COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 327 



to these nerves, a branch of the facial will be found crossing the anterior horn 

 near its dorsal end and passing into the muscles lying along the posterior border 

 of the mandible. 



Make a median longitudinal incision through the whole floor of the mouth 

 and pharyngeal cavities and open the two flaps so that the roof of these cavities 

 is revealed. Locate the vagus (really vago-sympathetic) trunk in the neck and 

 trace it anteriorly to its point of exit from the skull, removing the mucous membrane 

 from the roof of the pharyngeal cavity. The vago-sympathetic trunk passes to 

 the dorsal side of the hypoglossal nerve seen above and there enters a ganglion, 

 the superior cervical ganglion of the sympathetic. F-om this ganglion numerous 

 branches pass out. Internal to the hypoglossal locate the glossopharyngeal 

 nerve, the carotid artery being situated between the two. Slightly anterior to 

 these will be found the facial nerve, as it exits from the skull. Its branches pass 

 to the muscles between the anterior horn of the hyoid and the lower jaw, one of 

 them curving over the ventral surface of the horn as noted above. The vagus 

 nerve proceeds posteriorly and supplies the heart and other viscera. It will be 

 noted that the facial, glossopharyngeal, and vagus nerves are much reduced, 

 owing to the loss of the lateral line system and the gill apparatus. Note, how- 

 ever, that these nerves continue to supply the remains of the visceral arches and 

 their visceral muscles. 



Add the cranial nerves to your drawing of the brain. 



5. Ventral aspect of the brain. Remove the brain from the skull and 

 examine the ventral surface. On the ventral surface of the diencephalon note 

 the optic chiasma, the infundibulum just behind this, with the hypophysis pro- 

 jecting ventrally from the latter. Note the roots of the abducens and hypogossal 

 nerves arising from the ventral surface of the medulla. 



6. Median sagittal section. Make a median sagittal section and study the 

 cut surface. Identify the fourth ventricle in the medulla, the aqueduct or passage 

 below the cerebellum, the optic ventricle in the optic lobe, the third ventricle in 

 the diencephalon. Note the increased size of the diencephalon as compared 

 with the elasmobranch, and the backward extension of the cerebral hemi- 

 sphere over the diencephalon. The diencephalon is divided into epithalamus, 

 thalamus, and hypothalamus as in the dogfish, each including the parts previ- 

 ously enumerated. Note that the cerebral hemisphere presents a solid medial 

 wall, called the septum. Cut into the roof or pallium of the hemisphere. Note 

 its cavity, the lateral ventricle, and the large mass protruding from the floor into 

 the ventricle; this mass is the corpus striatum. 



The functions of the parts of the turtle brain are similar to those stated for 

 the elasmobranch brain. The cerebral hemisphere is still largely olfactory, 

 although its lateral surface is beginning to assume the functions characteristic 

 of the mammalian hemisphere. 



Draw the section. 



