576 OP SECRETION AND EXCRETION. 



Some of them, also, have a tendency to assume a crystalline form ; which is 

 considered by Dr. Prout to indicate their unfitness to enter into the composition 

 of organized tissues. With regard to some of the chief of these, there is suffi- 

 cient evidence of their existence, in small quantity, in the circulating Blood ; but 

 it is also clear, that they exist there as products of decomposition, and that they 

 are destined to be separated from it as speedily as possible. If their separation 

 be prevented, they accumulate, and communicate to the circulating fluid a posi- 

 tively deleterious character. Of this we have already seen a striking example 

 in the case of Asphyxia ( 574); and the history of the other two principal 

 excretions, the 'Bile and Urine, will furnish evidence to the same effect. As 

 a general fact, then, it may be stated that the materials of the Secretions pre- 

 exist in the Blood, in a state nearly resembling that in which they are thrown 

 off by the secreting organs ; but that the materials of those secretions which are 

 destined to perform some particular function within the economy, are derived 

 from the substances which are appropriated to its general purposes; whilst those 

 of the excretions are the result of the destructive changes that have taken place 

 in the system, and cannot be retained in it without injury. 



619. The composition and uses of the principal Secretions which are elabo- 

 rated for special purposes within the economy, have already been partly described 

 in connection with the functions to which they respectively minister ; and the 

 remainder will hereafter come under notice in the same manner. It is here in- 

 tended, however, to consider that important system of Excretory operations 

 which serves to maintain the purity of the circulating fluid ; by removing from 

 it those products of the disintegration of the tissues which are not capable of 

 serving any purpose in the nutrition of the system, and which even act upon it 

 as poisons ; and also by withdrawing the products (apparently of a similar cha- 

 racter) of the decomposition of those surplus alimentary materials which, not 

 being required for the nutrition of the tissues undergo retrograde metamorphosis 

 without having ever undergone the process of organization. The process of 

 Respiration, as already pointed out ( 535), is in part to be regarded as one of 

 an excretory character, though the peculiar manner in which it ministers to the 

 removal of carbon and hydrogen from the system, and its subserviency to other 

 purposes, necessitate its separate consideration. The true Secreting processes 

 which are to be regarded as more or less completely excretory, are the separation 

 of bile by the Liver, that of urine by the Kidneys, that of perspiration by the 

 Skin, and that of fecal matter by the glandulae of the Intestinal surfuce. The 

 sum total of these, with the addition of the carbonic acid and watery vapor 

 poured from the Lungs, and of the indigestible matter rejected in the form of 

 feces, must be equal to the total amount of the solid and fluid ingesta, and of 

 the oxygen which disappears from the inspired air; the weight of the body 

 remaining the same. The experiments of Dr. Dalton on his own person gave 

 the following as the proportional quantities discharged through the principal 

 channels of excretion. 1 The mean quantity of solid and liquid Aliment taken 

 into the system daily (during 14 days in spring) being 91 oz., or about 5f Ibs., 

 the average amount of Feces (including part of the solid matter of the bile) was 

 5 oz. ; the average amount of Urine was 48 oz. daily ; and, as the total weight 

 of the body remained the same, the quantity of fluid and solid matter excreted 

 by the Skin and the Lungs must have been 37 \ oz. At other periods of the 

 year, a variation was observed ; especially in the relative amount of fluid passing 

 off by the Urine, and by Cutaneous exhalation. A more elaborate series of 

 researches, however, has been recently made by M. Barral ; a part of whose 

 results has been given in the preceding chapter. The following are the average 

 daily amounts of the several components of the food consumed, and of the 



" Edinburgh New Philosophical Journal," 1832, 1833. 



