NECTURUS 71 



absent. Slit open one of the lungs and note the thinness of the 

 walls. Examine carefully the structure and mechanism of the 

 glottis. 



Exercise 8. Draw the lungs. 



Remove the lungs and the urogenital organs from the body. 



The Nervous System. This system is composed of the follow- 

 ing divisions: (1) the central nervous system, consisting of the 

 brain and the spinal cord 5(2) the peripheral nervous system, con- 

 sisting of the cranial and spinal nerves and the sympathetic nerv- 

 ous system; and (3) the special sense organs. 



The Special Sense Organs : the Auditory Organs. The ear con- 

 sists of a membranous labyrinth, or inner ear, alone, no external 

 opening being present. It is relatively of large size and may be 

 studied in this dissection. The auditory capsule, which incloses the 

 labyrinth on each side, is composed largely of cartilage and forms 

 the hinder part of the skull. 



The membranous labyrinth is made up of the following parts: 

 ( 1 ) the vestibule, an irregular sac occupying the ventral and middle, 

 portions of the capsule and formed of two main divisions, the up- 

 per utriculus and the lower sacculus; (2) the semicircular canals, 

 three delicate tubes which rise from the utriculus and extend to 

 the dorsal wall of the capsule; and (3) the ductus endolymphat- 

 icus, a delicate tube which passes dorsally from the sacculus. 



The auditory capsules will be recognized from the outside as 

 a pair of prominences at the hinder end of the skull. Remove 

 the skin and muscles from the dorsal side of the skull and care- 

 fully shave off the hard bone and the cartilage which form the 

 roof of one of the auditory capsules. The semicircular canals 

 will first come into view. Two of these are medial in position 

 and one lateral. The two medial canals are known as the anterior 

 and the posterior vertical canal. The anterior one is the larger ; 

 its forward end, close to its junction with the vestibule, is en- 

 larged and forms the ampulla. The posterior canal has an ampulla 

 at its hinder end. The lateral canal is called the horizontal canal ; 

 its ampulla, which is at its forward end, is close to that of the 

 anterior vertical canal. 



