18 General Part. 



The sympatlietic nervous system, present in many animak, consists 

 of a series of small ganglia and nerve trunks, from whicli nerves are given o£B 

 to tlie digestive tract (in the Vei-tehrata to other viscera also and to the vascular 

 system). The sympathetic system is comparatively independent of the central 

 nervous system, although it is connected with it ; the muscle cells of an organ 

 supplied by it, contract without impulse from the central system, they are 

 involuntary muscles, and receive the necessary stimulus from the sympathetic 

 ganglion cells. 



5. Sense Organs. 



An animal receives impressions from the outer world by 

 means of the sense organs, whicli are usually modified tracts 

 of epidermis. They are always closely connected with the nervous 

 system either by nerve fibres springing from ganglion cells 

 (within or external to the central nervous system) and ramifying 

 amongst the cells of the sense organ (Fig. 17, ep, andep'); or by 

 fibres originating in the sense cells of such an organ (p. 17) and 

 passing to the ganglia (Fig. 17, sa). 



Sense organs may be classified as simple or complex; to the former 

 class the organs of touch, of smell, and of taste are usually refeiTed, 

 since they are of a simpler structure than the organs of hearing and sight, 

 which are often very complicated. 



The sense of touch contrasts with the other senses in that it 

 is distributed over the whole, or the greater part, of the surface of 

 the body ; the entire skin, especially the epidermis, is, therefore, a 

 sense organ. In some cases, e.g., in the Vertebrata, nerve fibres pass 

 to these portions of the skin and ramify amongst the epidermal cells ; 

 in others, e.g., amongst Annelida and MoUusca, special tactile cells, 

 each furnished at its free surface with one or more hair-like 

 processes,* and continued at the other end into a nerve fibre,t occur 

 amongst the ordinary epidermal cells. In some animals the skin 

 is furnished with warty or filamentous processes, provided with 

 numerous tactile cells, or with a rich nerve supply, and, therefore, 

 important as tactile organs. 



Amongst Vertebrates, certain tactile nerve fibres, as already mentioned, 

 innervate the epidermis, but others terminate in the dermis. The latter are 

 sometimes enveloped in concentric laminse of connective tissue, pacinian bodies,^ 

 or the nerve endings may be of a different form. 



Olfactory Organs are acted upon by gaseous matters in a 

 peculiar way ; they can be ascribed with perfect certainty to only a 

 small proportion of forms, viz., existing terrestrial animals. They are 

 formed of epithelium, derived from the epidermis, in which there are 

 tall, thin, sensory cells, each with a tuft or tufts of sensory hairs at 



* Sometimes these tactile, and other such, sensory cells are very like ciliate 

 epithelium ; the hairs are, however, not actively, but merely passively movable. 



t Sometimes only a narrow external part of the sense cell is situated between the 

 other epidermal cells, whilst the thicker part, with the nucleus, lies in the subjacent 

 connective tissue. 



J These may also occur internally — e.g. in the mesentery of the cat. 



