Cells and Tissues. 



9 



central cavity, wherein is contained the substance separated from the 

 protoplasm {e.g., mucus), and this escapes to the exterior through an 

 aperture. Secreting cells commonly consist of a long thin neck, 

 wedged between the adjacent epithelial cells, and a wide sac-shaped 

 inner part, lying in the deeper tissues. All secreting cells derived 

 from the general epithelium are called glandular cells, for 

 any part of the animal body which forms a secretion is considered 

 to be a gland. Sometimes glandular activity is distributed over 

 a large, continuous area of the epithelium, which is usually 

 invaginated into the subjacent connective tissue ; such are true 

 glands. In its simplest form the gland is a flat patch of cells, 

 a pit, a small sac, or a longer tube ; but the tube itself may form 

 evaginationSj and these on their part may possess branches, and so 



1 nm^iiSMxn 



3 4 



Fig. 9. Diagrams of different true glands. The secreting cells are dotted. 1 The 

 ' simplest form ; the secreting cells are not invaginated. 2 — 5 Different kinds of simple 

 glands. 6 Branched tabular gland. 7 Eaoemose gland. — Orig. 



on ; SO that a compound gland is made up of a complex system of 

 canals (the walls of which consist of a single layer of epithelium), 

 held together and supported by connective tissue [see below). In 

 these large glands, the ends of the branches only secrete, whilst the 

 rest of the system serves as a reservoir and as a means of exit. 

 Such a distinction into secreting portion and duct, may be 

 noticed even in very simple glands. Sometimes the ends of the 

 canals exhibit a rounded enlargement, when the gland is called 

 racemose, to distinguish it from the tubular gland, which is 

 without this expansion. 



Tor the development of sensoiT' epithelium, see " Sense-organs." 

 2. Skeletal tissue is characterized by the great prominence 

 of the intercellular substance. In an early stage of develop- 

 ment it consists, like epithelia, of cells only; but later, the cells, 

 though they always remain simple and undifferentiated themselves, 

 secrete some form of intercellular substance which constitutes the 

 chief part of the hard structures, and gives them their great 

 importance in the animal body. Of skeletal tissues, connective 



