386 MANUAL OF PHYSIOLOGY. 



For the details of the dietetic value of milk, see Chapter V 011 



Food, p. 103. 



EXCRETIONS. 



The term excretion is commonly used to denote a gland fluid 

 the chief constituents of which are manufactured by other tissues, 

 and are of no use in the economy, but, on the contrary, require to 

 be continually removed in order that their accumulation in the 

 blood may not give rise to injurious consequences. These effete 

 matters are the outcome of the various chemical changes in the 

 tissues, whence they are always collected by the blood and carried 

 to the glands which preside over their elimination. 



The next form of cutaneous glands is commonly arranged 

 among the excretory organs, though their more important func- 

 tion, as will hereafter appear, is to supply surface moisture for 

 the purpose of regulating the temperature. 



SUDORIFEROUS GLANDS. 



The sweat glands are distributed all over the cutaneous surface, 

 but in some parts, such as the axilla, perineum, etc., they are 

 both more abundant and larger than elsewhere. They are simple 

 tubes extending in a more or less wavy manner through the skin, 

 and ending in a rounded knot formed of several coils of the tube 

 some way beneath the corium, where they are surrounded by a 

 capillary plexus. The tube is lined with glandular epithelium, 

 and its basement membrane is beset with longitudinally arranged 

 smooth muscle fibres. 



The secretion of sweat is always going on, though it does not 

 constantly appear as a moisture on the surface, because the 

 amount produced is only just equal to the amount of evapora- 

 tion that takes place. In this case it is spoken of as insensible 

 perspiration. Under certain circumstances the sweat collects on 

 the surface and becomes obvious as fluid sensible perspiration 

 which bathes the skin, being produced more rapidly than it can 

 be evaporated. The quantity of secretion necessary to become 

 sensible varies with the dryness and heat of the air, that is, with 

 the rapidity with which evaporation takes place. It happens, 

 however, that the very circumstances which tend to assist evapo- 



