THE NERVES. 805 



However this may be, it must be remarked that the special anatomical and 

 physiological characteristics of the sympathetic nerves, should not cause them to 

 be considered as a system indejxiudent of the first, or cerebro-spinal nerves. The 

 fibres composing both have, in fact, a common origin in the spinal cord — or, 

 rather, those of the ganglionic nerves emanate from tlie nerve-cords of animal 

 life. In the considerations which follow, we will therefore omit this distinction 

 of the nerves into two groups. 



(The nerve-fibres in the sympathetic system of nerves are chiefly non-medul- 

 lated, and form the preponderance of this kind of nerve-fibre. Sympathetic 

 nerves are whiter than cerebro-spinal nerves, or have a pale-grey hue.) 



Origin of the Nerves. — We ought to distinguish in these cords their 

 real or deep origin, and their superfirial or apparent origin. 



The latter is represented by the point of issue of the roots of the nerves, 

 which are ordinarily spread in a fan-shape, then united — generally after a very 

 brief course — into a single trunk, which offers at its commencement a ganglionic 

 enlargement, if fibres of general sensibility enter into its constitution. The 

 inferior spinal roots issue from the bottom of the inferior collateral furrow of the 

 spinal cord ; the superior roots from slightly within the superior collateral furrow. 

 Their real origin is the point of departure of these roots in the substance of the 

 cerebro-spinal axis. The nuclei of the nerves have been the object of much 

 investigation during recent years, and we therefore know the majority of them. 

 It may be remarked that the cells of the nuclei of the spinal nerves are larger 

 than those of the grey matter of the medulla oblongata. Otherwise, the motor 

 cells diminish in volume from below upwards — that is, from the lumbar region 

 to the medulla oblongata (Pierret). 



Distribution of Nerves. — The nerve-tnmks, formed by the radicles of 

 which we have just spoken, issue in pairs from the foramina at the base of the 

 cranium or in the walls of the spine, to be distributed to all parts of the body by 

 dividing into successively decreasing branches. 



Those among these branches which ramify in the organs of animal life, 

 generally follow the track of the deep vessels or the subcutaneous veins, and are 

 always found most superficial. Their ramescence is effected in a very simple 

 manner, by the successive emission of the fasciculi composing the principal 

 trunks, until these are completely expended. These branches pursue their 

 course nearly always in a direct line ; only some — as the ramifications of the 

 two principal nerves of the tongue — describe very marked flexuosities, with 

 the same protective intention as the arteries of that organ. Anastomoses some- 

 times join these l)ranches to one another ; and anastomoses — frequently compli- 

 cated—unite many nerves together, forming what are called plexuses. But in 

 these anastomoses, no matter how complicated they may be, there is never any 

 fusion of the nerves, but merely aggregation of their fibres, which always pre- 

 serve their independence, characters, and special properties. These anastomoses, 

 then, differ essentially from those of arteries, and never permit two trunks to 

 mutually supplement each other when the course of one is interrupted. 



The nerves sent to the organs of vegetative life, and which arise from the 

 two subspinal chains in the formation of which nearly every pair of nerves 

 concurs, comport themselves in their distribution in a slightly different manner. 

 They are enlaced around arteries, forming on these vessels very complicated 

 plexiform networks, and yet the fibres composing them are as absolutely inde- 

 pendent as in the anastomoses above described. 



