770 THE NERVOUS SYSTEM 



connected with it bundles of nerve fibres. Some of these fibres bear impulses from neighboring 

 ganglia or from the tissues of the neighboring organs and transmit them to the cell-bodies of the 

 ganglion; others arise from the cell-bodies in the ganglion and bear impulses to the central system 

 or, in case of the sympathetic, to other ganglia or to the tissues of the peripheral organs. Nec- 

 essarily, the larger the ganglion, the larger will be the bundles of fibres connected with it. 



Nuclei may be considered in two general classes: (1) Recipient nuclei or nuclei of termina- 

 tion, and (2) Nuclei of origin. 



A nucleus of termination is an accumulation of cell-bodies in which the axones of a given 

 fasciculus or of a nerve root terminate, that is, cell-bodies which, by synapses, receive the im- 

 pulses borne by the terminating axones. In most cases the impulses transferred to a nucleus so 

 named are sensory in character. The nucleus may be considered as a defined region in which 

 neurones of the next order are interpolated in a given nerve pathway or system of neurone 

 chains. Fasciculi in the spinal cord which bear impulses to the cerebrum have their nuclei 

 of termination in the medulla oblongata, and the sensory or afferent axones of the cranial 

 nerves find their nuclei of termination upon entering the central system. 



A nucleus of origin is an accumulation of cell-bodies of neurones which give origin to the 

 axones going to form a given nerve root or a fasciculus. Strictly speaking, a nucleus of ter- 

 mination for one nerve tract is the nucleus of origin for another, the next link in the neurone 

 chain. However, the term is commonly used to distinguish a group of cell-bodies giving rise to a 

 motor nerve tract. Thus each motor cranial nerve has its nucleus of origin within the central 

 system. The central grey substance of the spinal cord is in the form of a column continuous 

 throughout the length of the cord and so the cell-bodies in the ventral horns of this column which 

 give rise to the motor or afferent roots of the spinal nerves are not considered as grouped into 

 nuclei of origin, one for each of the motor roots. 



The dorsal root of each spinal nei^ve is afferent or sensory in function and its axones arise as 

 processes of cell-bodies comprising the spinal ganglion of the nerve. The afferent or sensory 

 fibres of the cranial nerves arise as processes of cell-bodies comprising the ganglia of the cranial 

 nerves, which ganglia are, in development and character, exactly homologous to the spinal 

 ganglia. 



The ventral root of each spinal nerve is efferent or motor in function and its fibres arise as 

 processes of cell-bodies situated in the ventral horn of the grey substance of the spinal cord. 

 The efferent or motor fibres of the'cranial nerves arise as processes of cell-bodies accumulated as 

 nuclei of origin in the grey substance of the encephalon, and homologous with those cell-bodies 

 of the ventral horns of the spinal cord which give origin to the ventral-root fibres. 



The general relation of the cerebrum (which includes the mesencephalon) to the remainder 

 of the nevous system is a crossed relation. Neurone chains from the general body to the cere- 

 brum, via the spinal nerves and cord and via the cranial nerves and medulla oblongata and pons 

 of one side, cross the mid-line to terminate in the opposite side of the cerebrum. Axones, and 

 neurone chains, arising in response in one side of the cerebrum, likewise usually decussate in 

 descending to terminate in the respective regions of the opposite side. 



Many of the names given nervous structures, prior to 1850 especially, instead of suggesting 

 something of their functional or anatomical significance, indicate nothing more than active 

 imaginations for accidental resemblances between the various structures of the nervous system 

 and objects in ordinary domestic environment. Also, quite often the name given a structure is 

 merely the name of some anatomist associated with it. The much needed elimination of these 

 old non-descriptive names is proving a very slow process. Attempts have often increased the 

 difficulty by making necessary the use of several names for a given structure instead of one. The 

 most recent and concerted attempt, the nomenclature known as the BNA (anatomical names 

 cho.sen by a commission appointed for the purpose which convened in Basle in 189.5), has been 

 adopted by modern text-books. It is here used in the form of the English equivalents of the 

 Latin terms, except in cases of those Latin terms which have become so commonly used as to be 

 considered words incorporated into the English language. The BNA has retained many of the 

 old names and, since a name should indicate something of the locality and significance of the 

 structure to which it is applied, it is not yet wholly satisfactory throughout. In applying the 

 names of a few fasciculi, the BNA in the following pages is slightly modified by so compounding 

 the name that the first word in the compound indicates the locality of origin of the fasciculus ana 

 the second, the locality of its termination. Thus, "Dorsal spino-cerehellar fasciculus" indicates 

 the more dorsally coursing of the fasciculi which arise from cell-bodies in the spinal cord and 

 terminate in the cerebellum. This principle applies to many of the BNA names without change, 

 as "lateral cerebrospinal fasciculus." 



THE CENTRAJ. NERVOUS SYSTEM 



The central nervous system [systema nervorum centrale] or organ is an 

 aggregation of nuclei, fasciculi and commissures — a large axis of grey and white 

 substance situated in the dorsal mid-line of the body— and the bundles of fibres 

 connecting it with the tissues of other systems and with the peripheral ganglia are 

 of necessity correspondingly large. So numerous are the axones connecting it 

 and so intimately are its neurones associated that a disturbance affecting any one 

 part of th(! system may extend to influence all other i)arts. The enlarged 

 cephalic extremity of this central axis, the brain or encephalon, is a special ag- 

 gregation of nuclei and masses of grey substance, many of which are much larger 

 than any found in the periphery. 



i 





