NATURE AND CONDITIONS OP THE RESPIRATORY PROCESS. 35T 



eases, notwithstanding that the general habits of the people would seem 

 peculiarly favourable to their production. 



640. There is another remarkable class of diseases, resulting from a 

 disordered condition of the nutritive processes ; those, namely of a 

 malignant nature. We not unfrequently meet with abnormal growths 

 of a fatty, cartilaginous, fibrous, or bony structure ; which appear to 

 originate in some perverted action of the part itself, and which have 

 little tendency to reappear in the same part, when they have been 

 removed, still less, to reappear in distant parts. But the various 

 forms of Malignant or Cancerous disease are peculiar in this, that 

 they are composed of cells, sometimes of a globular form (see Fig. 18), 

 sometimes elongated or spindle-shaped, having a power of rapid multi- 

 plication, and not capable of changing into any kind of normal tissue. 

 When a truly cancerous growth has once established itself in any part 

 of the body, it may increase to an unlimited extent, obtaining its 

 nourishment from the blood-vessels in its neighbourhood, and destroy- 

 ing the surrounding parts by its pressure, as well as by drawing-off 

 their supply of aliment. When it has developed itself to a consi- 

 derable degree in one part, it is very liable to make its appearance 

 in others, especially when the original growth has been removed ; and 

 hence the judicious surgeon is disinclined to remove a Cancerous 

 growth of any but the most limited kind ; knowing that the disease is 

 almost certain to reappear. There is a strong analogy between such 

 Cancerous growths, and the low forms of Fungoid Vegetation, which 

 develope themselves in the interior of the higher Plants, and even in 

 Animal bodies ; and in both cases, the disease may be propagated by 

 inoculation from one individual to another. But still it appears pro- 

 bable that Cancerous disease, like tubercular, is of constitutional origin; 

 and the peculiar tissue which characterizes it, is perhaps to be regarded 

 simply as the manifestation of the presence of a morbid matter in the 

 blood, which is thus removed from the circulating current ; just as fatty 

 matter is removed by an increased formation of Adipose tissue, or as 

 the elements of the excretions are eliminated by an increased growth of 

 the gland- cells of which they are the appropriate pabulum. 



CHAPTER VIII. 



OF RESPIRATION. 



1. Essential Nature and Conditions of the Respiratory Process. 



641. THE function of Respiration essentially consists in an inter- 

 change of oxygen and carbonic acid, between the blood of the Animal 

 and the surrounding medium ; carbonic acid being given out by the 

 blood, and oxygen entering in its stead. It has been already noticed 

 ( 84) that this function is performed likewise by Plants ; although, 

 in consequence of their deriving a large part of their food from the 



