PETROMYZON 27 



into the central nervous system. These nerve-cells are not 

 in the nerve-tube but just outside it. They lie on the track 

 of the sensory dorsal nerves and form swellings or ganglia. 

 In Petromyzon and all higher forms, on every dorsal nerve- 

 root, whether cranial or spinal, there is a ganglion. The 

 ventral nerves consist of nerve-fibres which are formed from 

 nerve-cells which lie inside the nerve-tube ; they therefore 

 do not have ganglia. 



In Petromyzon there are two important primitive features 

 to note in connexion with the nerves. One is that the dorsal 

 and ventral roots of each segment do not join together, but 

 remain separate. The other is that the nerves are simple and 

 uncovered by insulating material, i.e. they are non-medullated. 



The nerves which innervate striated muscles go straight 

 from the central nervous system to the muscle. On the other 

 hand, those nerves which supply the smooth muscle-fibres of 

 the gut and of the arteries (and in higher forms certain other 

 structures also) do not run direct from the central nervous 

 system to the muscle. Instead, they run to other nerve-cells, 

 and these run to the muscle. These latter nerve-cells form 

 part of the autonomic nervous system (see Chapter XXXI). 

 In Petromyzon this system is only feebly developed. It is 

 represented by some groups of nerve-cells along the gut, 

 supplied by the intestinal branch of the vagus (see p. 47), and 

 also by some cells close to blood-vessels near the spinal cord. 



Skull and Skeleton. — The brain, paired sense-organs, and 

 roots of the cranial nerves are protected by a case of cartilage 

 forming the skull. This is characteristic of all forms above 

 Amphioxus which are therefore referred to as Craniata. The 

 brain is surrounded by the cranium proper ; the sense-organs 

 are protected by capsules. The nasal capsules are fixed on to 

 the front of the cranium by connective tissue ; the auditory 

 capsules are firmly fused on to the sides of the cranium by 

 cartilage. The spinal cord enters the skull at the hind end 

 through the foramen magnum, but in Petromyzon the nerves 

 of the fourth and following segments (glossopharyngeal and 

 vagus) come out from the brain behind the hindmost limit of 

 the skull. This shows that the process of cephalisation or 



