1120 GENERAL ANATOMY OR HISTOLOGY. 



relations with the nervous centres ; by this means, also, groups of muscles may be 

 associated for combined action. 



The sympathetic nerves are constructed in the same manner as the cerebro- 

 spinal nerves, but consist mainly of non-medullated fibres, collected into funiculi, 

 and enclosed in a sheath of connective tissue. There is, however, in these nerves 

 a certain admixture of medullated fibres, and the amount varies in different 

 nerves, and may be known by their color. Those branches of the sympathetic 

 which present a well-marked gray color are composed more especially of gelatinous 

 nerve-fibres, intermixed with a few medullated fibres ; while those of a white 

 color contain more of the latter fibres and a few of the former. Occasionally, the 

 gray and white cords run together in a single nerve, without any intermixture, as 

 in the branches of communication between the sympathetic ganglia and the spinal 

 nerves, or in the communicating cords between the ganglia. 



The nerve-fibres, both of the cerebro-spinal and sympathetic system, convey 

 impressions of a twofold kind. The sensory nerves, called also centripetal or 

 afferent nerves, transmit to the nervous centres impressions made upon the 

 peripheral extremities of the nerves, and in this way the mind, through the 

 medium of the brain, becomes conscious of external objects. The motor nerves, 

 called also centrifugal or efferent nerves, transmit impressions from the nervous 

 centres to the parts to which the nerves are distributed, these impressions either 

 exciting muscular contraction, or influencing the processes of nutrition, growth, 

 and secretion. 



Origin and Termination of Nerves. By the expression " the termination of 

 nerve-fibres " is signified their connection with the nerve-centres, and with the 

 parts they supply. The former are sometimes called their origin, or central 

 termination ; the latter their peripheral termination. The origin in some cases is 

 single that is to say, the whole nerve emerges from the nervous centre by a single 

 root ; in other instances the nerve arises by two or more roots, which come off 

 from different parts of the nerve-centre, sometimes widely apart from each other, 

 and it often happens, wljen a nerve arises in this way by two roots, that the 

 functions of these two roots are different ; as, for example, in the spinal nerves, 

 each of which arises by two roots, the anterior of which is motor and the posterior 

 sensory. The point where the nerve root or roots emerge from the nervous centre 

 is named the superficial or apparent origin, but the fibres of which the nerve 

 consists can be traced for a certain distance into the nervous centre to some por- 

 tion of the gray substance, which constitutes the deep or real origin of the nerve. 



The manner in which these fibres arise at their deep origin varies with their 

 functions. The centrifugal or efferent nerve-fibres originate in the nerve-cells of 

 the gray substance, the axis-cylinder processes of these cells being prolonged to 

 form the fibres. In the case of the centripetal or afferent nerves the fibres grow 

 inward either from nerve-cells in the organs of special sense (e. g., the retina) 

 or from nerve-cells in the ganglia. Having entered the nerve-centre, they branch 

 and send their ultimate twigs among the cells, without, however, uniting with them. 



Peripheral Terminations of Nerves. Nerve-fibres terminate peripherally in 

 various ways, and these may be conveniently studied in the sensory and motor 

 nerves, respectively. Sensory nerves would appear to terminate either in minute 

 primitive fibrillce or networks of these ; or else in special terminal organs, which 

 have been termed peripheral end-organs, and of which there are several principal 

 varieties, viz., the end-bulbs of Krause, the tactile corpuscles of Wagner, the 

 Pacinian corpuscles, and the neuro-tendinous' and neuro-muscular spindles. 



Termination in Fibrillae. When a medullated nerve-fibre approaches its termi- 

 nation, the white matter of Schwann suddenly disappears, leaving only the axis- 

 cylinder, surrounded by the neurilemma, and forming a non-medullated fibre. 

 This, after a time, loses its neurilemma, and consists only of an axis-cylinder, 

 which can be seen, in preparations stained with chloride of gold, to be made up 

 of fine varicose fibrils. Finally, the axis-cylinder breaks up into its constituent 

 primitive nerve-fibrillse, which often present regular varicosities and anastomose 



