THE CRANIAL NERVES. 1219 



nerves, although directly or indirectly connected with the cerebro-spinal axis, pre- 

 sent peculiarities and, as the system of sympathetic nerves, are accorded, at least for 

 convenience of description, a certain degree of independence. While by no means 

 all of the spinal nerves contribute splanchnic branches such branches being given 

 off especially by the thoracic and upper lumbar nerves they all receive sympathetic 

 filaments, which form, therefore, integral parts of the somatic nerves. From the 

 sympathetic neurones of the gangliated cords axones pass, by way of the gray rami 

 communicantes (page 1357), to the trunks of the spinal nerves and thence by these 

 are carried to all parts of the body for the supply of the involuntary muscle occur- 

 ring within the blood-vessels and the integument and for the cutaneous glands. .Fur- 

 thermore, it must be remembered, that although the predominating constituents of 

 a spinal nerve may be axones derived from anterior horn root-cells and destined for 

 voluntary muscle, such trunk also contains a number of afferent fibres which convey 

 impulses received from the neuromuscular and neurotendinous sensory endings, the 

 nerve-trunks reckoned as ' ' motor ' ' in all cases, when analyzed, being found to con- 

 tain sensory and sympathetic fibres as well as efferent ones. 



THE CRANIAL NERVES. 



The cranial nerves (nervi cerebrales) include twelve pairs of symmetrically 

 arranged nerve-trunks, which are attached to the. brain and, traced peripherally, 

 escape from the skull by passing through various foramina at its base to be distrib- 

 uted for the most part to the structures of the head. 



The point at which a cranial nerve is attached to the surface of the brain is 

 designated its superficial origin ; the group of more or less deeply situated nerve- 

 cells with which its fibres are directly related is often spoken of as its deep origin. 

 From what has been said (page 1278) concerning the position of the cell-bodies of 

 motor and sensory neurones, it is evident that only the motor fibres of the cranial 

 nerves spring from nerve-cells within the cerebro-spinal axis, while the fibres con- 

 ducting sensory impulses arise from nerve-cells situated within ganglia lying outside 

 the central nervous axis and somewhere along the course of the nerve-trunks. It 

 follows, therefore, that the term ' ' deep origin, ' ' as applied to the cell-groups within 

 the brain, can properly relate only to the origin of motor fibres ; the cell-groups with 

 which the sensory fibres come into relation after entering the brain-substance are in 

 reality nuclei of reception, or of termination, and not of origin. The sensory 

 impulses so received are transmitted to various parts of the brain by the more or less 

 complex paths afforded by the neurones of the second, third, or even higher order. 

 In addition to their relation to the deep nuclei, whether of origin or of reception, the 

 fibres of every cerebro-spinal nerve are directly or indirectly influenced by neurones 

 situated within the shell of gray matter that covers the cerebrum. The position of 

 these higher cortical centers, as they are termed, is known with considerable 

 accuracy for many groups of nerves, but regarding others more definite data con- 

 cerning cerebral localization must be awaited. 



Bearing in mind the foregoing distinctions, for convenience we may follow the 

 conventional description in which all the nerves are regarded as passing away from 

 the brain, the direction in which they convey impulses, centripetally or centrifugally, 

 being for the time disregarded. 



On leaving the surface of the brain at its superficial origin, each cranial nerve, 

 invested by a sheath of pia mater, traverses for a longer or shorter distance the sub- 

 arachnoid space, pierces the arachnoid and from the latter acquires an additional, 

 but usually not extensive, sheath. It then enters a canal in the dura mater that 

 leads to the foramen in the skull, through which the nerve escapes from the cranium, 

 invested by a sheath prolonged from the dura which is continuous with the epi- 

 neurium covering the nerve-trunk. The position of the dural aperture and that of the 

 foramen by no means always correspond, some of the nerves, notably the fourth and 

 sixth, pursuing an intradural course of some length before gaining their osseous exit. 



According to the order in which they pass through the dura lining the cranium, 

 the pairs of cranial nerves are designated numerically from the first to the twelfth. 

 They are further distinguished by names based upon their distribution or functions. 



