ELASMOBRANCnn 435 



branches. There is also a system of lymph spaces, which supple- 

 ments the blood circulatory system. 



Respiratory System 



The gills in the wall of the pharynx are constantly bathed in 

 water forced through from the mouth. An exchange of carbon 

 dioxide for oxygen is made by the blood as it passes through the 

 capillaries of the gills. This is made possible by diffusion of these 

 gases through the membranes of the gill lamellae. The gills are 

 supported by cartilaginous gill arches. 



Nervous System 



The central nervous system consists as usual in vertebrates of the 

 hrain and spinal cord. The brain includes two large olfactory lobes 

 at its anterior, followed by two cerebral hemispheres, a depressed 

 diencephalon, a pair of large optic lobes, a well-developed cerebellum, 

 and behind this the tnedulla oblongata. There is a very slight con- 

 striction between olfactory lobes and cerebrum. The cerebellum is 

 divided into quarters by a longitudinal groove and a transverse 

 groove. It covers a part of the optic lobes as well as the anterior 

 portion of the medulla oblongata. On the lateral walls of the medulla 

 are located the acusticolateral areas, including the earlike auricles. 



The cavity within the medulla, which opens to the dorsal surface 

 beneath and behind the cerebellum, is the fourth ventricle. There 

 are ten pairs of cranial nerves which are numbered and named from 

 anterior to posterior: I, olfactory; II, optic; III, oculomotor; IV, 

 trochlearis; V, trigeminus; VI, abducens; VII, facial; VIII, audi- 

 tory ; IX, glossopharyngeal ; X, vagus. The olfactory nerves extend 

 from the olfactory lobes ; optic from diencephalon and optic lobes; 

 oculomotor from ventral side of optic lobes or midbrain; trochlear 

 from dorsal side of optic lobes between them and cerebellum; the 

 trigeminus, abducens, facial, and auditory all from the anterior 

 portion of the medulla oblongata; the glossopharyngeal from a 

 more posterior part of the sides of the medulla. These last two 

 supply the gills, lateral line, and certain viscera. 



The spinal cord is tubular and somewhat flattened. It extends the 

 length of the vertebral column and gives off paired spinal nerves seg- 

 mentally. 



