4SS GENERAL ANATOMY. 



point from which they spring and upon which they vegetate, 

 if we may so express it: this origin is only the central extre- 

 mity of the nerve, or that by which it is connected with the 

 nervous centre. It is for all the nerves in the spinal marrow 

 and in the medulla oblongata; no one arises from the lobes of 

 the brain nor from the cerebellum. The olfactory nerve is 

 not even an exception to this rule; this nerve arises from a 

 prolongation of the spinal marrow, which, in animals, consti- 

 tutes the olfactory bulb. Sometimes foetuses are found de- 

 prived of the brain, and in which notwithstanding the olfac- 

 tory with the spinal marrow and the crura cerebri exist, as 1 

 have had occasion to observe lately. Bichat, in saying that 

 all the nerves arise from the medulla, makes an exception for 

 the optic and olfactory which does not really exist. 



The origin of the nerves is often more deeply situated than 

 it appears at first; so that the point from which they detach 

 themselves is often not their true origin: the fifth pair, for ex- 

 ample, does not arise from the pons varolii, from which it ap- 

 pears to come, for the pons varolii does not exist among ovi- 

 parous animals, where the origin of this nerve notwithstand- 

 ing is the same as in the mammalia. We need not, however, 

 seek to pursue the origin of the nerves beyond the reach of 

 the senses, and suppose them to set out from the brain or from 

 the cerebellum, as has been done to support hypothetical ex- 

 planations. 



It has been asked if the nerves cross each other at their ori- 

 gin; and it has been affirmed without hesitation that it is so, 

 to explain pathological phenomena in which the cause and the 

 effect, both seated in the nervous system, present a sort of 

 crossing. Let us see what observation teaches on this subject. 

 There is no sensible crossing in the nerves of the medulla ob- 

 longata. It is the same with those which arise from it where 

 it is prolonged into the cranium, except perhaps the optic 

 nerves, in which there appears to exist at least a partial cross- 

 ing. Authors, in effect, do not agree as to the mode of union 

 of these nerves. Their crossing, admitted by some, denied by 

 others, is evident in fishes; but in man, although in most cases 

 the atrophy of one of these nerves continues on the opposite 

 side, observers worthy of credit assure us that they have seen 



