THE SKIX. cci 



the mucous membrane which opens on the surface, and which may be 

 simple, as in certain glands at the root of the tongue and in the pharynx, 

 or complex and multilocular, as in the tonsils. 



On the hypothesis that these bodies are really secreting glands, it has been pre- 

 sumed that they are occasionally opened by dehiscence for the discharge of their 

 contents. According to another view they are dependencies of the lymphatic system, 

 and there are various analogies and indications of relationship which might be 

 adduced in favour of this opinion. On the other hand, it is not easy to see what 

 special connection there can be between the lymphatic system and the collections 

 of these bodies at the root of the tongue and in the tonsils, where, indeed, their 

 presence is more reconcileable with the notion of their being secreting organs ; in 

 short, it must be confessed that the question as to their function has still to be 

 answered. 



Nerves. The mucous membranes are supplied with nerves, and endowed 

 with sensibility ; but the proportion of nerves which they receive, as well 

 as the degree of sensibility which they possess, differs very greatly in 

 different parts. As to the mode of distribution and termination of their 

 nerves, there is nothing to be said beyond what has been already stated in 

 treating of the nerves in general. 



Secretion. Mucus is a more or less viscid, transparent, or slightly turbid fluid, of 

 variable consistency. It is somewhat heavier than water, though expectorated mucus 

 is generally prevented from sinking in that liquid by entangled air-bubbles. Examined 

 with the microscope, it is found to consist of a fluid, containing solid particles of 

 various kinds, viz., 1. Epithelium-particles detached by desquamation ; 2. Mucus- 

 corpuscles, which are bodies resembling much the pale corpuscles of the blood ; 

 3. Granules and molecules occasionally. The viscidity of mucus depends on the 

 liquid part, which contains a peculiar substance, named by the chemists mucin. 

 This ingredient is precipitated and the mucus rendered turbid by the addition of 

 water or a weak acid, but it may be partly redissolved in an excess of water, and 

 completely so in a strong acid. This mucin is soluble in alkalies, and its acid 

 solutions are not precipitated by ferrocyanide of potassium. Little can, of course, 

 be expected from a chemical analysis of a heterogeneous and inseparable mixture of 

 solid particles with a liquid solution, such as we find in mucus, which is, moreover, 

 subject to differences of quality according to the part of the mucous membrane whence 

 it is derived. Examined thus in the gross, however, the nasal mucus has been found 

 to yield water, mucin, alcohol-extract with alkaline lactates, water-extract with traces 

 of albumen and a phosphate, chlorides of sodium and potassium, and soda. Fat has 

 been obtained by analysis of pulmonary mucus, reputed healthy. 



Regeneration. The reparatory process is active in the mucous membranes. 

 Breaches of continuity occasioned by sloughing, ulceration, or other causes, readily 

 heal. The steps of the process have been examined with most care in the healing of 

 ulcers of the large intestine, and in such cases it has been found that the resulting 

 cicatrix becomes covered with epithelium, but that the tubular follicles are not re- 

 produced. 



THE SKIN. 



The skin consists of the cutis vera or corium, and the cuticle or epidermis. 



The epidermis, cuticle, or scarf-skin, belongs to the class of epithelial struc- 

 tures, the general nature of which has been already considered. It forms a 

 protective covering over every part of the true skin, and is itself quite in- 

 sensible and non-vascular. The thickness of the cuticle varies in different 

 parts of the surface, measuring in some not more than -g^th, and in others 

 from ^jth to -j^th of an inch. It is thickest in the palms of the hands and 

 soles of the feet, where the skin is much exposed to pressure, and it is not 



