128 



NORMAL HISTOLOGY. 



FIG. 168. Portion of submaxillary gland 

 of rabbit, showing distribution of nerves 

 to the alveoli. X 290. (Retzius.) 



cortex of the suprarenal body, a basement membrane is wanting, a delicate 

 reticulum and the wall of the vessel alone intervening between the blood- 

 stream and the protoplasm of the cells. Although subject to local deviations, 

 as in the liver, the veins follow the general course of the arteries, the larger 



blood-vessels, together with the duct-tubes, 

 the lymphatics and the nerves, occupying 

 the tracts of connective tissue between the 

 lobes and the lobules. The lymphatics 

 are represented by the trunks which accom- 

 pany the ducts within the interlobular 

 tissue. Within the lobules the lymph- 

 channels become less definite until, finally, 

 they are recognizable only as the lymph- 

 spaces between the bundles of connective 

 tissue separating the alveoli. 



The nerves distributed to the larger 

 glands include both medullated and non- 

 medullated fibres which follow the arteries 

 and ducts, around which they form plexuses. 

 Along these strands sympathetic ganglion- 

 cells occur, sometimes singly but more often 

 grouped as microscopic ganglia, from which 

 sympathetic fibres proceed to the muscle of the blood-vessels and ducts and 

 to the alveoli. Upon reaching the latter, the nonmedullated fibres break up 

 into end-plexuses surrounding the alveoli; the ultimate distribution includes 

 epilcmmar and hypolemmar ji brillcz, the former lying upon and the latter 

 beneath the basement membrane. The hypolemmar fibrillae, derived from 

 the extra-alveolar plexus, pass through the basement membrane and end in 

 fine varicose threads between the gland-cells. 



Development. Since glands are only ex- 

 tensions of the mucous membrane or integument 

 upon which they open, their development begins 

 as an outgrowth or budding from the epithelium 

 covering such surfaces. In the simple tubular 

 glands the minute cylinders are closely placed and 

 composed of densely packed cells. In the case 

 of the larger compound glands, as the salivary 

 or pancreas, the first anlage consists of a solid 

 cylindrical plug which, penetrating into the meso- 

 derm, soon begins to branch. The ends of the 

 terminal divisions enlarge and eventually become 

 the alveoli. Meanwhile the surrounding meso- 

 blast undergoes condensation and forms the 

 interlobular and other septa, as well as the general 

 envelope, or capsule, thereby giving definite form 

 to the glandular aggregation. The vascular and 

 other structures usually found within the inter- 

 lobular tissue are secondary and later formations. 

 The development of the gland involves a double process of active growth, 

 the extension of the epithelial processes and a coincident subdivision of the 

 latter by the mesoderm to form the units of the organ. The lumen of the 

 gland appears first in the main excretory duct, from which it extends into 

 the secondary tubes and, finally, into the alveoli. 



Duct, 

 with 

 lumen 



Alveoli, 

 still solid 



FIG. 169. Section of foetal oral 

 mucous membrane, showing devel- 

 oping tubo-alveolar gland. X 50. 



