378 MANUAL OF PHYSIOLOGY. 



The secretion is produced continuously in small amount, but is 

 subject to such considerable and sudden increase, that at times it 

 cannot all escape by the nasal duct, but is accumulated in the eyes, 

 until it overflows to the cheek as tears. This excessive secretion 

 may be induced by the application of stimuli to the conjunctiva, 

 the lining membrane of the nose, or the skin of the face, or by 

 strong stimulation of the retina, as when one looks at the sun. A 

 similar increase of secretion follows certain emotional states con- 

 sequent on grief or joy. These facts show that the secretion of the 

 gland is under nervous control, the impulses stimulating secretion 

 commonly starting either from the periphery, and passing along 

 the sensory branches of the fifth or along the optic nerve, or 

 from the emotional centres in the brain, and arriving at the 

 gland in a reflex manner. The amount of secretion can also 

 be augmented by direct stimulation of the lachrymal nerves, 

 so that in all probability these are the efferent channels for the 

 impulse. 



Mucous GLANDS. 



In connection with mouth and stomach secretions, mention has 

 been made of glands which are elongated saccules lined with re- 

 fracting cells (Fig. 165). They are distributed over all mucous 

 membranes, and are the chief source of the thick, tenacious, clear, 

 alkaline, and tasteless fluid called mucus. 



This material contains about 5 per cent, of solid matters, of which 

 the chief is mucin, the characteristic material of mucus, which 

 swells up in water and gives the peculiar tenacity to the fluid. 

 It is precipitated by weak mineral and acetic acids ; and, as the 

 precipitate with the latter does not redissolve in an excess, this 

 acid becomes a good test to distinguish it from its chemical allies. 

 Mucin is not precipitated by boiling. Mucus also contains traces 

 of fat and albumin, and inorganic salts, viz., sodium chloride, 

 phosphates and sulphates, and traces of iron. 



The fluid is secreted either by the special mucous glands, or it 

 may be produced by the epithelium of the mucous surfaces. 

 The cells produce in their protoplasm a quantity of the secretion, 

 which may often be seen to swell them out to a considerable 

 extent. This clear fluid is then expelled, and the altered cells 



