Encyclopaedia Britannica > Volume 3, Athens-BOI

AKATOMY.]
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,
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 Elern. of Embryology; see also Huxley, Anat. Vert. Anim.,
p. 301, figs. 90, 91.
2 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
Sauropsida is also given in the text.
725
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 Chelonia 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 pecten 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 Amphisbasnoidca 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 Amphisbcenaf). 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 Balamiceps. It is generally sagitti-
form 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. dt., 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.
The alimentary canal of Birds may have several dilata-
3 See Macgillivray, Brit. 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. lil ;
Huxley on “ Incus and Stapes,” p. 398 ; and Parker, var. loc.
BIRDS