302 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



spinal cord into the fin. Only the first four or five of these, situated on the dorsal 

 side of the bag formed by the posterior cardinal sinus, are united by cross- 

 branches to form a true plexus, the posterior ones passing directly into the fin. 



Skate: A large number of ventral rami supply the pectoral fin, the anterior 

 ones uniting to a plexus. Strip off the pleuroperitoneum at the level of the sub- 

 clavian artery and note there the enormous nerve trunk of the brachial plexus. 

 It is formed by the union (within the neural canal) of a large number of ventral 

 rami. This will be seen later. Follow out the nerve trunk to the pectoral fin. 

 It lies along the posterior side of the curved cartilage (propterygium) which is 

 situated in the pectoral fin about halfway from the mid-dorsal line to the margin. 

 Cut through skin and muscles on the dorsal side of the animal along the posterior 

 and lateral side of this cartilage and expose the trunk. It supplies only the 

 anterior part of the pectoral fin. The posterior part as already noted is supplied 

 by direct ventral rami, not forming a plexus. The lumbosacral plexus for the 

 pelvic fin is located as follows. Remove the skin from the base of the fin on the 

 dorsal side. This exposes a fan-shaped layer of muscles. Cut through this and 

 just ventral to it will be found a number of nerves which diverge into the fin 

 muscles. 



2. The sense organs. — For the rest of the section a large separate head will 

 generally be provided. In that case the specimens used up to this point may be 

 discarded. A very careful dissection of this head, on which the student will be 

 graded, is required. 



a) The ampullae of Lorenzini: It has already been noted that the skin of 

 the head is perforated by pores, from which mucus exudes under pressure. 

 Note the distribution of the pores. Remove a piece of skin from a region bear- 

 ing pores (in the skate from the ventral side of the head) and note that each 

 pore leads into a canal of varying length lying beneath the skin. Each canal, 

 named the canal of Lorenzini, terminates in a little bulb, the ampulla of Lorenzini, 

 which is supplied by a nerve, a delicate white fiber easily seen attached to the 

 ampulla. The function of this sensory apparatus appears to be the perception 

 of vibration and pressure in the surrounding water. 



b) The lateral line system: In fishes and Amphibia (larval stages only of land 

 Amphibia) there is present a system of sense organs, called the lateral line system, 

 which is related to the aquatic mode of life. This system is completely lost, 

 together with the nerves which supply it, in the land vertebrates. It consists 

 of the lateral line canals and the lateral line nerves. The lateral line canals in 

 fishes consist of tubes situated on the inner surface of the skin onto which they 

 open by pores. In the canals are sensory cells which closely resemble the sen- 

 sory cells of the ear, and, in fact, the lateral line system appears to be related 

 to the internal ear both morphologically and functionally. The function of the 

 lateral line system is believed to be the perception of water vibrations of low 

 frequency. 



