AVES. 



375 



arbor vitae as in Mammalia, and two small floccular lobes. The extinct 

 Cretaceous birds had large olfactory lobes, and small hemispheres, their 

 brain resembling to a certain extent that of the Alligator. The spinal 

 cord has a lumbar swelling with the posterior fissure widely open and filled 

 by gelatinous tissue. The cervical sympathetic is double, one part accom- 

 panying the common carotids, the other running in the vertebrarterial 

 canal. The fore-part of the sclerotic is obtusely conical, and contains a 

 ring of bones ; the hind-part spheroidal, and there is in some birds a bony 

 plate in it near the entrance of the optic nerve. A vascular pigmented 

 process, the pecten or marsupium, projects into the vitreous humour in the 

 line of the choroidal fissure and is absent only in Apteryx. The muscular 

 fibres of the iris are striated, and the radial portion of the striated ciliary 

 muscle, known as Crampton's muscle, large, a bird having apparently ex- 

 ceptional powers of accommodation. The movements of the third eyelid are 

 governed by two special muscles, the quadratus and pyramidalis, and its 

 gland, the Harderian gland, is large. The stapes or columella auris is well- 

 developed, provided with processes at its outer end, and it has an epihyal 

 element added to it. The posterior and the horizontal semicircular canals 

 unite where they cross, and the cochlea contains no otoliths, but has a 

 cuticular membrane, the homologue of the membrana Corti of the Mam- 

 mal. The tympanic cavity is continued into the lower jaw by a membra- 

 nous or bony canal, the siphonium. A nasal gland, lying on the nasal bone 

 or extending to the frontals above the orbit, pours its secretion into the 

 nose. 



All living birds are edentulous. But in some embryo Parrots there 

 are dental papillae attached to the periosteum of the jaws, supplied by 

 vessels and nerves, and capped by cells of the rete mucosum. These cells 

 become cornified, and contain air. Three such papillae are found on the 

 maxillae, and ten in shallow alveoli on the mandibles of Melopsittacus. 

 They are however hidden in every instance beneath the horny covering of 

 the bill. In the Lamellirostres ( = Chenomorphae] the edges of the bill are 

 raised into oblique lamellae, and in the Mergansers the edges of the bill are 

 produced into pointed processes, supported by corresponding bony processes. 

 In Odontopteryx from the London Clay there are similar bony processes, 

 large and small intermingled. Hesperornis and Ichthyornis from the Creta- 

 ceous strata possess numerous true teeth, composed of dentine and enamel 

 with large pulp cavities. In the former the praemaxillae are edentulous, 

 and the teeth lodged in grooves of the maxillae and mandibles. They have 

 fangs composed of osteodentine. The succession is lateral as in Mosa- 

 saurus and the Crocodile among Reptilia. The teeth of Ichthyornis are 

 implanted in sockets, and the succession is vertical as in Deinosauria. 

 Archaeopteryx from the Solenhofen slates also has teeth which probably 

 resemble those of Hesperornis. The tongue is well-developed, especially in 



