102 GENERAL ANATOMY. 



The eyelids are formed at the end of third month, as small cutaneous folds, 

 which come together in front of the globe and cohere. This union is broken 

 up, and the eyelids separate before the end of fetal life. 



The lachrymal canal appears to result from the non-closure of a fissure which 

 exists between the external nasal process and the maxillary process (p. 98). 



Development of the Ear. The first rudiment of the ear appears about the same 

 time as that of the eye, in the form of a vesicle (primitive auditory vesicle, Fig. 

 60, 11) situated close on the outside of the third cerebral vesicle, though not 

 communicating with it. It is formed by a depression of the epithelium over 

 the second pharyngeal arch, which becomes converted into a closed sac. From 

 this vesicle the internal ear is developed. The auditory nerve is described either 

 as a projection from the third cerebral vesicle, or as an independent formation 

 which unites with both, and thus establishes a communication between the 

 cerebral and the auditory vesicles. The middle ear and Eustachian tube con- 

 stitute the remains of the first pharyngeal or branchial cleft. The formation 

 of the ossicles of the tympanum has been already pointed out, viz., the \ncus 

 and malleus from Meckel's cartilage, and the stapes, with its muscle, from the 

 second pharyngeal arch. These parts project into the first pharyngeal cleft, 

 which remains occupied by connective tissue during the whole of foetal life, 

 according to Kolliker. The rnembrana tympani forms across the cleft, dividing 

 it into an outer and inner portion. The pinna, or external ear, is developed 

 from the soft parts covering the first pharyngeal arch. 



Development of the Nose. Two fossae (olfactory fossae) have been already 

 spoken of, which are found below and in front of the ocular vesicles and the 

 upper maxillary projection (Fig. 60, 2, 3). They appear about the fourth week. 

 Their borders become prominent, and the fossae deepen, except at the lower 

 part, where they lead by a groove (olfactory groove) into the buccal cavity. 

 This groove is bounded by the internal and external nasal process. As the 

 superior maxillary projection increases, the olfactory groove is transformed 

 into a deep canal, the rudiment of the two superior rneatus of the nose. As the 

 palatine septum is formed, the buccal cavity is divided into two parts, the upper 

 of which represents the inferior meatus of the nose, while the lower forms the 

 mouth. The soft parts of the nose are formed from the coverings of the frontal 

 projection, and of the olfactory fossae. The nose is perceptible about the end 

 of the second month. The nostrils are at first closed by epithelium, but this 

 disappears about the fifth month. 



The olfactory nerve, as above pointed out, is a prolongation, at first in the 

 form of a hollow stalk, from the anterior cerebral vesicle. 



The Development of the Teeth is spoken of in the body of the work. 



Development of the Skin, Glands, and Soft Parts. The epidermis is produced 

 from the external, the true skin from the middle blastodermic layer (Fig. 53, 

 19, ?,0). About the fifth week the epidermis presents two layers, the deeper 

 one corresponding to the rete mucosum. The subcutaneous fat forms about the 

 fourth month, and the papillae of the true skin about the sixth. A considerable 

 desquamation of epidermis takes place during foetal life, and this desquamated 

 epidermis mixed with a sebaceous secretion constitutes the vernix caseosa, with 

 which the skin is smeared during the last three months of foetal life. The nails 

 are formed at the third month, and begin to project from the epidermis about 

 the sixth. The hairs appear between the third and fourth month in the form 

 of a depression of the deeper layer of the epithelium, which then becomes 

 inverted by a projection from the papillary layer of the skin. The papilla 

 grows into the interior of the epithelial layer, and finally, about the fifth month, 

 the foetal hairs (lanugo) appear first on the head and then on the other parts. 



