ML L v/ auditory nerve root 



ary branch, Vj ^ ^ '^°° ^tic capsule 



facial nerve (VII) root I '^ 



superficial ophthalmic Vll^ \^ trigeminal (V) root 

 eye 

 opthalmicus profundus (V ' 



glossopharyngeal nerve IX 

 X (root and ganglion) 



palatine branch of VII 



hyomandibular branch VII 



Figure 13-8. Distribution of the cranial nerves of Ambystomo. (In part after Strong, 1 895) 



4-20) but above the stapes in Necturus. The chordae tympani 

 separates anterior to the stapes in Necturus and passes below 

 that bone. In the others it separates behind the stapes and 

 passes forward and downward well below the stapes. Ex- 

 planations of these relationships can be based on changes in 

 position of the tympanic membrane and the middle-ear 

 cavity and on the development of different processes extend- 

 ing outward from the footplate. In the amniotes the mid- 

 dle-ear cavity is not strictly the spiracular pouch but rather a 

 posteroventral diverticulum of that pouch. In salamanders 

 the process of the footplate might be equated with the dor- 

 sal process of the columella of reptiles, but it could also be a 

 neomorph in this group. 



The first, or the first and second, spinal nerves give rise 

 to the hypoglossal nerve (XII). The spinal accessory is a 

 part of the tenth nerve. However, there is a problem of 

 terminology here; the "vagus" of the amphibian could be 

 identified as the vagoaccessory. 



In its development, the brain of the amphibian follows 

 the pattern of the mammal with the exception that only a 

 cerebral flexure develops (the pontine and cervical flexures 

 are only temporary in the mammal). 



Choanate fishes 



Ojpnoon The brain of the dipnoan Protopterus is not unlike 

 the amphibian (Figure 13-9). The telencephalon of this 



group is peculiar in that the olfactory bulbs lie dorsal to the 

 anterior ends of the elongate cerebral lobes. A "prepara- 

 physis" arises froi» the roof anterior to the velum transver- 

 sum. 



The diencephalon is relatively small with a large saccus 

 dorsalis of chorioid tissue forming its roof On this is a 

 pineal body, whose stem extends down and back toward the 

 posterior commissure. The habenular nuclei are prominent 

 and lie to either side of the pineal stem. The habenulae are 

 joined by a (superior) commissure. The hypothalamus has 

 slight inferior lobes. The optic lobes are fused to form a 

 single midline mass. The cerebellum is a narrow transverse 

 ridge as in the amphibian. 



Neoceratodus is somewhat different in that the cerebral 

 hemispheres are smaller and with membranous dorsal and 

 medial walls. This membranous part forms a thick, folded, 

 glandular chorioid plexus which continues as the roof of the 

 diencephalon. In the young, the dorsal and medial walls of 

 the lobes are nervous, the adult condition is achieved by an 

 eversion suggesting that of the actinopterygian. The optic 

 lobes are slightly separated. 



Acfinisfian The brain of Latimeria is distinct from that of 

 the dipnoans. It is extremely small as compared with the 

 cranial cavity (a parallelism to the situation in Acipenser); 

 except for the olfactory bulbs it lies entirely behind the dor- 

 sum sella, in the occipital part of the skull. The olfactory 



THE CONDUCTING AND INTEGRATING SYSTEM 



391 



