124 ZOOLOGY SECT. 



middle vesicle (pn.) is in connection with the small left ganglion 

 habenulge. The optic nerves differ from those of the higher 

 classes in the fact that each passes directly to the eye of its 

 own side. 



The spinal cord (Figs. 752 and 757, my.} is flattened and baiid- 

 like. The dorsal roots of the spinal nerves alternate with the 

 ventral and do not unite with them to form a trunk. There is 

 no sympathetic. The hypoglossal-is the first spinal nerve. 



Sensory Organs.- -The external nostril (Fig. 752, na-" Fig. 754, 

 na. ap.} leads by a short passage into a rounded olfactory sac 

 (Fig. 752, na, Fig. 754) placed just in front of the brain and having 

 its posterior wall raised into ridges covered by the olfactory or 



na.a^^ 



\ sjb ol ffP P n * L l.gn.hb 



\\ ^^=W. Jrtf-lcrb.h/ MJ 



, ,, JVv.10 

 med.obl ^ 



Nv.4 N 



FIG. 754. Petromyzon. Side view of brain with olfactory and pituitary sacs in section. 

 cblm. cerebellum; crb. //. cerebral hemisphere; dien. diencephalon ; /. fold in nasal tube; 

 gl. nasal glands ; inf. iufundibulum ; I. fjn. hb. left ganglion habenulse ; rned. obi. medulla 

 oblongata ; na. ap. nostril ; nch. notochord ; Nv. 1, olfactory nerve ; Nv. 2, optic ; Nr. 3, oculo- 

 motor ; Nv. k, trochlear ; Nv. 5, trigeminal ; No. 6, abducent ; Nv. 7, facial ; Nv. 8, auditory ; 

 Nv. 10, vagus ; N*\ 12, hypoglossal ; olf. cp. olfactory capsule ; olf. I. olfactory lobe ; olf. m. m. 

 olfactory mucous membrane ; opt. 1. optic lobe ; pn. middle pineal body ; pn'. inferior pineal 

 body ; pn. e. pineal eye ; pty. b. pituitary body ; pt>/. p. pituitary pouch ; sp. median septum 

 of olfactory sac ; sp. 1, dorsal root of first spinal nerve. (Combined from figures by Ahlborn 

 and Kaenische.) 



Schneiderian membrane (Fig. 754, olf. m. m.). From the bottom 

 of the sac is given off a large pituitary pouch (Fig. 752, no.' . 

 Fig. 754, pty. _/;>.) which extends downwards and backwards, be- 

 tween the brain and the skull-floor, passes through the basi-cranial 

 fontanelle, and ends blindly below the anterior end of the 

 notochord. 



The relations between the olfactory sac, the pituitary pouch, and 

 the pituitary body are very remarkable. In the embryo, befoiv 

 the stomodseum (Fig. 755, A, stdm.) communicates with the mesen- 

 teron, two unpaired ectodermal invaginations appear in front of 

 the mouth. The foremost of these is the rudiment of the olfac- 

 tory sac (olf. s.). The other, which is situated between the olfactory 

 sac and the mouth, is the pituitary sac (pty. s.), which in this case 



