MUCOUS SOFTENING. 37 



It is still doubtful whether we are justified in calling them 

 ^^ solutions;" some authors wholly deny the solubility of mucus 

 in the physical sense of the term. So much is certain, that 

 mucus is a member of the colloid group of Graham* and that 

 among colloids it takes a leading place. Mucus possesses hardly 

 any diffusive power ; this property has important bearings on its 

 development in the organs generally, and it is of especial moment 

 Avith regard to softening. We may assert, from an a priori con- 

 sideration of its colloid nature, that mucus, as such, can never 

 pass from the blood-vessels into the tissues, or from the tissues 

 into the blood-vessels. The homogeneous wall of the capillaries 

 would not allow it to pass in either direction. 



Now, as we never find any trace of this substance in the 

 blood, we are justified in assuming that mucus is invariably a 

 local product, and that it will remain where it is first formed 

 until it is either removed mechanically, or transformed into some 

 material capable of being absorbed. 



This incapacity for reabsorption is of little moment when the 

 development of mucus takes place on a free surface. To this 

 category belong not only the physiological secretions of mucous 

 membranes, in which the epithelial cells act as mucus-secreting 

 organs, but also some of those processes which come under the 

 head of softening, e.g. the mucous metamorphosis of fibrinous 

 false membranes, and other fibrinous deposits occurring in the 

 course of inflammatory disorders of the respiratory tract. But 

 the incapacity for reabsorption displayed by mucus gives rise to 

 marked and characteristic morphological effects when mucus is 

 developed in the interior of the body, whether in a closed ca^'ity 

 or as a substitute for tissues which have undergone destruction, 

 as in the case of the softening which we are now considering. 

 In such cases it qualifies mucus for the fulfilment of a task which 

 is performed in a higher degree only by " colloid " matter. It 

 enables it, namely, while itself amorphous, to form an integral 

 constituent even of permanent textures, serving to fill up all in- 

 terstices with its semi-fluid substance. 



* " Annalen der Chemie u. Pharmacie," Bd. 121, s. 1. Graham draws 

 attention to the difference in diffusive capacity between colloids and 

 crystalloids. The crystalloids diffuse rapidly and with case ; the colloids 

 little or not at all. To the latter category belong gum, starch, dextrin, 

 'mucus, albumen, and gelatin {Ldmstoffe). 



