678 THE CEREBRAL NERVES. 



of the head) , enlargement of the ear and increased rapidity in the growth of the 

 hair were observed, together with hypertrophy of the muscular coat of the veins, 

 of the cartilage, and of the horny skin, with atrophy of the epidermis; further, 

 diminution in the size of the cerebral hemisphere of the corresponding side, perhaps 

 in consequence of the pressure exerted by the dilated vessels. Lewaschew ob- 

 served hypertrophy of the leg and foot in the sequence of long-maintained chemical 

 irritation of the sciatic nerve in dogs, and also the development of aneurysmal 

 dilatation of the vessels. 



In man, irritation or paralysis of the nerves or degeneration of the gray matter 

 of the spinal cord is not rarely attended with alterations in the pigment of the 

 skin, and of the nails and hair and in their growth, as well as cutaneous eruptions, 

 for example herpes zoster after inflammation of the spinal ganglia or nerves. 

 and a tendency to bed-sores; further, rare affections and degenerations of the 

 joints (in cases of tabes). Local diseases of the brain have been observed to 

 be attended, with unilateral derangement in the growth of the hair and the nails. 



(d) Inhibitory nerves, which suppress or diminish a movement or secretion 

 already present. 



Examples are found in the vagus as the inhibitory nerve of the movement 

 of the heart, the splanchnic as that of the movements of the intestine, the vaso- 

 dilators as inhibitory nerves of the unstriated muscle of the vessels. 



II. CENTRIPETAL NERVES. 



(a) Sensory nerves, which convey sensory impressions to the central organ 

 by means of special end-apparatus. 



(6) Nerves of special sense. 



(c) Reflex or excito-motor nerves, which, when stimulated at the periphery, 

 conduct the irritation to the center, within which this excitation is transmitted 

 to the centrifugal fibers (I, a, b, c, d} , so that the activity of the latter is manifested 

 as reflex movement, reflex secretion or reflex inhibition. 



III. INTERCENTRAL NERVES. 



These connect ganglionic centers one with another for the communication of 

 the excitation among them, for example in the coordinated movements, for in- 

 stance of the eyes and of widespread reflexes. 



THE CEREBRAL NERVES. 



All cranial motor nerves arise from their cerebral nuclei of origin as 

 neurites of ganglion-cells in the same way as the fibers of the anterior 

 roots of the spinal cord arise from the ganglia of the anterior horns. 

 The sensory cerebral nerves have their actual origin in the bipolar cells 

 of the peripheral ganglia of the sensory nerves. Into each of these eel's 

 a cellulipetal dendrite enters from the region endowed with sensation, 

 while a cellulifugal neurite passes from the cell to the brain, where it 

 comes into contact with the terminal ramifications of the sensory nucleus 

 of origin 



I. OLFACTORY TRACT AND BULB. 



The strand-like triangular-prismatic olfactory tract, situated upon the inferior 

 surface of the frontal lobe, becomes enlarged on the cribriform plate of the ethmoid 

 bone to form the olfactory bulb, which is the analogue of a special portion of the 

 brain that exists in different vertebrates with a well-marked power of smell. 

 From the bulb there pass through the cribriform plate between 15 and 20 olfactory 

 filaments, which continue first between the mucous membrane and the periosteum 

 and into the mucous membrane itself only in the lower third of the olfactory 

 region. The structure of the bulb, as well as the relations of the olfactory nerves, 

 are discussed on p. 914. 



The origin of the olfactory fibers may be traced as follows: (i) To the forni- 



