BRAIN, CRANIAL NERVES. 



107 



fibres with the right of two thickenings on the superior com- 

 missure, called the right ganglion habenulae. The ventral and 

 smaller vesicle also presents in its lower wall, though not so 

 markedly, features which recall retinal structure. It is hollow 

 and is connected with a small solid body on which it lies, and 

 which is the anterior part of the small left ganglion habenulae 

 (Fig. 55, 3). These two bodies, though in contact, are apparently 

 not connected. The pineal stalk is connected to the roof of the 

 brain just in front of the posterior commissure, while the 

 parietal organ is attached just anterior to the superior com- 

 missure. The pineal body lies close to the roof of the skull, 

 and the skin above it is not pigmented (Fig. 

 56). 



The hypophysis or pituitary body is dorso- 

 ventrally flattened and follicular in structure. 

 It lies beneath the infundibulum. 



In Myxinoids the corpora bigemina are present in 

 the normal form and there is no thin place in the 

 roof of the iter. The thalamencephalon is not nearly 

 so prominent, the optic lobes being approximated to 

 the cerebral lobes. The anterior part of the brain is 

 solid, the central canal not extending beyond the mid- 

 brain. A small space in the region of the thalamen- 

 cephalon may be made out but it is quite isolated 

 from the iter. A pineal body, has, so far, not been 

 found in the myxinoid brain. 



Fig. 56. — Dorsal view 

 of the head of Pe- 

 tromyzon planeri 



(after Ahlborn). Na 

 external nasal aper- 

 ture ; Ep position of 

 the epiphysis (the 

 non-pigmented char- 

 acter of the skin at 

 this spot is not clearly 

 shown). 



The cranial nerves are fairly normal in 

 their arrangement. There are ten pairs, but 

 in Myxinoids the third, fourth and sixth 

 appear to be entirely absent, in correspond- 

 ence with the absence of eye-muscles. The optic nerves, which 

 are very small in Myxinoids, appear not to cross, the chiasma 

 being hidden in the substance of the brain. In Petroniyzon the 

 sixth nerve arises close to the fifth, and supplies the inferior 

 rectus as well as the external rectus. There is said to be no 

 lateral line branch of the vagus in Myxinoids, but in the lam- 

 preys this nerve is well developed and reaches to the end of 

 the tail. It is, however, in an unusual position, being placed 

 far from the skin close to the neural sheath of the vertebral 

 column, and it appears to be connected with the posterior 

 roots of the spinal nerves dorsal to which it passes. 



