TISSUES. 57 



5. They stain deeply with nuclear stains. 



6. Their chief function is to secrete. 



7. The absence of blood- and lymph- vessels. 



8. The presence of free nerve endings. While this 

 is of no diagnostic microscopic value, it is physiolog- 

 ically an important relation to bear in mind. In 

 wounds and old sores the epithelial border is the 

 most sensitive part and should be carefully manipu- 

 lated to avoid inducing pain. 



Glands. Much literature has been contributed 

 the last years relative to the proper conception as 

 to what constitutes a gland. The prevailing opin- 

 ion seems to be that any structure which secretes or 

 puts out a product that is not used directly in the 

 metabolism of the body should be called a gland. 

 If the fluid is a waste, the product is an excretion ; 

 if it has a utility, it is a secretion. Accordingly, 

 mucous and synovial and serous membranes are 

 glandular structures as well as the liver, the pan- 

 creas, or the kidney. Furthermore, the simplest 

 form of a gland is a single secreting cell situated 

 apart by itself, and such unicellular glands are quite 

 common in invertebrates and are represented in man 

 by the goblet cells found in mucous membranes. 

 Epithelial cells are the chief secreting cells of the 

 body, and these cells, therefore, form the glandular 

 tissue of all glands except the lympho- glandules, 

 which is a connective-tissue production. The 

 lymph glands thus constitute a class entirely by 

 themselves as distinguished from all other forms, 

 which may be called epithelial glands. As one of 

 the important functions of lymph glands is to con- 

 tribute white blood-corpuscles and thus scatter its 



