TEE CRANIAL OR ENCEPHALIC NERVES. 809 



parotideal branches of the facial nerve ; then proceed as if preparing the muscles of the tongue 

 and pharynx and the arteries of the eye. Finally, the branches of the fifth pair, hypoglossal, 

 glosso-pharyngeal, and ocular nerves are exposed by freeing them from the tissues surround- 

 ing them. 



To follow more easily the ramifications of the cranial nerves in their intra-osseous course, 

 the head may be macerated in a bath of diluted nitric acid. The bones being softened, are 

 more easily cut and chiselled, while at the same time the nerves themselves are rendered more 

 firm and apparent by the dissolution of the connective tissue. 



1. FiEST Paik, or Olfactory Nerves (Figs. 423, 424). 



The first cranial pair is constituted by the olfactory lobules, the anterior 

 extremities of which give off a great number of nerve-filaments ; these pass 

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

 brane lining the bottom of the nasal fossae. 



Each olfactory lobule is connected with the brain by two roots — an external 

 and an internal — both composed of white substance (Fig. 424). The external 

 commences by a grey-coloured convolution which borders, externally, the tempo- 

 ral lobe of the hemisphere. The internal, followed from before backwards, turns 

 round in the interlobular fissure, in front of the optic commissure, to mix with 

 the cerebral convolutions. These two roots circumscribe a triangular space 

 occupied by the extra-ventricular nucleus of the corpus striatum, which they 

 embrace. In brains which have been macerated for a long time in alcohol, it is 

 easy to see that the fibres of these roots are continuous, in very great part, with 

 those of the corpus striatum, and leave with the fasciculi of the isthmus, which 

 radiate and spread through the grey matter of the corpus striatum. 



According to Meynert and Luys, a portion of the radicular fibres of the 

 olfactory nerves intercross in the substance of the white commissure of the 

 encephalic isthmus. 



After the union of its two roots, the olfactory lobule is constituted by a wide 

 white band that passes forward on the inferior face of the hemisphere, and soon 

 terminates in a very elongated oval dilatation lodged in the ethmoidal fossa. 

 This bulb is formed by grey substance on its inferior face, and white substance 

 on the superior. It is said to be a flattened ganglion laid on a band of white 

 substance, which at first represents in itself the olfactory lobule. 



We have already seen that this lobule is hollow, and that it communicates 

 with the lateral ventricles of the brain. This peculiarity, added to the special 

 features of its external physiognomy, might, it appears to us, give rise to doubts 

 as to the real nature of the lobules in question. It is evident that they are not 

 nerves, but rather dependencies of the brain ; and it is only conformable to 

 custom that we describe them here as the first pair of cranial nerves. 



The real olfactory nerves are the filaments which arise from the inferior face 

 of the ganglion or oJfactory bulb, and which traverse the cribriform plate to reach 

 the mucous membrane of the nose. Their number corresponds to the ethmoidal 

 foramina. At first very soft, delicate, and easily torn, they are enveloped on 

 their passage through these apertures by a very strong neurilemma, which gives 

 them great solidity. Some, in ramifying, descend on the septum nasi ; others — 

 and these are the most numerous — divide on the ethmoidal cells, where they form 

 fine and more or less plexuous tufts among the no less interesting divisions of 

 the ethmoidal branch of the ophthalmic artery. Their terminal extremities do 

 not descend below the upper third of the nasal fossae, but remain confined to the 

 bottom of these cavities. 



