114 



PRINCIPLES OF ANIMAL BIOLOGY 



is not so controlled. In general, smooth muscle tissue is involuntary 

 and striated muscle is voluntary, but this distinction fails in part, for 

 the heart muscle of vertebrates is striated but not voluntary. 



Nervous Tissues. — Nervous tissues are those which have as their 

 chief function the reception of stimuli and transmission of impulses. These 

 tissues include the perceptory cells of the auditory, visual, olfactory and 

 gustatory organs and the cells by which the sensations of heat, cold, pres- 

 sure and so on are perceived. They also include the nerves, ganglionic 

 masses, and central nervous system which includes brain and spinal cord 

 in the vertebrates and analogous organs in the invertebrates. As men- 

 tioned in the discussion of epithelial tissues, some epithelial cells receive 

 stimuli and transmit impulses, and may therefore also be classed with the 

 nervous tissues. The receptive cells of audition, vision, smell and taste are 

 of this epithelial character. Examples of nerve cells are illustrated in 

 Fig. 78. 



Fig. 78. — Three kinds of nerve cells. A, from ventral horn of spinal cord of an ox; 

 B, from cortex of cerebrum of a cat; C, Purkinje cell from cerebellum of a cat; d, dendrite; 

 nev, axon; nu, nucleus; nuc, nucleolus. {B and C from Golgi preparations. Original.) 



ORGANOLOGY 



Organology comprises the study of the organs of the body and their 

 arrangement into systems. Under this head the great systems of organs 

 of the bodies of higher invertebrates and vertebrates are considered in 

 this chapter with special reference to certain chosen types. The digest- 

 ive, respiratory, circulatory, excretory, sexual, locomotor, and nervous 

 systems are discussed seriatim, and while structural aspects are especially 

 considered it is not possible to ignore function. It should be borne in 

 mind throughout the discussion that the various systems are intimately 

 related and that no system can be properly discussed with respect either 

 to its structure or to its function without involving one or more of the 

 pther systems. An understanding of the relations of these systems to 



