THE BRAIN 



l6l 



Neuromeres. — In its development the brain shows evident traces 

 of having once been a segmented structure, consisting of a series of 



Fig. 167. — Median section of brain of calf, based on a figure by Butschll. a, 

 -queduct; ac, anterior commissure; cc, corpus caliosum; /, fornix; k, habenula; hy, 

 Hypophysis; t, infundibulum; tm, intermediate mass ('soft commissure') ;wi,mammillary 

 body; ob, olfactory bulb; oc, optic chiasma; ol, optic lobes; p, pinealis; pc, posterior 

 commissure; r, recessus suprapinealis; s, septum pellucidum; ///, IV, third and fourth 

 ventricles. 



Fio. z68. — Diagram of the segments (neuromeres, myotomes, etc.) of the head, 

 after Neal. A, anterior myotome; a, abducens nerve; b, branchial clefts; /, facial 

 nerve; g, glossopharyngeal nerve; Aj hypoglossal nerve; /, lens, surrounded by layers 

 of eye; n, nasal pit, near it the terminalis nerve; 0, oculomotor nerve; op*, ophthalmicus 

 profundus part of fifth nerve; os*, ophthalmicus superficialis part of fifth nerve; ot, 

 otocyst; s, spiracular cleft; t, trigcmmal nerve; ta, truncus arteriosus; tr, trochlearis 

 nerve; I-VIII, neuromeres; 1-6, myotomes. 



segments or neuromeres metamerically arranged. Concerning the 

 number of these and their relations there has been an enormous dis- 



