GLANDS. 



stroma and of the blood-vessels. Surfaces highly endowed with general and tactile 

 sensibility are provided with a generous supply of twigs containing medullated fibres. 

 As the latter pass towards their ultimate destination (for convenience assuming that 

 all are peripherally directed) they lose their medullated character and, as naked axis- 

 cylinders, form the siibepithelial plexuses, from which delicate filaments pass into the 

 papillae, where they terminate either as free club-shaped or special sensory endings. 

 It is probable that in places the nerves penetrate between the epithelial cells forming 

 the layers next the basement epithelium and terminate in varicose free endings. 



GLANDS. 



Certain of the epithelial cells lining the mucous membranes of the body become 

 modified to assume the role of secretion-forming organs or glands, the products of 

 which are poured out upon the free surface and keep the latter moist. The latter 

 purpose is secondary in the case of many important glands, as the parotid, pancreas, 



FIG. 1285. 



Diagram showing types of glands, a-e, tubular; f-i, alveolar or saccular. a, simple; i>, coiled; c-d, increasingly 

 complex compound tubular; e, tubo-aiveolar ; f, simple ; g-k-i, progressively complex compound alveolar. 



or liver, since these organs supply special secretions for particular ends. Aggrega- 

 tions of the secreting elements vary greatly in size, form, and arrangement, as well 

 as in the character of their products. 



The simplest type is the unicellular gland found in the lower forms; in principle 

 this is represented in man and the higher animals by the goblet-cells seen in pro- 

 fusion in mucous membranes covered with columnar epithelium. The secretion 

 poured out by these goblet-cells serves to protect and lubricate the surface of the 

 mucous membranes in which they occur. The term ' ' gland, ' ' however, usually 

 implies a more highly developed organ composed of a collection of secreting epithe- 

 lial elements. 



Glands are classified according to their form into two chief groups, the tubular 

 and the alveolar, each of which occurs as simple or compound. It should be empha- 

 sized that in many instances no sharp distinction between these conventional groups 



