300 Veterhiary Obstetrics 



come so intimately fused that they fail to open at the proper 

 time, a condition which is not usually subject to remedy. In 

 other cases, the cornea fails to undergo the normal transforma- 

 tion into a transparent body, but remains opaque. In yet other 

 instances, there is seen growing from the neighborhood of the 

 lacrymal duct long hairs, which irritate the eye but which are, 

 at times, subject to surgical removal. In still other instances, 

 there is a contraction of the inner, or mucous, layer of the eye- 

 lid or a comparative overgrowth of the external skin, by which 

 means the eyelids are inverted, producing entropium. 



Thk E.\r. 



The first traces of the ears consist of open pits opposite to the 

 hind-brain at an early period in the life of the mammalian em- 

 bryo, usually at about ten to fifteen days. These pits .sink in 

 deeply until they come in close contact with the hind-brain and 

 early acquire a communication with the auditory nerves. The 

 mouths of the pits soon close and the invaginated portion be- 

 comes separated from the external epiblast, to constitute the 

 auditory vesicles, in which the essential parts of the ear, the 

 semicircular canals, vestibule, cochlea, etc., are developed. 



The Eustachian tube is formed from the hyomandibular pouch, 

 which extends out from the pharynx as a diverticulum. At one 

 time it pushes out against the epiblast and is .separated from the 

 exterior by a very thin membrane consisting of epiblast exter- 

 nally, in immediate contact with the inner layer of hypoblast, 

 with no mesoblast between. Later, a layer of mesoblast grows 

 in between the epiblast and hypoblast and the", three constitute 

 the tympanic membrane. This pouch does not normally reach 

 the surface at any time during fetal life, but occasionally opens 

 abnormally in various mammalian animals, to constitute gill-slit 

 fistula, which rarely persists at the time of birth in a manner an- 

 alogous to the open gills of fishes and other similar animals. In 

 the solid ungulates, there is a large infundibulum formed in con- 

 nection with the Eu.stachian tube, which is known as the guttural 

 pouch, or air sac. The slit-like communication of this guttural 

 pouch with the pharynx is sometimes abnormal in the new-born 

 foal, which causes air to become impacted in it (tympany of the 

 guttural pouch), which may so press upon the larynx as to 

 strangle the young animal. 



