THE CRANIAL OR ENCEPHALIC NERVES. 705 



solution has its source in a misunderstanding, and is really not founded on 

 tacts,_ wJuch are the same for everyone, their appreciation only varyins 

 Nothing IS more easy to prove. Look, in the first instance, at the optic 

 nerve, ihis, according to some, proceeds from the corpora quadrigemina 

 and thalami optici ; it therefore arises from the brain. No doubt, if these 

 two parts of the encephalon be considered as a portion of the hemispheres: 

 butthis isfar from proved, and in an anatomical and physiological point 

 yew It IS not so. As the corpora quadrigemina and thalami optici form 

 part ot the isthmus, it is natural to look upon it as the source from which 

 the second pair of nerves arise. With regard to the first pair, their fibres 

 are also connected with those of the isthmus, across the corpus striatum, as 

 will be proved hereafter. But we are far from denying their connections 

 with the hemispheres (see tlie description of the first pair), and which arc 

 very intimate ; though this proves nothing against our opinion. Therefore 

 it is, that, in recognising in the disposition of the roots of the olfactory 

 nerve conditions altogether special, we admit that the isthmus of the 

 encephalon is the common point of departure for all the cranial nerves— an 

 important and capital fact, and which constitutes, for the great category of 

 encephalic nerves, a true family character. 



Among the other points relative to the origin of these pairs of nerves, 

 are the following : All the encephalic nerves appear to be connected at 

 their origin with the fasciculi of the isthmus, whose properties they share. 

 All are equally connected with a centre of grey substance placed in the 

 texture of the isthmus, and named by Stilling the nucleus of tie nerves. 

 The majority originate by converging filaments, the anterior of which come- 

 from the brain, and the posterior from the side of the spinal cord. 



Preparation of the cranial nerves. — Four preparations are necessary to study the 

 cranial nerves : 



1. An encephalon extracted after opening the cranium by its base, and hardened 

 by prolonged immersion in alcohol or very diluted acetic acid. Tliia piece permits ihe 

 origin of the nerves to he studied (Fig. 322). 



2. The superficial nerves of the head; these are the auricular nerves, and the 

 divisions of the subzygomatio plexus, with the suborbital and mental branches, as well 

 as the superficial ramuscules of the three nerves of the ophthalmic branch of ti.e fifth 

 pair (Fig. 110). 



3. A piece disposed as in figure 386, for the study of the maxillary nerves. To prepare 

 it, the greater part of the masseter muscle should be removed in dissecting the masseteric 

 nerve; the globe of the eye must be extirpated, the orbital and zygomatic processes 

 excised, the two maxillary sinuses opened, and the branch of the inferior maxillary bcine 

 chiselled oflf as in the figure. Lastly, the anastomosis of tlie lacial with the siib- 

 zygomatic nerve is dissected by cutting away the parotid gland. 



4. The deep nerves, including those of tlie globe of the eye ; this preparation shonld 

 be made by following exactly the instructions given for dissecting the arteries of the 

 head. Figure 335 will serve as a guide for details. 



Tlie pneumogastric and spinal nerves, which are not included in these considerations, 

 should be prepared and studied at the same time as the great sympathetic, Wheu 

 treating of the latter, we will refer to them. 



1. First Pair, or Olfactory Nerves. (Figs. 327, 329.) 



The first cranial pair is constituted by the olfactory lobes, whose 

 anterior extremities give off a great number of nervous filaments, which pass 

 through the cribriform foramina to ramify in that part of the pituitary 

 membrane lining the bottom of the nasal fosssB. 



Each olfactory lobe is connected with the encephalon by two roots, an 

 external and an internal, both composed of white substance (Fig. 322). 



