foramen of Munro with the third ventricle), which is little more than a fissure between the very thin inner wall of the lobe and its thick outer part, which contains the corpus striatum. The corpora striata are united by an anterior commissure, which is not of large size. The thinning of the inner wall of the lobes, from the margin of the foramen of Munro backwards, which gives rise to the fissure of Bichat in the Mammalia, extends for a very short distance in the Sauropsida, even in Birds. The olfactory lobes are usually elongated, and contain ventricles continuous with those of the prosencephalic hemispheres. In all the Sauropsida the motor nerves of the tongue pass through a foramen in the occipital bone. Hence twelve pairs of cranial nerves are present, except in the Ophidia, which possess no spinal accessory nerves. The lateral cutaneous branches, so gene rally sent to the trunk by the pneumogastric in the Ichthyopsida, are absent, but the pneumogastric gives a recurrent branch to the larynx. The third, fourth, and sixth nerves arise quite independently of the fifth. The
sympathetic is well developed.[1]
Organs of Sense.
Birds possess nasal glands, which attain a large size, and lie more usually upon the frontal bone, or in the orbits, than in the nasal cavity. In the Snakes and Lizards these bones lie between the septo-maxillaries above and the vomers below. In Birds, wherever placed, the duct opens near the same region as in the Reptiles. In many kinds, especially Passerines, the bones that should cover them are really present, but are rudimentary and attached to the vomer. These are the septo-maxillaries.
The eye in many Birds, as in the extinct Ichthyosauria, attains very great absolute and relative dimensions. Birds possess, like many Reptiles, a nictitating membrane.[2] In the Lizards a short, thick muscle (bursalis) is attached to the inner and posterior wall of the orbit, and ends in a fibrous sheath. A tendon, one end of which is attached to the presphenoidal region of the inner wall of the orbit, passes backwards through the sheath, and then forwards, to be attached to the nictitating membrane. When the muscle contracts, it necessarily pulls the latter over the eye. A Harderian gland is always developed, and a lachrymal gland very generally, but not always. In the Chelonia, muscular fibres (forming the so-called pyramidalis muscle) arise from the inner side of the eyeball, and, arching over it at the optic nerve, are inserted partly into the outer edge of the nictitating membrane, partly into the lower eyelid. The Crocodilia have a pyramidalis muscle taking the same origin and course; but it sends no fibres to the lower eyelid, its tendon being inserted alto gether into the nictitating membrane. The third arrange ment, which in a manner brings together the first and the second, is that seen in Birds. A pyramidalis muscle, arising from the inner and under surface of the eyeball, soon ends in a tendon which sweeps round the upper and outer surfaces of the sclerotic to the nictitating membrane, as in the Crocodiles. But there is also a bursalis muscle, which however arises, not, as in Lizards, from the wall of the orbit, but from the upper surface of the sclerotic itself, whence it passes backwards and ends in a fibrous sheath which encloses the tendon of the pyramidalis. The con traction of the muscle necessarily tends to draw the tendon of the pyramidalis away from the optic nerve. A tubercle is sometimes developed from the sclerotic above the en trance of the optic nerve, and prevents the tendon of the pyramidalis from shifting forwards and inwards. The eyeball is always turned by four recti and two obliqui muscles. The superior oblique does not pass over a pulley. The Chdonia and most Lacertilia have a more or less com pletely developed retractor or choanoid muscle. A ring formed of bony plates is developed in the fore-part of the sclerotic in Lacertilia, Chelonia, Ichthyosauria, Dicyno- dontia, Pterosauria, and Aves ; but not in Ophidia, Plesio- sauria, or Crocodilia. The iris and tensor choroidei contain striated muscular fibres. A pecteu is very generally de veloped. It attains a large size, and becomes much plaited in Aves. Even in Birds, the sclerotic is cartilaginous.
In the organs of hearing, also, the Bird is best studied as a culmination of the Sauropsida,
Only Crocodilia and Aves possess a rudiment of an ex ternal ear. The Ophidia and the Amphisbwnoidca have no tympanic cavity. In some Chelonia, in Sphenodon, and in the Chameleons, the tympanic membrane is covered by integument, but a tympanic cavity exists. In Lacertilia the tympanic cavities communicate by wide openings with the pharynx ; but in Chelonia, Crocodilia, and Aves, the communicating passages, reduced in size, become eustachian tubes. In the Chelonia these curve backwards, downwards, and inwards, round the quadrate bones, and open separ ately on the roof of the mouth. In the Crocodilia there are three eustachian tubes one median, and two lateral. In Aves there is but one eustachian aperture, answering to the median of the Crocodilia ; and, as in the latter group, each eustachian tube usually traverses the osseous base of the skull to join its fellow in the common aperture.
The stapes is a columelliform bone, the outer end of which is attached to the tympanic membrane when the latter is developed, but lies among the muscles when there is no tympanic cavity (Snakes and Amphisboenae], All Sauropsida possess a fenestra rotunda, as well as a fenestra ovalis ; and all have a cochlea, which is never coiled spirally, and is more rudimentary in the Chelonia than in other groups. Three semicircular canals, an anterior and a posterior vertical and an external horizontal, are con nected with the membranous vestibule. In Aves, the anterior vertical canal is very large in proportion to the others, and the adjacent crura of the two vertical canals overlap before they unite with one another.[3]
Alimentary Organs.
Well-developed sub-lingual, sub-maxillary, and parotid glands appear in Birds, and the sub-lingual glands attain an immense size in the Woodpeckers (Macg., Brit. Birds, vol. iii. plate 15). The tongue varies greatly, being some times obsolete (as in the Crocodile). It is small in the Totipalmatce and in Balceniceps. It is generally sagitti- f orm and papillate at the base ; but it is thick, and even emarginate in the Rapaces (Macg., Brit. Birds, vol. iii. plates 19, 20). In the Picidce (op. cit., plate 15), where the hyoid bones are extremely elongated and the tongue pre- hensible to a marvellous degree, the true lingual part is a small arrow-head, covered with a prickly, horny sheath ; these prickles are reverted, like a Snake s teeth.
- ↑ 1 See Owen, Art. "Aves," Cyclop. Anat. Phys., pp. 298, 299; Macgillivray, Brit. Birds, vol. i. p. 48, and vol. iii. plate 18 ; Owen " On Brain of Apteryx," Trans. Zool. Soc., vol. vii. plates 45 and 46, p. 381. For the development of the Fowl s brain, see Foster and Bal four s Elem. of Einbryology ; see also Huxley, Anat. Vert. Anim., p. 301, figs. 90, 91.
- ↑ See Macgillivray, Brit. Birds, vol. iii., plate 17, p. 146, for excel lent figures and descriptions of the Bird s eye ; also Owen, article "Aves," Cyclop. Anat. and Phys., p. 303. For its development, Foster and Balfour s work, p. 97. The Bird s eye being a more highly specialized Reptilian organ, its condition in the lower types of the Sauropsidv, is also given in the text.
- ↑ See Macgillivray, Bnt. Birds, vol. iii. plate 18, figs. 2 and 3, p. 156 ; Owen, article "Aves," in Cyclop. Anat. and Phys., p. 308, fig. 141. For development, see Foster and Balfour s work, p. Ill ; Huxley on " Incus and Stapes," p. 398 ; and Parker, var. loc.
