BRAIN. 



321 



central vermis and two small lateral lobes. There is a parietal 

 organ in Sphenodon (Fig. 178) and Lacertilia, which is described 

 below under Lacertilia. There are twelve pairs of cranial 

 nerves except in snakes in which the spinal accessory is absent. 

 The facial is not united with the trigeminal, and the glosso- 

 pharyngeal is an independent nerve though it has several con- 

 nections with the vagus. The 

 hypoglossal or twelfth cranial nerve 

 passes out through a foramen or 

 sometimes more than one foramen 

 the exoccipital bone. The 9th 



in 



10th, and 11th 

 skull together. 



nerves leave the 



The 3rd nerve gives off a twig to the 

 muscle of the upper eyeUd. The 6th 

 supplies the muscles of the nictitating 

 membrane and the retractor bulbi. The 

 5th nerve has two roots, a smaller motor 

 and a larger sensory. The ophthalmic 

 nerve which sometimes has a special 

 ganglion corresponds to the R. ophthal- 

 micus superficiahs portio trigemini of 

 fishes. The 7th has become mainly a 

 motor nerve ; it gives off a palatine nerve, 

 which may anastomose with the superior 

 maxillary, and a mandibular branch which 

 enters into similar relations with the 

 inferior maxillary. The main nerve 

 passes back dorsal to the columella auris 

 and suppUes the muscles of the hyoid, the 

 cutaneous muscles of the neck, and the 

 mylohyoid. The 7th nerve sends off 

 anastomosing branches to the 9th (Jacob- 

 son's anastomosis). The 10th nerve 

 possesses a ganglion of the trunk as well 

 as of the root. 



The spinal accessory is a part of the 

 vagus which becomes distinct in Sauro- 

 psida and Mammalia. It arises by several 

 roots from the spinal cord between 



the dorsal and ventral spinal nerve-roots as far back as the third 

 spinal nerve, and passes forwards through the foramen magnum into the 

 skull which it leaves in association with the vagus. It supphes the 

 trapezius and other muscles. Part of its fibres enter the vagus. 



The hypoglossal may be regarded as being generally homologous with 

 the spino-occipital nerves (ventral vagus roots) of fishes. It represents 

 a variable number of anterior spinal nerves which have lost their dorsal 

 roots and become associated to form an additional cranial nerve. 



Fig. 177. — Brain of the Alligator, 

 dorsal view (after Rabl Riickhard), 

 Cb cerebellum ; Mh optic lobes 

 (corpora bigemina) ; Mo medulla 

 oblongata ; / olfactory lobes ; II 

 optic, IV trochlear, V trigeminal, 

 VIII auditory, IX glossopharyn- 

 geal, X vagiis, X/ spinal accessory 

 nerves ; IC, 2C first and second 

 spinal nerves ; Vh cerebrum. 



Z.— II. 



