paraphysis 



velum tronsversum 



habenular commissure 



palliol commissure 



parietal organ 



parietal nerve 



saccus dorsalis 



chorioid plexus entering foramen of Monroe^ 



cerebrum 



anterior commissure 



olfactory buib. 



medulla oblongata 

 trochlear cftiosma 



pars nervosa of pituitary 

 ^\\ anterior lobe 



Figure 13-4. Medial view of the right half of the brain of tocerto vivipora, (After von Kupffer) 



covering most of the surface of the cerebrum of the mam- 

 mal is only a dorsolateral band in the reptile (Figure 13-5). 

 The archipallium (hippocampal cortex) covers the dorsal 

 and medial aspects, while ventrolaterally there is an exten- 

 sive paleopallium (olfactory area). The pallial commissure 

 is not developed as a corpus callosum. The cerebral lobes 

 are proportionally smaller than in the mammal and the ol- 

 factory bulbs project anteriorly from them. 



From the extreme posterior part of the roof of the telen- 

 cephalon, a chorioid invagination occurs which enters the 

 cerebral ventricles and a preparaphysis extends upward; 

 behind this is a hippocampal commissure in the velum trans- 

 versum. Behind the velum is an irregularly digitated dorsal 

 sac, a parietal (parapineal) eye on a long stalk, and a 

 pineal evagination (epiphysis). The pineal structure is 

 sometimes partly glandular. Both parietal and pineal organs 

 are lacking in the crocodilian but a dorsal sac is present; 

 only a well-developed pineal is present in the bird. 



In some lizards and Sphenodon the parietal organ is 

 developed as a light sensitive "eye." There are clear lens 

 cells above, and below are sensory cells which connect through 

 ganglion cells with cells of an external ganglion. The fibers 

 of the external ganglion form a parietal nerve which extends 

 down to the left habenular ganglion. The parietal eye is 

 enclosed ventrally by pigment cells and lies in the parietal 

 foramen of the skull, close beneath translucent scales of the 

 skin. In early fossil reptiles the skull has a parietal foramen 

 suggesting that this eye was well developed. 



The remainder of the diencephalon is generally like the 



mammal. The optic nerves decussate completely (only half in 

 the mammal). There is a lateral geniculate nucleus at 

 which a part of the optic nerve fibers synapse with cells 

 connecting with the posterior part of the cerebral cortex 

 (visual area). There is a massa intermedia in the turtle but 

 not in the other reptiles or birds. 



Behind the diencephalon, nearly in contact with the cere- 

 bral lobes, are large optic lobes. Behind the optic lobes is a 

 fairly large cerebellum and behind this a medulla with a 

 chorioid plexus in its roof A pons is present in the bird, and 

 suggested in the turtle. 



The cranial nerves of the reptile are like those of the 

 mammal. The vomeronasal division of the olfactory, which 

 innervates the Jacobson's organ, is distinct in the turtle or 

 lizard, absent in the alligator or bird. Jacobson's organ is an 

 accessory olfactory and taste organ. In addition, there is a 

 parietal nerve which, like the optic, represents a brain tract. 

 In some lizards (Iguana) there is a pair of parietal nerves 

 entering the right and left habenular nuclei and a pineal 

 nerve entering the posterior commissure on the right. The 

 remainder of the nervous system agrees with the mam- 

 malian plan both in its structure and development. 



Amphibians 



The brain of the amphibian differs from that of the rep- 

 tile only in the general reduction of the cerebral lobes and 

 cerebellum (Figure 13-6). There are large olfactory bulbs be- 



388 



THE NERVOUS SYSTEM 



