COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 301 



' sac sinks internally and by constrictions differentiates into various chambers and ducts 

 which will be seen in the dissection. The middle ear appears in Amphibia. It is a chamber 

 produced by an evagination from the first visceral pouch and remains connected with the 

 pharyngeal cavity by the stalk of the outgrowth called the auditory (Eustachian) tube. 

 The middle ear contains little bones for transmitting the sound; there are three of these in 

 mammals, which were treated with the skull. The external ear begins in reptiles and birds 

 and is complete in mammals. It consists of a passage invaginated from the region of the 

 first gill slit; this passage is the external auditory meatus. The inner end of this passage 

 comes in contact with the wall of the middle ear, the two walls then fusing to form a mem- 

 brane of double origin, the tympanic membrane, commonly called the eardrum. The 

 external orifice of the meatus is surrounded by an outgrowth, the pinna (generally called 

 "ear"), for catching sound waves. 



For more complete accounts of the development, comparative anatomy, and functions 

 of the central nervous systems and sense organs, the appropriate chapters in K, W, or Wd 

 should be consulted. 



B. THE NERVOUS SYSTEM AND SENSE ORGANS OF ELASMOBRANCHS 



i. The spinal nerves and fin plexi. Remove all of the viscera including the 

 kidneys and reproductive organs from the pleuroperitoneal cavity. Note 

 against the dorsal coelomic wall dorsal to the pleuroperitoneum the white nerves 

 passing out at segmental intervals. These are the ventral rami of the spinal 

 nerves. They lie along the myocommata. In the spiny dogfish they are 

 buried in the muscle and will be revealed by cutting along the myocommata. 

 Farther laterally they. emerge to the internal surface. Trace the ventral rami 

 into the hypaxial muscles. 



In the regions of the paired fins the ventral rami supply the muscles of the 

 fins and are more or less united with each other to form a plexus. The plexus 

 for the posterior appendage is the lumbosacral plexus, for the anterior appendage, 

 the cermcobrachial plexus. These plexi are as follows: 



Dogfish: The lumbosacral plexus to the pelvic fin is found by cutting 

 through the skin on the dorsal side of the base of the fin. On carefully separat- 

 ing the fin muscles from those of the trunk the nerves of the plexus are seen as 

 white cords passing into the base of the fin. They are more or less imbedded 

 in connective tissue which should be carefully cleaned away. There are ten 

 nerves passing into the fin of which, however, only the last ones are united by 

 cross-branches to form a plexus. The first of the ten is called the collector nerve. 

 Trace it forward and note that it is formed by the union of branches from the 

 ventral rami anterior to the fin. 



The cervicobrachial plexus to the pectoral fin is located by cutting through 

 the skin at the base of the fin on the ventral side. On separating the skin from 

 the muscles of the trunk nerves will be seen passing in the connective tissue 

 to the pectoral fin. Proceed carefully forward, carrying your cut into the coelom 

 at the side of the esophagus. The plexus is then seen to consist of a number of 

 nerves (nine in the smooth dogfish, eleven in the spiny species) passing from the 



