ORGANIC STRUCTURE : MORPHOLOGICAL 47 



an effector organ. In the simple nerve-net, or diffuse nervous 

 system, some of the elements of the latter are connected with 

 receptors and others are connected with muscles, or other effector 

 organs. In the higher animals there is a centrahzed nervous 

 system, or systems, and these central parts are the ganglia. Nerves 

 leading into the ganglia are afferent nerves and those leading out 

 are efferent nerves. The brain of the vertebrates is an aggregate 

 of ganglia. The cerebral ganglia of the invertebrates corresponds 



SpiTial 

 cord 



VtsiuiL 



jSpiTiaZ 

 nerves 



NcLsol 

 orgojv 



^ ''or^ "^^^^^ 

 Receptor ^ 



Cerebral 

 gojiglicL 



E-Pferent 



Afferent 



"^^"^ ^ ,%iC^' neurone 

 3 {J. rue Tierue 



centre) 



Se^mBTvtal 

 ^cxJigHcL 



V Effector 



^v 



Fig. 13. 



I, Diagram of the central nervous system of a vertebrate ; 2, diagram of the central nervous 

 system of a worm ; 3, an element of the ner\ous system of a vertebrate. 



with the vertebrate brain. The element of the whole nervous 

 system of either the vertebrate brain or the invertebrate nerve- 

 centre consists of at least two neurones (see Section 13 and 

 No. 3 of Fig. 13). 



\2g. The Sense-Organs. Whatever it may be, a sense- 

 organ consists of one or more receptors. A simple receptor is 

 the branched, or otherwise modified termination of a nerve-fibre 

 and these terminations are situated in the skin, or in some other 

 bodily part where physical stimuli impinge upon them. As a 

 rule the nerve-terminations are provided with accessory parts : 

 thus the visual receptors are the nerve-terminations in the retina 

 of the eye, but the eyeball, the lens, the iris, etc., are the accessory 

 parts which modify and direct the stimuli impinging on the 

 nerve-terminations . 



