HISTOLOGY 23 



minute vibratile hairs of cilia (£) which create currents. In glandu- 

 lar epithelium the cells, usually cubical or columnar, are specialized 

 for the elaboration of secretions to be used by the animal or of waste 

 products (excretions) to be voided from the body. 



j/ Glands. — The chief kinds of glands may be mentioned here. All have for 

 fPthsil function the extraction and_elaboration of c erta in products fro m the blood, 

 7 consequently they have a good blood~supply. Glands may be unicellular or 

 I multicellular according as they consist of isolated cells or of many cells. In uni- 

 cellular glands (abundant in the digestive tract) each cell passes its own secretion 

 directly to the place where it is to be used (fig. 22, u). 



Multicellular glands occur where a large amount of secretion is necessary in a 

 limited space, hence they are not on the surface but at some deeper point, and 

 their product is conveyed to the desired place by a duct. Multicellular glands 

 are of two structural kinds. In the tubular gland the whole is approximately of 

 the same diameter throughout, with little differentiation of gland and duct. It 

 may be simple (A) or coiled (B) or branched (C, D), these modifications serving 

 to increase the secreting surface. In acinous glands (D, E) there is a marked 

 difference between gland and duct, the glandular part forming an enlargement 

 (acinus) on the end of the duct. Both simple and compound acinous glands 

 are common. 



Still another type of gland, the ductless or 'internal secretion' gland, occurs. 

 In this there is no duct, the secretion elaborated by the cells passing by osmose 

 into the blood-vessels. These secretions, collectively known as hormones, have 

 recently acquired great prominence from their influence on different organs. 



Nervous Tissues 



Nervous tissue has for its function the correlation of the animal 

 with its environment. In order to accomplish this it must provide 

 for the recognition of stimuli from without, the inauguration of 

 other impulses within itself and the transfer of both to other parts. 

 The essential constituent of the tissue is the nerve cell, ganglion ceU 

 or neuron, to which are added others of a supportive (glia cells) or 

 nutritive character. As the parts to be connected by the nervous 

 tissues are often remote from each other the neuron is not compact 

 like most other cells, but gives off long processes from the central 

 mass, these processes differing in their terminations. Some end in 

 places where they can only receive stimuli, others where the stimuli 

 can only cause parts to act. Thus the processes are physiologically 

 divisible into afferent and efferent tracts, the body of the cell being, 

 besides a place for the regulation of the nutrition of the neuron, the 

 locality for the regulation and correlation of impulses, and apparently 

 in many cells, for the inauguration of new impulses. 



v^ 



