THE DIGESTIVE SYSTEM 
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gland, pancreas, or liver is formed. Such an organ may retain 
its connection with the epithelium from which it was derived, 
through a more or less elongated duct, the lumen of which on the 
one hand leads from the lumina of the various tubules or alveoli 
of the gland, and on the other hand opens into the lumen of the 
alimentary canal. The extensive and repeated branching of a 
gland gives rise to lobes which are thus composed either of masses 
of tubules convoluted for greater compactness, or of branched 
tubules ending in alveolar expansions (acini), or of clusters of 
alveoli, according to the nature of the gland. Each ultimate 
alveolus or tubule will thus communicate with the intralobular 
duct, and these in turn will lead into interlobular ducts which lead 
finally into the common duct of the gland. 
Salivary Gland (compound tubulo-alveolar). 1 —Review the 
general external form and appearance of a salivary gland with its 
many lobes and its discharging duct, noting its investment of 
connective tissue which holds the lobes together and conveys the 
blood vessels and nerves to the gland tissue. 
Study microscopically, sections of a salivary gland. Note 
the division into lobes, each exhibiting a compact mass of sections 
through the tubules and alveoli, among which may be seen 
occasional sections through the intra lobular ducts and their 
branches. Observe that the parts are held together by loose 
connective tissue, which is the interstitial tissue as distinguished 
from the parenchyma or true glandular tissue. Study under 
high power for details of structure as are peculiar to the gland in 
question, such as the form of lumen, the shape, size and arrange¬ 
ment of cells, nature of nucleus and cytoplasm, in the case both 
of the alveoli and of the ducts, and the distribution in the tubules 
and acini of serous and mucous cells. Note the presence of the 
zymogen granules in the serous cells, ready to be poured out as 
the liquid secretion of the gland. Draw such details as you see 
clearly and understand. 
Liver (modified type of tubular gland).—Examine macroscopi- 
cally pieces of pig liver hardened in alcohol or formalin, make fresh 
sections of it in various directions and note that it is a compact 
1 Sections of the pancreas may be substituted for this study, as a type of tubulo- 
alveolar gland. 
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