BEAIX. 103 



hemispheres, with a single body wedged in between their 

 posterior half; in Petromyzon, at least, the vascular tissue 

 leading to an epiphysis seems to be connected with this body. 

 Then follows the lobus ventriculi tertii, distinctly paired in 

 Myxiuoids, less so in Petromyzon. The last pair are the 

 corjpora quadrigemina. According to this interpretation the 

 cerebellum would be absent in Myxinoids, and represented in 

 Petromyzon by a narrow commissitre only (Fig. 45, 5), stretching 

 over the foremost part of the sinus rhomboidahs. In the 

 Myxinoids the medulla oblongata ends in two divergent swell- 

 ings, free and obtuse at their extremity, from which most of 

 the cerebral nerves take their origin. 



The Nerves which supply the organs of the head are either 

 merely continuations or diverticula of the brain-substance, or 

 proper nerves taking their origin from the brain, or receiving 

 their constituent parts from the foremost part of the spinal 

 chord. The number of these spino-cerebral nerves is always 

 less than in the higher vertebrates, and their arrangement 

 varies considerably. 



A. Nerves which are diverticula of the hrain (Figs. 41-45). 



The olfactory nerves {first pair) always retain their inti- 

 mate relation to the hemispheres, the ventricles of which are 

 not rarely continued into the tubercle or even pedicle of the 

 nerves. The different position of the olfactory tubercle has 

 been already described as characteristic of ^ome of the orders 

 of fishes. In those fishes in which the tubercle is remote 

 from the brain, the nerve which has entered the tubercle as 

 a single stem leaves it split up into several or numerous 

 branches, which are distributed in the nasal organ. In the 

 other fishes it breaks up into branchlets spread into a fan-like 

 expansion at the point, where it enters the nasal cavity. The 

 nerve always passes out of the skull through the ethmoid. 



