Book II. ANIMAL PHYSIOLOGY. 293 



1968. Besides those substances which animals wake rise of as food, voter is likewise employed as drink, 

 and as the vehicle of nutritious matter. Salt is necessarily mixed with the drink of the inhabitants of 

 the ocean, and is relished by man and many other animals. Other inorganic substances are likewise 

 employed tor a variety of purposes. Many savages make use of steatite and clay along with their food. 

 The common earthworm swallows the soil, from which, in its passage through the intestines, it extracts 

 its nourishment. 



196R In so?ne cases, substances are swallowed fur other purposes than nourishment. Stones are retained 

 in the stomach of birds to assist in triturating the grain. The wolf is said to satisfy his hunger by filling 

 his stomach with mud. 



Sect. II. Of the Circulating System. 



1970. The food being reduced to a pultaceous mass, and mixed with a variety of secreted 

 fluids, by means of the digestive organs, is in this state denominated chyme. This mix- 

 ture exhibits a chemical constitution nearly approaching that of blood, into which it is 

 destined to be converted, by the separation of the useless from the useful part. This is 

 effected by certain vessels called lacteals, which absorb the nutritious part of the chyme, 

 and convey it to a particular receptacle. Another set of absorbents, the lymphatics, 

 take up all the substances which have been ejected from the circulation, and which are 

 no longer necessary in the particular organs, and communicate their contents to the store 

 already provided by the lacteals. The veins receive the altered blood from the extremities 

 of the arteries or the glands, in which they terminate, and proceed with it towards the 

 lungs, to be again aerated. In their progress they obtain the collected fluid of the other 

 absorbents, and, in the lungs, again prepare the whole for the use of the system. Thus, 

 during the continuance of life, the arteries supply the materials by which the system is 

 invigorated and enlarged, and oppose that tendency to decay, produced by the influence 

 of external objects. The process continues during the whole of life, new matter is daily 

 added, while part of the old and useless is abstracted. The addition is greatest in early 

 life, the abstraction is greatest in old age. 



1971. This continued system of addition and subtraction has led some to conclude, that a change in th 

 corporeal identity of the body takes place repeatedly during the continuance of life; that none of th.- 

 particles of which it consisted in youth remain in its composition in old age. Some have considered the 

 change effected everv three, others every seven, years. This opinion, however, is rendered doubtful by 

 many well known facts. Letters marked on the skin by a variety of substances frequently last for life. 

 There are some diseases, such as small-pox and measles, of which the constitution is only once susceptible; 

 but it is observed to be liable to the attack of these diseases at every period of human life. 



Sect. III. Of the Reproductive System of Animals. 



1972. Animals are reproduced in consequence of the functions of certain organs, with 

 the exception of some of the very lowest in the scale. In those animals which possess 

 pectdiar organs for .ne preparation of the germ or ovum, some are androgynous (man- 

 woman), and either have the sexual organs incorporated, and capable of generating 

 without assistance, or the sexual organs are distinct, and the union of two individuals is 

 necessary for impregnation : others have the sexual organs separate, and on different 

 individuals. The voting of such animals are either nourished at first by the store of 

 food in the egg, or by the circulating juices of the mother. Those species in which the 

 former arrangement prevails are termed oviparous, while the term viviparous is restricted 

 to the latter. 



1973. In all animals it is the business of the female to prepare the ovum or germ, and 

 bring it to maturity. For this purpose, the germ is produced in the ovarium, farther 

 perfected in the uterus or matrix, and finally expelled from the system through the 

 vagina. The office of the male is to impregnate the germ by means of the spermatic 

 fluid. This fluid is secreted in the testicles, transmitted by the spermatic ducts, and 

 finally conveyed by the external organ to its ultimate destination. 



1974. Among the viviparous animals, the reproductive organs present many points of 

 resemblance, and appear to be constructed according to a common model. It is other- 

 wise with the sexual organs of the oviparous tribes. These exhibit such remarkable 

 differences in form and structure that it is impossible to collect them into natural groups, 

 or assign to them characters which they have in common. 



1975. The manner in which the eggs of birds are impregnated by the male has not been 

 satisfactorily determined. With the exception of the cicatricula, a female bird, in the absence 

 of the male, can produce an egg. The conjunction of the sexes, however, is necessary 

 for the impregnation of the egg, and the effect is produced previous to the exclusion. 



1976. In many kinds of fishes and reptiles, the yolks, after being furnished with their 

 glair, are ejected from the body of the female, and the impregnating fluid from the male 

 is afterwards poured over them. Impregnation can be effected readily in such cases, by 

 the artificial application of the spermatic fluid. 



1977. Impregnation in insects appears to take place while the eggs pass a reservoir 

 containing the sperm, situated near the termination of the oviduct in the volva. 



1978. The most siniple mode of hatching is effected by the situation in which the eggs are placed by the 

 mother, after or during their exclusion. ' In this mode a place i* usually selected where tin- eggs will be 



U :! 



