1482 A MANUAL OF ANATOMY 



sheaths to the various cranial nerves, which blend with their 

 perineurium. It gives rise to two web-like expansions, namely, 

 the tela choroidea inferior and tela choroidea superior. 



The tela choroidea inferior is situated in the lower part of the 

 roof of the fourth ventricle, and from it are derived the choroid 

 plexuses of that ventricle. The tela choroidea superior, or velum 

 interpositum, is an invagination of the pia mater through the trans- 

 verse fissure beneath the splenium of the corpus callosum. It lies 

 underneath the body of the fornix, and its deep surface is covered by 

 the ependymal lining of the third ventricle, the latter forming the 

 roof of that cavity. The tela choroidea superior furnishes the 

 choroid plexuses of the two lateral and third ventricles. 



The pia mater of the encephalon differs from the pia mater of 

 the spinal cord in being thinner and less adherent to the nervous 

 substance-. The greater thinness is due to the fact that it is 

 destitute of the outer layer which characterizes the spinal pia 

 mater. 



Structure. — The pia mater of the encephalon consists of a single layer of 

 areolar tissue, which contains a great many small bloodvessels, these being 

 derived from the larger vessels, lying in the subarachnoid space. 



The Cranial Nerves. 



The cranial nerves are arranged in twelve pairs. They have 

 received numerical names according to the order in which they 

 leave the cranial cavity from before backwards, and they are alsa 

 known by functional names. The different pairs of nerves are as 

 follows, in order from before backwards : 



First, or olfactory. Seventh, or facial. 



Second, or optic. Eighth, or auditory. 



Third, or oculo-motor. Ninth, or glosso-pharyngeal. 



Fourth, or trochlear (pathetic). Tenth, or pneumogastric (vagus). 



Fifth, or trigeminal (trifacial). Eleventh, or spinal accessory. 



Sixth, or abducent. Twelfth, or hypoglossal. 



The cranial nerves are connected to certain parts of the encephalon. 

 and these connections constitute their superficial or apparent origins. 

 The fibres, however, can be traced to certain collections of grey 

 matter, which are called nuclei. From the deep positions occupied 

 by these nuclei they constitute the deep origins of the nerves. 



First or Olfactory Nerve. — ^This nerve is a complex structure, and 

 consists of (i) the olfactory lobe, and (2) the olfactory filaments or 

 nerves, which are about twenty in number. 



The olfactory lobe has been already described (see p. 1397). 



The olfactory nerves are non-medullated. They arise as the 

 axons of the olfactory cells of the olfactory mucous membrane of 

 the nasal fossa, and enter the cranial cavity through the foramina 

 of one half of the cribriform plate of the ethmoid bone. Thereafter 

 they enter the grey matter on the ventral or inferior aspect of the 

 olfactory bulb, and terminate in arborizations which intermingle 



