616 OF SECRETION. 



case of Asphyxia ( 779) ; and the history of the other two principal Excre- 

 tions, 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 

 oft' by the secreting organs: but that the materials of those secretions, which 

 are only destined to perform some particular function in the economy, are 

 derived from the substances which are appropriated to its general 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. 



821. 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 

 other account can be given, than that which refers them to the special endow- 

 ments of the cells, which are the real instruments of the process. When the 

 ultimate structure of Glands 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. Of Mucous membrane 

 ( 178), the structure is in some instances almost equally simple ; but in gene- 

 ral the secreting surface is extended, by the inversion of the membrane, into 

 a large number of little open sacs or follicles (Fig. 45), which are lined with 

 epithelium-cells, and copiously supplied with blood-vessels, and which are 

 equally concerned with the external superficies, in the elaboration of the pro- 

 tective secretion 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 clus- 

 tered 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 modifica- 

 tion 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 ever^ gland may be found under a variety of forms, 

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

 other, is far simpler in the lower classes of Animals than in the higher; the 

 number of effete compounds, to be excreted from the circulating fluid, is much 

 smaller; and the variety of purposes, for which special secretions are required, 

 is much less. Hence, for almost every Gland, there is a part of the Animal 

 scale below which it does not exist ; and when it mal>es its first appearance, 

 it almost invariably presents a character nearly as simple, as that of the least 

 complex glanular structures in the higher animals. Thus the Pancreas in 

 Fishes (Fig. 257), the Mammary Gland in the Ornithorhyncus (Fig. 222), the 



Salivary glands in the Echinodermata, 



Fig. 222. and the Urinary organs of Insects, are 



nothing more than follicles more or less 

 extended, and having separate orifices. 

 Again, in Insects, we find that all the 

 glands, the Liver and Salivary glands, 

 as well as the Kidneys and Testes, 

 have the form of prolonged tubes ; 

 whilst in Mollusca, all the secret- 

 Mammary Gland of Ornithorhyncus. ing organs, the Urinary and Genital, 



as well as the Biliary and Salivary, 



consist of multiplied vesicles connected with a ramifying duct. Moreover, it 

 is a well-ascertained fact that, even in the highest organisms, the functions of 

 Glandular structures (especially of those concerned in Excretion) are to a 



