146 ANATOMY OF VERTEBRATES. 



superocclpital into the back part of the falx 1 : the tentorium re- 

 ceives a bony plate in many Delphini. 2 In Seals both the tento- 

 rium and hind part of the falx are ossified, and a thick ridge 

 enters the fore and under part of the falx between the rhinence- 

 phalic fossae. The tentorium is ossified in the Carnivora to the 

 extent, and in the families, noted in vol. ii., where the conditions 

 of such bony plate are discussed at p. 506. 3 A short tentorial 

 ridge projects anterior to the cerebellar fossa of the petrosal in 

 Lemur macaco.* The tentorial margin of the petrosal is slightly 

 produced in Cebus, and to a greater extent in Ateles. In other 

 Quadrumana, as in Man, the sole ossification co-extended with any 

 part of the dura mater is that called ( crista galli ' in Anthropo- 

 tomy. An unossified process from the middle of the posterior 

 border of the tentorium, extending from the internal occipital crest, 

 projects into the notch between the hemispheres of the human 

 cerebellum, and is termed ' falx minor ' and ' falx cerebelli.' 



§ 211. Nerves of Mammals. — The olfactory nerves are absent 

 in all the Cetacea save those with baleen, in which they are few 

 and small; they are present in all other Mammals, and are sent oft 

 in greater number from their cerebral centre — the rhinencephalon 

 — than in lower Vertebrate classes. 5 The Ornithorkyjichus is the 



1 xliv. p. 442. 2 lb. No, 2500, p. 453. 



3 A more extensive series of comparisons of the interior of the skull has tended to 

 rectify the physiological view entertained at the period of the publication of the 

 posthumous edition of the * Legons d'Anatomie Comparee,' of Cuvier, vol. ii. p. 290 ; 

 vol. iii. p. 155. * xliv. p. 722. 



5 Anthropotomists still describe the connections and course of the ' crura rhinen- 

 cephali ' as the origins of the olfactory nerve ; although they recognise that, ' unlike 

 other nerves, a large proportion of grey matter is mixed with the white fibres,' &c. 

 (lxii". vol. ii. p. 583, 186G), and might rectify the notion by many weightier 

 anatomical conditions. Some even maintain the view by such remarks as the 

 following : ' As it is known that in the first development of the ear the peripheral 

 part or vestibular expanse, as well as the rest of the acoustic nerve, is originally 

 formed by the extension of a hollow vesicle from the first or hindmost foetal encephalic 

 compartment, so in the case of the cms cerebri, although the peripheral or distributed 

 part (crus rhinencephali or olfactory nerve) is of separate origin from the hemispheric 

 bulb, this latter part is comparable in its origin with the acoustic vesicle.' I have 

 paraphrased the argument of the editors ©f lxii" (vol. ii. p. 584), to show that 

 development, as a vesicle in connection with nervous centres, is no ground of homology 

 or homotypy. Whenever a false homology has to be maintained, the earliest and 

 obscurest phenomena of embryonal development are usually resorted to in support of 

 such view. 



The terminal expansion of the acoustic nerve is in an organ which begins as 'a 

 follicle or hollow vesicle;' the terminal expansion of the optic nerve is also in a 

 vesicle; and the true olfactory nerves expand terminally on what began as a follicle or 

 vesicle, which form is retained, little altered, in Fishes. The vascular pituitary 

 membrane supporting that expansion is the homotype of the choroid supporting the 

 l-etina. No doubt the cerebellum is at first a vesicle, as is the optic lobe, and the 

 hemisphere, and the olfactory lobe; and each may claim to be regarded as the 



