INTRODUCTION, 19 
inclosed by the skin of the tadpole; and the tadpole, to become a frog, 
parts with its tail, mouth, and branchie. The child, at its birth, loses its 
placenta and membranes; at a certain period its thymus gland nearly dis- 
appears, and it gradually acquires hair, teeth, and beard; the relative size 
of its organs is altered, and its body augments in a greater ratio than its 
head, the head more than the internal ear, &c. 
The place where these germs are found, and the germs themselves are 
collectively styled the ovary; the canal through which, when detached, 
they are carried into the uterus, the oviduct; the cavity in which, in many 
species, they are compelled to remain for a longer or shorter period pre- 
vious to birth, the wterus; and the external orifice through which they 
pass into the world, the vulva. Where there are sexes, the male impreg- 
nates the germs appearing in the female. The fecundating liquor is 
called semen; the glands that separate it from the blood, testes; and when 
it is requisite it should be carried into the body of the female, the iatro- 
ductory organ is named a penis. 
Of ihe Intellectual Functions of Animals. 
The impression of external objects upon the mx, the production of a 
sensation or of an image, is a mystery into which the human understand- 
ing cannot penetrate; and materialism an hypothesis, so much the more 
conjectural, as philosophy can furnish no direct proof of the actual exist- 
ence of matter. The naturalist, however, should examine what appear to 
be the material conditions of sensation, trace the ulterior operations of the 
mind, ascertain to what point they reach in each being, and assure himself 
whether they are not subject to conditions of perfection, dependent on the 
organization of each species, or on the momentary state of each individual 
body. 
To enable the mE to perceive, there must be an uninterrupted commu- 
nication between the external sense and the central masses of the medul- 
lary system. It is then the modification only experienced by these masses 
that the mE perceives: there may also be real sensations, without the ex- 
ternal organ being affected, and which originate either in the nervous chain 
of communication, or in the central mass itself; such are dreams and vi- 
sions, or certain accidental sensations. 
By central masses, we mean a part of the nervous system, that is so 
much the more circumscribed, as the animal is more perfect. In man, it 
consists exclusively of a limited portion of the brain; but in reptiles, it 
includes the brain and the whole of the medulla, and of each of their parts 
taken separately, so that the absence of the entire brain does not prevent 
sensation. In the inferior classes this extension is still greater. 
The perception acquired by the mx, produces the image of the sensation 
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