BRAIN AND NERVE 89 



consider it as a structure — an assemblage of nerve cells and nerve 

 fibres arranged in a most complex manner. 



The General Scheme. — The central nervous system of a verte- 

 brate animal consists of masses of nerve cells called ganglia, 

 and these are aggregated together as the hrain, the spinal cord. 

 and the sympathetic ganglia. They are connected together by 

 commissural (or connecting) strands of nerve fibres, and they are 

 also connected with the sense organs, muscles, and other parts 

 of the body by nerves. The latter are bundles of nerve fibres 

 varying in diameter from about j inch (the great sciatic 

 nerve of the leg), down to fine threads which are just 

 visible to the naked eye. The nerve fibres themselves are just 

 beyond the limits of unaided vision. A nerve issuing from the 

 brain or spinal cord, or from a sympathetic ganglion, breaks up 

 into finer branches, and these branches divide again and again until 

 the whole is decomposed into its constituent fibres. The latter end 

 in muscle fibres or among the cells of a gland or some other organ. 

 In a sense organ, such as the eye or ear, there are great numbers 

 of nerve cells, from each of which a single nerve fibre starts. 

 These individual fibres become united into a nerve, and the latter 

 (as in the case of the optic or auditory nerves) may go directly 

 to the brain, or they may join with the bundles of fibres coming 

 from other sense organs, and so reach the brain or spinal cord. 



Thus we have two main divisions of the nervous system: 

 (1) The central parts, consisting of the ganglia with their con- 

 nections; and (2) the peripheral parts, consisting of the nerves 

 going from the organs of sense into the brain and spinal cord, and 

 of the nerves going out from the brain to the muscles, glands, and 

 viscera. The peripheral nerves, whether outgoing or ingoing, are 

 distributed to every part of the body, forming a network of fibres 

 which is everywhere a very fine one, but which becomes finer and 

 finer the more important is the organ to which they are distributed. 



Now a very diagrammatic and undetailed picture of the 

 nervous system would show the following parts (Fig. 21): 



We see that the whole consists of the brain, which is divided 

 into several principal parts: the cerebrum or great brain, the 

 cerebellum or little brain, and the medulla. The spinal cord is to 

 be regarded as the prolongation backward of the medulla. From 

 the brain there issue out ten pairs of cranial nerves, which consist 

 of fibres either going to or coming from the parts of the head, 

 face, and neck (with a few organs in the body cavities). From 



