1 62 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



cussion and a great lack of unanimity of opinion. Apparently there 

 is the best support for the following statement. There are at least 

 eight neuromeres in the brain (figs. 145, 168), five in front of the ear, 

 one corresponding to the ear in position, and at least two behind 

 that organ. The first of these gives rise to the fore-brain and the 

 parts arising from it, as described on page 148. The second develops 

 into the mid-brain; the third is in the region of the cerebellum, while 

 the fourth and fifth are at the anterior end of the medulla oblongata 

 where the trigeminal and facial nerves arise. The sixth, besides 

 being related in position to the ear, gives ofif the glossopharyngeal 

 nerve, and the rest are connected with vagus nerve. Some claim two 

 '~Y fore-brain neuromeres and a larger number for the postotic region. 



Envelopes (MENDSfCEs) of the Central Nervous System 



Both brain and spinal cord are surrounded by envelopes (menin- 

 ges) of connective tissue which support and protect them, and 



Fig. 169. — Section of spinal cord of dogfish (Acanthias). The section is a little 

 oblique, passing through a dorsal root of a spinal nerve (d) on one side and a ventral 

 root (v*) on the other, c, cartilage of the vertebral column; tn, meninz primitiva; 

 p, perchondrium (endorhachis of the spinal canal. In the cord are shown the dis- 

 tribution of white and gray matter, the central canal and some of the divisions of the 

 white matter into fibre tracts. 



also, by carrying blood-vessels, provide for their nourishment- 

 These meninges become more complicated with ascent in the verte. 

 brate series. The canal of the vertebral column and the cavity of the 

 skull are lined with a layer of connective tissue, the endorhachis, 

 which is really the periosteum or perichondrium of the skeletal parts 

 and hence not a true meninx. In the fishes (fig. 169) there is a single 

 envelope, the meninx primitiva, which bears the blood-vessels and 

 lies close upon the spinal cord. Between it and the endorhachis is a 



