OF SECRETION IN GENERAL. 509 



constituents of the Blood, either pure, or but slightly altered ; thus, in the 

 Lachrymal fluid, the Saliva, the Pancreatic juice, the Serous fluid of areolar 

 tissue and of serous and synovial membranes, we 'find little else than Albu- 

 minous and Saline matter, derived at once from the blood. The Casein, 

 which is the most characteristic ingredient of milk ( 680), is but a slightly 

 altered form of Albumen ; and some curious evidence has recently been ob- 

 tained, that this alteration commences in the. Blood, and goes on during 

 pregnancy as a preparation for lactation-.* On the other hand, the character- 

 istic ingredients of the Excretions are very different in character from the 

 normal elements of the Blood. They are all of them completely unorgani- 

 zable ; and they possess, for the most part, a simple atomic constitution. Some 

 of them, also, have a tendency to assume a crystalline form, which is consi- 

 dered 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 sufficient 

 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 separa- 

 tion be prevented, they accumulate, and communicate to the circulating fluid 

 a positively deleterious character. .Of this, we have already seen a striking- 

 example in the case of Asphyxia ( 546) ; 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 Se- 

 cretions 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 secre- 

 tions, which are only destined to perform some particular function in the 

 economy, are derived from the substances which are appropriated to its gene- 

 ral purposes ; whilst those of the excretions are the result of the changes that 

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

 650. Of the reason why certain compounds forming part of the circulating 

 Blood, are separated from it by one organ, and others by a different one, no- 

 thing whatever is known ; arid there is nothing in the evident structure of 

 Glands that can afford any clue to the attainment of such knowledge. When 

 their ultimate structure is considered, it is found to be neither more nor less 

 than a vascular membrane, covered with epithelium-cells, and made up into 

 various forms for convenience of packing. Of such a membrane, in its most 

 expanded state, that which composes the walls of the Serous cavities, or of the 

 Synovial capsules, affords a good example ; and the fluid which these cavities 

 contain is secreted by it. Of Mucous membrane, the structure is in some in- 

 stances almost equally simple ; but in general the secreting surface is extended 

 by the inversion of the membrane into a large number of little open sacs or folli- 

 cles, which are copiously supplied with blood-vessels, and are equally con- 

 cerned with the external superficies, in the elaboration of the protective secre- 

 tion that covers these membranes. In the most complex form of gland, we 

 find nothing but a very obvious modification of this structure. Either the 

 sacs are prolonged into coeca or blind tubes, as is the case in the Human 

 Kidney or Testis ; or they are very greatly 'multiplied, and are clustered 

 together (just like currants upon a stalk) upon efferent ducts common to 

 several of them, as is seen in the Parotid. Now, that the particular modi- 

 fication of structure, which the Gland may present, has no essential connection 

 with the character of the Secretion it is destined to form, is evident from this 

 circumstance, that almost every gland may be found under a variety of forms, 

 in different parts of the Animal series. The Secreting system, like every 



*See Dr. G. Bird, in Guy's Hospital Reports, vol. v. 

 43* 



