THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 615 



and on the other reposes with its plate-like base in the fenestra 

 ovalis. 



The view here adopted that the stapes belongs to the second, the 

 malleus and incus to the first visceral arch is supported by the 

 important relation of the nerves in their distribution to the musculus 

 stapedius and to the tensor tympani, as has recently been rightly 

 pointed out by RABL. The muscle of the stapes is supplied from the 

 nerve of the second visceral arch, the nervus facialis ; it forms part 

 of a group embracing the m. stylohyoideus, and the posterior belly 

 of the digastric; the muscle of the malleus receives a branch of the 

 trigeminus, irhich is the nerve of the mandibular arch. 



The separation of the territories of innervation prevails, moreover, with the 

 muscles of the palate, one of which the tensor veli palatini arises in front 

 of the Eustachian tube the remnant of the first visceral cleft and is 

 therefore supplied by the n. trigeminus, whereas the levator veli palatini and 

 azygos uvulae lie behind it, and, because belonging to the hyoid arch, receive 

 branches from the n. facialis (RABL). 



At first all the auditory ossicles lie imbedded in a soft gelatinous 

 tissue outside the tympanic cavity, which still has the form of a 

 narrow fissure. These conditions are not altered until after birth. 

 The tympanic cavity, taking in air, then becomes enlarged, its 

 mucous membrane is evaginated between the auditory ossicles, 

 and the gelatinous tissue just mentioned undergoes a process of 

 shrinkage. Auditory ossicles and chorda tympani thus come to 

 lie apparently free in the tympanic cavity ; accurately considered, 

 however, they are only crowded out into it, for even in the adult 

 they are enclosed in folds of the mucous membrane, and by means 

 of these they preserve their original and genetically established 

 connection with the wall of the tympanic cavity. 



Up to the present stage the construction of the head-skeleton is, 

 on the whole, simple. In the third stage of development, on the 

 contrary, upon the beginning of the process of ossification, it attains 

 in a short time a high degree of complication, which is effected 

 especially by the development of two entirely different kinds of 

 bone, one of which has been called primordial bone, the other 

 covering bone (Deck- oder Belegknochen). 



Primordial bones are such as are developed out of the cartilaginous 

 skeleton. Either there arise centres of ossification within the carti- 

 lage after softening and dissolution of its matrix, as was described 

 in the ossification of the vertebral column, the ribs, and the sternum, 

 or the perichondrium alters its formative activity, and secretes, in 



