732 SPECIAL PHYSIOLOGY. 



between the blood and the tissues and glands, its renovation from the 

 lymph and chyle, and the rapidity of its purification from the poisonous 

 or injurious chemical products of the disintegration of tissue, by the 

 excretory processes, are very surprising. When imperfectly elabo- 

 rated, or purified, by the formative, nutritive, and secretory or excretory 

 processes, it becomes unhealthy, and a possible source of disease. 

 Emotional and other disturbances of the nervous centres may, through 

 their influence over these processes, also render the blood 'unhealthy 

 or even poisonous. General disorder ensues, and the functions, espe- 

 cially those of the liver, alimentary mucous membranes, kidneys, skin, 

 and mammary glands, are vitiated. Cutaneous and other local dis- 

 eases arise. Further, the blood may become the vehicle of miasmatic 

 and malarious poisons, or the seat of zymotic decompositions, and so 

 fevers, simple, exanthematous, intermittent or remittent, typhoid or 

 choleraic, may ensue. / 



THE BLOOD GLANDS. 



In Nutrition, certain materials are attracted to, and assimilated by 

 the tissues, from the common nutrient plasma of the blood, and the 

 materials so attracted, are removed from the blood. In the act of 

 secretion, as for example, in that of saliva and bile by the salivary 

 glands and the liver, various other materials are separated from the 

 blood. Nutrition arid secretion are, indeed, intimately allied, the 

 former being a secretive process, and the latter a nutritive process ; 

 hence nutrition is sometimes termed nutrient secretion. Diminished or 

 increased activity or arrest of the nutritive processes in certain tis- 

 sues, such as the nervous or muscular systems, may affect the blood 

 quite as seriously as errors in the secreting processes ; and the healthy 

 balance of both functions is necessary for the preservation of the 

 normal constitution of the blood. 



All secreting glands, however, possess special channels, called ducts, 

 which open either upon the exterior of the body, as in the case of the 

 cutaneous and mammary glands, or into some internal cavity, as, e. </., 

 the salivary and gastric glands, and by which the materials separated 

 from the blood are conveyed away, though some of them may be more 

 or less completely reabsorbed. But in the nutrition of tissues, such 

 as muscle or nerve, the materials separated from the blood are not 

 carried away by ducts, but remain, for a time, as part of the body, 

 and are only reabsorbed when they have performed their proper func- 

 tions, and, in doing so, have undergone further change. 



Now, there exist in the Vertebrata generally, and in Man, certain 

 peculiar organs, which, from their compact form, general appearance 

 and relations, and highly vascular character, have been called glands ; 

 but they have no secreting orifices, channels, or ducts proceeding from 

 them, to open on the surface, or into the cavities, of the body. These 

 organs include the spleen, the supra-renal bodies, the anterior portion 

 of the pituitary body, the thyroid body, and the thymus. From being 

 destitute of ducts, they are named the ductless glands ; from their ob- 

 vious connection with the process of sanguification, they are called 



