142 



NORMAL HISTOLOGY. 



FIG. 171. 



mucosa to break up into capillaries which enclose the tubules and 

 acini in close net-works, lying outside but in intimate relation with 

 the basement-membrane, an arrangement favoring the passage of 

 substances from the blood into the protoplasm of the secreting cells, 

 which are thus placed between the blood-current on the one hand 

 and the lumen of the gland on the other. 



Numerous lymphatic spaces are contained within the connective 

 tissue surrounding the acini and the tubules, some of the clefts being 

 immediately beneath the membrana propria and in close relation with 

 the gland. 



The nerve-supply of glandular structures is often very rich. The 

 nerve-trunks accompany the larger blood-vessels in the submucous 

 tissue and give off numerous small bundles which follow the smaller 

 arteries in their distribution to the mucosa, where they form delicate 

 plexuses about the acini and the tubules immediately outside the 

 basement-membrane. The exact mode of the final termination of 

 the nerves and their relation to the individual secreting cells are still 

 matters for investigation ; whether the fibres pierce the basement- 

 membrane to terminate among the glandular epi- 

 thelium, while probable, must be regarded as still 

 unproved. 



The development of glands proceeds from 

 the epithelial tissue of the young mucous mem- 

 brane, the penetrating cylinder of epithelium rep- 

 resenting ectodermic or entodermic tissue, except 

 in those cases where the glands are formed in 

 connection with the parts of the genito-urinary 

 tract derived entirely from the mesoderm. 



The first trace of the glands consists of a 

 cylindrical ingrowth of the epithelium into 

 the subjacent mesodermic tissue, both the tubular 

 and the saccular glands alike starting as simple 

 epithelial processes. Where, however, the struct- 

 ure is destined to become a gland of the racemose 

 type, the branching cords of epithelial elements 

 early indicate the nature of the future gland as 

 distinguished from one of the compound tubular 

 variety ; since, in this case, the terminations of 

 the epithelial masses soon become markedly ex- 

 panded and club-shaped, from which dilatations 

 the ultimate divisions or primary alveoli of the racemose glands are 

 extended secondarily. The epithelial cords, at first solid, later 

 acquire a lumen which extends as far as the terminal compartments 

 of the gland. Sometimes, as conspicuously instanced by the liver, 



Developing salivary 

 gland from fifteen-day 

 rabbit embryo. The 

 ectodermic ingrowth has 

 divided into secondary 

 branches which termi- 

 nate in slightly expanded 

 club - shaped ends : e, 

 epithelium of oral sur- 

 face ; m, young connec- 

 tive tissue of future tunica 

 propria into which the 

 epithelium grows. 



