512 THE CRANIAL NERVES. 



nerves is in some instances very marked. The fifth pair or trigeminus 

 emerges from the tuber annulare in two distinct bundles or roots, of 

 which one is sensitive, the other motor ; the sensitive root presenting 

 soon afterward a well developed ganglion, with which the fibres of the 

 motor root do not mingle. This nerve beyond the ganglion, therefore, 

 contains both motor and sensitive fibres, and is distributed both to 

 muscles and to the integument. In a similar manner the glosso-pha- 

 ryngeal nerve is joined, beyond its ganglion, by motor fibres from the 

 facial; and the pneumogastric receives abundant communications from 

 the spinal accessory and other motor nerves. Both the sensibility and 

 motion therefore of the parts to which they are distributed are provided 

 for, in a manner not essentially different, by both the cranial and spinal 

 nerves. 



The other points, both of difference and analogy, in the cranial nerves, 

 relate to their origin and distribution. Their apparent origin, that is, 

 the point at which they become detached from the surface of the brain, 

 is not their real origin ; but in every case their fibres can be traced from 

 this point inward, between other longitudinal or transverse tracts of 

 white substance, until they reach a mass of gray matter, often placed at 

 a considerable distance and in quite a different locality from their 

 apparen origin. All the cranial nerves, excepting the olfactory and 

 the optic, are thus found to originate from a mass of gray substance 

 upon and beneath the floor of the fourth ventricle, and extending forward 

 to surround the aqueduct of Sylvius. This layer of gray substance is a 

 continuation of that in the spinal cord ; but while in the cord it has the 

 form of a central mass with lateral anterior and posterior horns, in the 

 medulla oblongata it takes the shape of a lamina occupying only the 

 posterior part of the cerebro-spinal axis. In its various divisions and 

 expansions, which are rarely, if ever, completely discontinuous from 

 each other, it forms the so-called " nuclei" of the cranial nerves. 



In their distribution, these nerves present also certain anatomical 

 features which are more apparent than real in their importance. The 

 oculomotorius, patheticus, and abducens emerge from the substance of 

 the brain at very different points, and, running forward through the 

 cranial cavity in the form of separate cords, are enumerated as three 

 distinct nerves. But they all originate from the layer of gray substance 

 already mentioned, two of them, the oculomotorius and the patheticus, 

 in close proximity to each other ; they all pass out of the cranium, into 

 the orbital cavity, by the sphenoidal fissure ; and they are all distributed 

 to the group of muscles moving the eyeball. In a physiological point 

 of view, therefore, they are branches of a single nerve, rather than three 

 separate trunks. Even when two or more nerves emerge from the 

 cranium by different foramina, like the three divisions of the trigeminus, 

 they are nevertheless, properly speaking, parts of the same nerve, if they 

 have similar physiological properties and are distributed to the muscles 

 or integument of the same regions. It is the ultimate distribution of a 

 nerve, and not its course through the bones of the skull, that determines 



