TERATOLOGY. 



Teratology is the science of monstrosities, or aberrations in 

 the development of the embryo. The subject is too extensive 

 for any definite consideration in a treatise upon obstetrics. The 

 extent of deviation from the normal may be of every gradation. 

 There may be merely a perceptible deviation from the normal, 

 which we usually designate a defect, or arrest, in development. 

 Or the deviation from the normal may be so extreme as to result 

 in an almost shapeless mass, such as the acardia, or "mole", 

 illustrated on page 289, or a double monster such as is illustrated 

 in Figs. 126 and 127. Such deviations are known as monsters 

 and, as a general rule, the departure from the normal is such 

 that it prevents the animal from living after birth or compromises 

 the existence of the mother in the act of birth, so that such ab- 

 normal individuals are rarely born alive or, should this occur, their 

 defects in structure are such as to nearly always cause their early 

 death. In the lesser deviations from the normal, we sometimes 

 fail to observe the defect at the time of birth and it is only when 

 it has caused disease that we are led to note the fundamental de- 

 fect in development. Such is well illustrated by the defects of the 

 teeth, which are mentioned on page 314. 



In the preceding chapter, under the head of Embryology, we 

 have mentioned numerous forms of monstrosities and defects in 

 development in order to draw a contrast between the normal 

 embryological development and the abnormal, or teratologic, de- 

 velopment. We have also aimed, in that chapter, to point out 

 the causes of these deviations, so far as they are well known, 

 and to forge a connecting link between teratology and disease. 

 Neither have we attempted to give an extended account of embry- 

 ology, but have aimed to aid the veterinary student, as far as 

 possible, to get the logical connection between embryology and 

 the subjects of obstetrics and surgery. The veterinary obstet- 

 rist needs know, so far as possible, the embryologic foundation 

 of certain defects, and of diseases arising from these in our do- 

 mestic animals, because intelligent handling of these must be 

 based upon such an understanding. 



"We shall have further occasion to refer to some forms of aber- 

 ration in development when dealing with the subject of dystokia, 

 since some of them lead to very great difficulty in parturition and 

 tax to. the utmost the resources of the obstetrist. We shall again 



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