37 



sometimes broad as in Scoliodon terrai-novse (Fig. 21), but more fre- 

 quently narrow and slit-like as in Squalus. 



The fibers composing the two great divisions of the olfactory nerve 

 enter the peripheral end of the olfactory tractus, forming with this, 

 near the base of the olfactory cup, a rounded enlargement called the 

 bulbus olfactorius. The central ends of the fila olfactoria divide within 

 the bulbus, into brush-like tufts, which come into close relation with 

 similar tufted endings of dendrites belonging to a different series of 

 cells (mitral cells), situated deeper in the bulbus. The rounded masses 

 formed by union of the union of the terminal tufts of the fila olfactoria, 

 and those of the dendrites of the mitral cells, are designated glomeruli 

 olfactorii. They mark the division between the neurons of the first 

 and second order, respectively, and are structural landmarks in con- 

 sidering the olfactory nerve. The neuraxons of the neurons of the 

 second order, pass backward through the tractus and reach various 

 central terminals. 



These are the well known relations of the olfactory nerve. It is 

 to be remarked, that the new nerve has no connection with glomeruli 

 or other parts of the olfactory lobe, but is made up of neurons of the 

 first order, extending from the olfactory membrane directly to central 

 terminals within the brain. 



In front, the prosencephalon of Squalus acanthias is divided by 

 a median furrow which extends to the ventral surface. Posteriorly, 

 the prosencephalon is undivided and merges gradually into the thalam- 

 encephalon, from the base of which extends the optic nerve (n. opt). 

 The epiphysis is not represented in the drawing. Behind the thalam- 

 encephalon come, in order, the mid-brain (ms'ewc), the cerebellum 

 (cbl.) and the medulla oblongata. 



The course of the new nerve may now be described. Merely for 

 purposes of description , it may be spoken of as passing from the 

 brain to the olfactary cup without prejudicing the question of the 

 source of origin of its fibres. Starting within median furrow from 

 two roots — an upper, slender, and a lower broader one — it passes 

 forward and laterally to the olfactory cup. In adults it loops down- 

 ward and sometimes appears on the ventral surface of the prosencepha- 

 lon, but in some adults, and in all earlier stages, it crosses the 

 anterior surface of the prosencephalon. It then curves in the angle 

 formed by the union of the olfactory tractus and the fore-brain, and 

 continues its course along the inner margin of the former, until it 

 reaches the base of the bulbus — here it has a ganglion. In front of 

 the ganglion it divides, unequally, into three stems (not shown in 



