26G ANATOMY OF VERTEBRATES. 



CHAPTER IV. 



NERVOUS SYSTEM OF EiEMATOCRYA. 



§ 51. Nervous tissues. — Nervous sii'ostance, like muscular, 

 ranks with the most complex of animal tissues in chemical con- 

 stitution, and possesses the greatest atomic weight : but the albu- 

 minous form of proteine here prevails. Nervous tissue presents 

 two formal characters ; one -^'csicular and grey in colour, the 

 other fibrous and white : but the neurine inclosed by neurilemma 

 being softer than myonine, and less definite in arrangement after 

 death, the nerve-fibre usually appears as a tube with white contents. 



Nervous siibstance has two principal dispositions ; one in 

 masses, called ' centres,' the other in threads, called ' nerves.' 

 The centres in Vertebrate animals constitute, according to their 

 relative size or position, the spinal chord (myelon), the brain 

 (encephalon), and ganglions. In these the vesicular, grey, or 

 dynamic form of tissue is associated with the fibrous, white, or 

 conductive form. Most nerves consist of the wliite filjres, and all 

 are internuncial in office, establishing a communication between 

 the centres and the various parts of the body. 



The centres, and their grey or vesicular constituent more 

 especially, appear to originate the nervous force : certain nerves 

 conduct it to the tissues, princi]ially muscular, on which it acts 

 by producing contraction ; other nerves carry the impressions 

 received at their distal ends to the centres : the first are termed 

 ' motory,' from the function they excite, and ' efferent,' from the 

 direction of conduction : the second are termed ' sensorv ' and 

 ' afferent.' Sensation, or the appreciation of the impression by the 

 individual, seems to follow only when the 'afferent' nerve conveys 

 its impressions to the brain ; when it stojis short in the myelon, 

 or ends in a ganglion, it may excite a corresponding or connected 

 'efferent' nerve to ])roduce motion, or a 'reflex' action, which 

 may then take place without sensation or volition. 



The myelon, the enceplialon, and their nerves, constitute tlie 

 'myelencephalous' or ' cere))ro-si)inal ' system, to which belong 

 the ganglions on the sensory roots of the spinal and trigeminal 

 nerves, and tliose in the glosso-pliaryngcal and vao-al nerves. 



