THE NEURAL TUBE AND ITS DERIVATIVES 



2 9 



be torn away so as to expose the third ventricle (Figs. 8, 9), is attached by its 

 caudal margin to a ridge containing a pair of knob-like thickenings, the habe- 

 nular nuclei and a commissure connecting the two (Fig. 11). From a point 

 just caudal to the middle of this commissure there projects forward over the 

 membranous roof of the ventricle a slender tube, the epiphysis cerebri or pineal 

 body, which comes in contact with the roof of the skull and ends in a slightly 

 dilated extremity. The epiphysis and habenular nuclei belong to the epithala- 

 mus. The thalamus forms the thick lateral wall of the third ventricle and is 

 traversed by the optic tracts on their way to the optic lobes. The hypothalamus 



Nasal sac 



Epiphysis 

 Superior oblique 



Trochlear nerve 

 Medial rectus 

 Superior rectus 

 Lateral rectus 

 Vestibule 



Spiracle- 



Semicircular canal 

 Glossopharyngeal nerve 



Vagus - 

 Branchial cleft i 



Superficial ophthalmic V, VII 

 Olfactory capsule 



Inferior oblique 



Maxillary V 

 Mandibular V 

 Palatine VII 



Spiracle 



Hyomandibular VII 

 Glossopharyngeal 



i. Branchial cleft 

 Vagus 



Spinal cord Lateral line branch of vagus 



Fig. 12. Dissection of the brain and cranial nerves of the dogfish, Scyllium catulus. The 

 eye is shown on the left side, but has been removed on the right. (Marshall and Hurst, Parker 

 and Haswell.) 



is relatively large in the shark and presents, in addition to a pair of laterally 

 placed oval masses, or inferior lobes, a thin walled vascular outgrowth, the saccus 

 vasculosus. Closely related to the ventral aspect of the hypothalamus is a gland- 

 ular mass, derived by a process of evagination from the oral epithelium, and 

 known as the hypophysis. For a picture of this structure in the adult dogfish 

 reference should be made to a paper on the subject by Baumgartner (1915). 

 On the ventral surface of the hypothalamus the optic nerves meet and cross in 

 the optic chiasma. 



The telencephalon includes all of the brain in front of the velum transfer sum, 



