liv Inti'oduction. 



can only be considered to be rudimentarily represented by a portion 

 of the inner wall of either lateral ventricle. The anterior com- 

 missure is not, the posterior is largely developed. The optic thalami 

 are smaller than the optic lobes. Tactile sensibility is limited to the 

 beak. Smell and taste are lowly developed. The cochlea of the 

 ear is a much simpler, the tympanum a much more extensive cavity 

 relatively than in Mammalia. The eyes are never absent, and, 

 though small in the Apteryx, are never rudimentary. The globe of 

 the eye consists of two segments, the anterior one of which is more or 

 less obtusely conical, whilst the posterior is spheroidal ; bony plates 

 inlaid in the anterior part of the sclerotica preserve the relative 

 conformation of the two portions. As in many lower Vertebrata, 

 a vascular process is prolonged from the choroid into the interior of 

 the bulb. This structure is known in Birds as the 'pecten,' or, 

 from its shape in Ratltae, as the ' marsupium \' and in many 

 grallatorial and aquatic Birds it reaches to the lens. It is absent 

 in Apteryx. The vitreous humour is relatively smaller than in 

 Mammals. Except in Owls and aquatic Birds, the lens is flat. 

 The ciliary muscle upon which the bird^s power of accommodating 

 the eye so as to obtain clear vision at very rapidly varying distances 

 depends, is, in correspondence with this need, composed of trans- 

 versely striped muscular fibres. The muscular fibres of the iris are 

 of similar character. A special muscular apparatus and a special 

 (Harderian) gland are developed, in relation with the third eyelid 

 or membrana nictitans. 



As in the Sub-kingdom Arthropoda, the sexes difier much ex- 

 ternally; and in the case of the Accipitres, the females, as is so 

 commonly observable in that Sub-kingdom, are larger than the 

 males. 



The testes are always retained within the abdomen anteriorly to 

 the kidneys; the left is occasionally the larger of the two. The 

 vasa deferentia are often dilated towards their terminations, but in 

 neither sex are there ever any accessory glands, distinct from and 

 appended to the generative canals by ducts, as are the Cowperian, 

 the prostatic glands and the vesiculae semiuales of many Mammals. 

 The right ovary is usually atrophied, and when it is persistent, as 

 in some Accipitres, its ova do not come to maturity. In the upper 

 part of the oviduct the albumen, in the lower the calcareous shell of 

 the egg is formed. Many young Birds are, as are also the young 



