membrane, 

 having blood-vessels richly distributed under its 

 attached surface, and actively growing cells on 

 its free surface. By foldings and reduplications 

 of these elements of structure, all secreting glands 

 are formed. The cells, however, are the active 

 agents. They select from the blood the materials 

 necessary, and form the secretion. Recent re- 

 searches have shewn that these cells are directly 

 influenced by the nervous system. The secre- 

 tion is found in their interior. They are de- 

 veloped, grow, live a certain time, drop off from 

 the membrane, and, becoming ruptured, the 

 secretion is set free. Thus secretion is not op- 

 posed to growth, as at one time supposed : it is 

 dependent on the growth of certain cells. 



13. EXCRETION. 



During the vital activity of the tissues, new par- 

 ticles are assimilated by the process of growth 

 already described. On the other hand, certain 

 materials are worn out, and becoming soluble, 

 pass into the blood. These effete matters must 

 be removed from the blood, and cast out of the 

 body, so that this important fluid may be kept in 

 a healthy condition. This process of removal is 

 the function of excretion. There are five great 

 channels of excretion : 



1. The Lungs. The lungs, as already described 

 under Respiration, separate from the blood car- 

 bonic acid and watery vapour. 



2. The Liver. This organ is the largest and 

 heaviest gland in the body, weighing, on an aver- 

 age, 65 oz. avoirdupois. It is situated on the right 

 side, beneath the lower ribs. It consists of five 

 lobes, of a dark reddish colour, and these lobes 

 are divided into lobules. The lobules are bound 

 together by areolar tissue, and their structure 

 is similar. The liver is supplied with the 

 blood from which it derives its secretions by 

 the portal vein, a vessel which collects all the 

 blood circulating in the stomach, spleen, and 

 intestines. The portal vein divides and sub- 

 divides in the liver, till it forms a plexus of minute 

 vessels between and in the lobules, from which 

 originate the radicles of the hepatic vein, a 

 vessel which carries the blood from the liver to 

 the ascending vena cava. The connective tissue 

 of the liver, and its vessels and nerves, are sup- 

 plied by a special artery, the hepatic artery. 

 The proper secreting structure of the liver con- 

 sists of numerous compressed cells, about the 

 nrWth of an inch in diameter, called hepatic 

 cells. These cells secrete materials from the 

 blood, which they elaborate into bile. This 

 secretion passes into minute ducts, the hepatic 



121 



