580 Dynamic Theory. 



served to be faintly contained upon the retina, which is formed by the 



unfolding of the nerve. ( See fig. 190.) 



Other Nerves. The fifth nerve, Trigeminus, is enormous in all fishes. 

 In the Lancelet the fifth nerve distributes many filaments to the ex- 

 panded sensitive integument, which represents the head and forms the 

 sides of the wide mouth, and also supplies the oral tentacles. In the 

 myxinoids the same nerve supplies both the muscles and the integuments 

 of the head, the tentacles, the nasal tube, the mucous membrane of the 

 mouth and tongue, the hyoid and palatal teeth, and the pharynx. The 

 trigeminus supplies the same parts in the Lamprey, but by fewer pri- 

 mary branches. ' ' That which sends filaments to the rectus externus and 

 rectus inferior of the eyeball, is continued forward beneath the skin, 

 and resolves itself into a rich plexus, which supplies the thick cirrate 

 border of the suctorial lip ; the nerves to the muscular parts of the 

 jaws and tongue arise distinct from the fifth nerve, and their trunk may 

 be regarded as a facial nerve. The auditory appears to be a primary 

 branch of the fifth, in Skate. Swann calls it glosso pharyngeal. In 

 the Cod, and all fishes above Dermopteri, the auditory is a distinct 

 nerve rising between the fifth pair and the vagus. " 



There is no spinal accessory ( eleventh pair ) in fishes or snakes, but 

 it is to be found 4n mammals, birds, and all reptiles, except snakes. 

 The general position of the points of exit from the brain of the nerves, 

 is practically the same in all the vertebrates in which they exist. The 

 spinal nerves generally correspond in number with the ribs. They are 

 in pairs, and pass out on each side through the vertebrae, or between 

 them. 



Reptiles. In a newt weighing 39 grains, the brain weighs ^ of a 

 grain, and in the large Sirens, Amphiumes and Menopomes the brain is 

 relatively still smaller. In these reptiles there is a fourth ventricle, as 

 in fishes. It is bounded at its upper end by a feeble rudiment of a 

 cerebellum, formed by the confluence and convergence of the sides of 

 the ventricle. The Axolotl has a long elliptical optic lobe, which gives 

 off small optic nerves below. It has the pituitary and pineal glands, 

 the latter large and extending from before the optic lobe upon the rear 

 of the interspace between the cerebral hemispheres. These cerebral 

 hemispheres are twice as long and broad as the optic lobe, are smooth 

 and hollow like Lepidosiren. Olfactory lobes are seated upon the fore 

 and outer part of the hemispheres. Cerebral ventricles continue into 

 the olfactory lobes. In serpents the cerebellum is smaller than the op- 

 tic lobes, and the optic lobes are less than one-fourth as large as the 

 cerebrum. The cerebellum covers the greater part of the fourth ven- 

 tricle, and is a semicircular lobe somewhat depressed or flattened. The 

 optic lobes are hollow, and are crossed by a transverse fissure near the 



