COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 361 



visceral motor nerve to the corresponding muscles. In addition, the vagus nerve is dis- 

 tributed extensively to the heart, lungs, stomach, and other viscera. The vagus arises from 

 the medulla. 



27. The eleventh or spinal accessory nerve of amniotes is probably a part of the vagus 

 isolated as a separate nerve. It springs from the medulla and upper spinal cord and is a 

 motor nerve to certain visceral muscles. 



28. The twelfth or hypoglossal nerve of amniotes is probably a spinal nerve which has 

 become included in the cranial cavity. It arises in the medulla and innervates the muscles 

 of the tongue. 



29. The chief sense organs of the head are the eyes, ears, and nose. 



30. The nose at first consists of a pair of olfactory sacs not communicating with the 

 mo.uth cavity. In land vertebrates they establish connections with the mouth cavity for 

 respiratory purposes. The nasal cavities thereafter have both respiratory and olfactory 

 functions, the latter being limited to the posterior regions of the cavities. 



31. In higher vertebrates, particularly mammals, the walls of the nasal cavities develop 

 complex outgrowths, the turbinals or conchae, for the purpose of increasing both respiratory 

 and olfactory surfaces. 



32. The eyes are compound structures. The nervous part of the eye is formed by an 

 evagination from the brain. The lens of the eye is an invagination from the adjacent 

 ectoderm. The coats of the eye, sclera, and chorioid, are formed in the surrounding mesen- 

 chyme. The eye is moved by muscles which are very constant in arrangement in the 

 different vertebrate classes, except that in mammals the superior oblique operates by means 

 of a pulley. 



33. The ear consists of internal, middle, and external portions. The internal ear is an 

 invagination from the ectoderm. It differentiates into the three semicircular ducts, the 

 sacculus, the utriculus, and the endolymphatic duct. Fishes and many urodeles possess 

 only the internal ear. 



34. The middle ear is added to the internal ear beginning with Amphibia. It consists 

 of a chamber developed by outgrowth from the first visceral pouch. The outer wall of this 

 chamber comes in contact with the skin, producing a double-walled membrane, the tympanic 

 membrane. Within the middle ear is a chain of ear bones, two in number in most vertebrates, 

 three in mammals. 



35. Beginning with reptiles the tympanic membrane sinks into the skull, leaving a 

 passage, the external auditory meatus, extending from the tympanic membrane to the 

 exterior. This passage is deepened in birds and mammals, and in the latter a fold of skin, 

 the pinna, develops around the external rim of the meatus. Pinna and meatus constitute 

 the external ear. 



36. The internal ear of mammals is more complicated than that of other vertebrates, 

 owing to the development of a spiral outgrowth, the cochlear duct, from the sacculus. 



