SPINAL CORD. NOSE. EYES. 109 



{zii.) which supplies the tongue muscles. It is called the hypoglossal, and 

 gives off near its origin a branch which supplies the anterior part of the 

 dorsal muscles. 



The spinal cord is flattened ; it has neither dorsal nor ventral 

 fissure, though traces of a dorsal fissure may be represented by 

 a fine tract of connective tissue which passes from the dorsal 

 side of the central canal to the dorsal surface of the cord. 



The spinal nerves have dorsal and ventral roots which unite 

 in Myxinoids, but not in Petromyzon. The posterior roots 

 possess a ganglion, which lies just outside the skeletogenous 

 wall of the neural canal. All the nerves are without the medul- 

 lary sheath and the motor fibres are larger than the sensory. 



In Petromyzon the dorsal root of the first spinal nerve enters the septum 

 between the fourth and fifth myomeres, the ventral root divides and supplies 

 the fourth and fifth myomeres. The motor root of the second spinal 

 nerve supplies the fifth and sixth myomeres, while the third and subse- 

 quent spinal nerves each supply one myomere only. 



Sense Organs. The external nostril and the nasal sac are 

 single and median, though the olfactory nerves are double. 

 From the ventral side of the nasal sac a tube — the nasopalatine 

 canal — is continued backwards between the brain and the skull 

 floor, passes through the basicranial fontanelle and ends 

 blindly on the ventral side of the anterior end of the notochord 

 in Petromyzontidae, whereas in Myxinidae the same tube opens 

 posteriorly into the mouth. The palatal opening of this 

 canal has nothing to do with the posterior nares of higher 

 vertebrates. It appears to be derived from the pituitary 

 invagination of the embryo, which arises in Marsipobranchs, 

 not as in most Vertebrates from the mouth, but as an 

 ectodermal invagination in front of the mouth, which secondarily 

 becomes connected with the nasal pit. It is for this reason 

 sometimes called the pituitary pouch. 



The eyes are normal in Petromyzon, and possess the usual 

 eye muscles. 



In the Myxinoids they are extremely reduced and without 

 eye-muscles. In Bdellostoma they are embedded in a spherical 

 fatty mass, and placed beneath the skin which is without 

 pigment immediately over them. In Myxine they lie deeper 

 within the muscles close to the skull wall. 



