PISCES 183 



connected with the Vllth and Xth nerves. These are brought 

 into relationship with the VHIth in the acoustic centre of the 

 medulla and act as distant receptors of movements of the water. 

 They are characterised by the secretion of mucus and have 

 been found to be allied to taste in function, and the presence 

 of the mucin gives rise to the impression that the organs may 

 act as receptors also of electrical waves produced by the move- 

 ments of living bodies even at a distance. 



This is only a rough sketch of the complicated architecture 

 of the nervous system and the associated organs for making it 

 more perfect. It is as much as the student can be expected 

 to make out by dissection and by the inspection of preparations. 

 It is important to note that the morphology of the nervous 

 system described above, which at once springs into place with 

 the advent of the Craniates, is preserved in essentials in all. 

 A due appreciation of the nature of the apparatus as a whole 

 may be obtained by considering its general structure. 



The motor roots of the spinal cord are related to the 

 muscles derived from the myotomes. In the embryo the 

 myotomes do not end at the anterior end of the vertebral 

 column. We know that they are present in the posterior 

 part of the skull, and we have good reason for believing that 

 the parachordal mesoderm is derived from myotomes which 

 either are not marked as such or have a fleeting history. With 

 relation to the myotomes of the posterior part of the skull, 

 which are apparent for a long period during development, 

 ventral roots exactly similar to those of the spinal cord are 

 formed. These undergo repression, both the myotomes and 

 the roots, in front ; but the survivors give rise to the muscles 

 of the tongue, or the corresponding muscles and the roots form 

 the nerve of these muscles, the Xllth or hypoglossal. These 

 are duly developed in the skate and its allies, but are lost. In 

 front of the hypoglossal region two other ventral motor roots 

 are developed in similar manner, the Vlth and the Illrd, 

 and these together with the dorsal IVth are directed to the 

 inner vation of the eye muscles. 



These muscles are late in appearance relatively, and are 

 developed from the mesoderm of the walls of certain cavities 

 which appear in the mesoderm and which are also late in 



