52 DESCRIPTIONS OF PREPARATIONS. 



bules ; and (2) semiplumes (penno-plumae) which have a stiff rachis and soft barbs 

 and lie at the outer margin of the pterylae and are covered by the contour feathers. 

 In a few birds down-feathers exist which grow persistently and break off at the 

 apex; whilst a powder or dust is poured out of the follicle lodging the tube. 

 They are known as powder-down feathers, and either occur scattered all over the 

 body e. g. in some Parrots, or restricted to limited tracts e.g. in Ardea. 



The tuft of feathers springing from the pollex constitutes the bastard wing, 

 The row of large wing-feathers is termed remiges, and is divisible into a set of 

 primaries and of secondaries attached, the former along the manus, the latter 

 along the ulna. They are covered above and below by the upper and lower wing 

 coverts. The large tail feathers are known as rectrices, and they are covered above 

 and below by the tail coverts. The feathers are not implanted irregularly into 

 the body but along certain tracts or pterylae between which are bare spaces or 

 apteria. 



The following special points should be noted in the internal anatomy. In 

 the central nervous system the small olfactory lobes; the cerebral hemispheres 

 pointed in front, broad behind, showing in horizontal section a huge corpus stri- 

 atum, a lateral ventricle reduced to a narrow chink and thin internal and pos- 

 terior walls : a small pineal gland, reverted, with walls composed chiefly of fibrous 

 tissue, and its extremity attached to the dura mater : the solid optic lobes widely 

 separate in the middle line where the cerebellum touches the cerebral hemispheres : 

 the cerebellum composed of a large median lobe and two small lateral floccular 

 lobes, the median lobe showing in longitudinal section an arbor vitae as in Mammalia : 

 the well-marked angle between the medulla oblongata and spinal cord. The 

 latter has a large lumbar swelling, in which a mass of neuroglia or substantia 

 reticularis lies immediately dorsal to the central canal, the posterior fissure 

 is widely open and the gap filled by a gelatinous tissue derived from the pia 

 mater. This lumbar swelling was of immense size in the extinct Stegosaurus 

 (Deinosaurid). The spinal cord ends with a filum terminale and the posterior 

 nerves form a cauda equina. The sympathetic system is double in the neck : one 

 part accompanies the vertebral artery and vein and is lodged in the vertebrarterial 

 canal : the other accompanies the carotid arteries on the ventral aspect of the neck. 

 The two parts are connected. 



The structure of the eye is peculiar in some points. The sclerotic coat has 

 an anterior conical portion containing a ring of bones, and a posterior spheroidal 

 portion. A pigmented vascular fold of membrane the pecten runs obliquely 

 forwards from the entrance of the optic nerve and projects into the vitreous 

 humour. The line of attachment marks the position of the embryonic choroidal 

 fissure. Its capillaries are continuous with those of the optic nerve, and not of 

 the choroid, and are contained within lymphatic sheaths. The nictitating mem- 

 brane is moved by two special muscles a quadratus or bursalis, and a pyrami- 

 dalis, which lie at the back of the eye and take origin from the sclerotic. The 

 former is a square muscle ending in a tendinous border, but the tendon is 

 tubular. Through the tube runs the cord-like tendon of the pyramidalis which is 

 inserted into the lower angle or edge of the nictitating membrane. When the 

 pyramidalis contracts, its tendon is prevented from pressing on the optic nerve 

 over which it runs by the simultaneous contraction of the quadratus. There is 

 a well-developed Harderian gland for the third eye-lid lying below the eye-ball. 



