SECRETION. 



461 



The proof that the constituents of the milk 

 pre-exist in the blood, is rather inferential 

 than direct. That the caseine (although so 

 like the albumen of the blood that we might 

 imagine it to be a mere modification of it, 

 effected in the act of secretion) is, in reality, 

 specially prepared in the circulating current, 

 would appear from the fact that, during preg- 

 nancy, a substance, Jciestein, having a close 

 relation to it, is eliminated by the urine, and 

 that this substance disappears from the urine 

 within a few days after parturition, the mam- 

 mary secretion being then fairly established. 

 Perhaps, however, the most remarkable evi- 

 dence to the same effect is afforded by cases 

 of metastasis of the mammary secretion, of 

 which an account will be presently given ; 

 and on the same kind of evidence rests the 

 proof of the pre-existence of the other cha- 

 racteristic elements of the mammary secretion 

 in the blood. 



With regard, however, to the elements of 

 other secretions, the evidence is less clear as 

 to the state in which they exist previously to 

 their elimination by the secreting apparatus. 

 The fact would appear to be, however, that 

 the solid constituents of most of them are 

 little else than constituents of the blood 

 itself, either pure or but slightly altered. 

 Thus in the lachrymal fluid, the saliva, 

 the gastric and pancreatic juices, and the 

 serous fluid of areolar tissue and of serous 

 and synovial membranes, we find little else 

 than saline matters, which are normal consti- 

 tuents of the serum of the blood, with one or 

 more organic compounds, that seem like 

 albumen in a state of change. The repres- 

 sion of these secretions does not produce any 

 deleterious effect upon the general system, 

 otherwise than as impairing or preventing the 

 performance of the function to which they 

 are subservient ; whence it may be inferred, 

 that the selection of the secreted products 

 from the blood is made in these cases, not for 

 the sake of purifying the circulating fluid from 

 any matter that would be noxious if retained, 

 bat merely for some minor purposes in the 

 economy, to which these simple fluids are 

 adequate. 



Metastasis of secretion. Although the 

 number and variety of the secretions become 

 greater in proportion to the increased com- 

 plexity of the nutritive processes in the higher 

 classes, and although each appears as if it 

 could be formed by its own organ alone, yet 

 we may observe, even in the highest animals, 

 some traces of the community of function 

 which characterises the general surface of the 

 lowest. It has been shown that, although 

 the products of secretion are so different, the 

 elementary structure of all glands is the same ; 

 that wherever there is afree secreting surface it 

 may be regarded as an extension of the gene- 

 ral envelope of the body, or of the reflexion 

 of it which lines the digestive cavity ; that 

 its epithelium is continuous with the epider- 

 mis of the integument, or with the epithelium 

 of the mucous membrane from which it is 

 prolonged ; and that the peculiar principles of 



the secreted products pre-exist in the blood, 

 in a form at least closely allied to that which 

 they assume after their separation. Now, it 

 may be stated as a general law in physiology, 

 that in cases where the different functions are 

 highly specialised (that is, where every one has 

 its special and distinct organ for its own pur- 

 pose alone), the general structure retains, more 

 or less, the primitive community of function which 

 characterised it in the lowest grade of develop- 

 ment* Thus, although the functions of 

 absorption and respiration have special organs 

 provided for them in the higher animals, they 

 are not altogether restricted to these, but 

 may be performed in part by the general sur- 

 face, which (although the special organ for 

 exhalation) permits the passage of fluid into 

 the interior of the system, and allows the in- 

 terchange of gases between the blood and the 

 air. In the same manner, we find that the 

 functions of secretion being equally performed 

 in the lowest animals by the whole surface, 

 whilst in the highest there is a complicated 

 apparatus of glandular organs, to each of 

 which some special division of the function is 

 assigned, either the general muco-cutaneous 

 surface, or some one of its subdivisions or 

 prolongations, is able to take on in some 

 degree the function of another gland whose 

 functions may be suspended. This truth was 

 well known to Haller, who asserted that 

 almost all secretions may, under the influence 

 of disease, be formed by each and every 

 secreting organ.f This statement, however, 

 needs to be received with some limitation, 

 and it would be probably safest to restrict it 

 to the excretions, whose elements pre-exist in 

 the blood, and accumulate there when the 

 elimination of them by their natural channel 

 is suspended. We shall now consider some 

 of the more remarkable examples of the me- 

 tastasis of secretion. 



It seems to be established by a great mass 

 of observations, that urine, or a fluid present- 

 ing its essential characters, may pass off 

 by the mucous membrane of the intestinal 

 canal, by the salivary, lachrymal, and mam- ' 

 mary glands, by the testes, by the ears, nose, 

 and navel, by parts of the ordinary cutaneous 

 surface, and even by serous membranes, such 

 as the arachnoid lining the ventricles of the 

 brain, the pleura, and the peritoneum. A 

 considerable number of such cases was col- 

 lected by Haller ij: : many more were brought 

 together by Nysten $ ; more recently Bur- 

 dach has furnished a full summary of the most 

 important phenomena of the kind || ; and Dr. 

 Laycock has compiled a valuable summary of 

 cases of urinary metastasis occurring as com- 

 plications of hysteria.^) The following table 



* See the author's " Principles of General and 

 Comparative Physiology," 2d ed, 243. 



f Elementa Physiologise, torn. ii. p. 369. 



+ Ibid. p. 370. 



Recherches de Physiologic et de Chimie patho- 

 logique, p. 265. 



|| Traite de Physiologic (Jourdan's Translation), 

 vol. viii. p. 248, et seq. 



f Edinb. Med. and Surg. Journ. 1838. 



