ORGANS OF THE BODY. 653 



The histogenic occurrences in connection with the development of the 

 cooroid, iris, and sclerotic require further investigation. The rudimentary 

 chrnea has been already alluded to, 133, and the mode of origin of the 

 fibres of the lens has been dwelt upon in the section quoted above. 



The lachrymal glands are developed in the same manner as most other 

 racemose secreting organs ; their cellular constituents springing from the 

 corneous germinal plate. The Meibomian follicles are developed at 

 rather a late period. 



322. 



We come now, finally, to the consideration of the organs of hearing, 

 the last of the apparatuses of special sense which shall occupy our 

 attention. 



On each side of the head we have an internal ear, consisting of the 

 labyrinth, which receives the impressions of sound, a midde ear for the 

 conducting of the waves of sound ; and an external ear, separated from 

 the last by the tympanum. 



We will commence our description with the latter. 



The outer ear is made up of the pinna and external auditory meatus. 

 The texture of its cartilage has been already described 108. The exter- 

 nal skin, likewise, which is here closely attached to the latter; and which 

 is quite destitute of fat, except in the lobe of the ear, needs no farther 

 notice. In the pinna of the ear we find numerous downy hairs, and 

 often considerable sebaceous follicles ( 304), as well as small sudoriferous 

 glands ( 302), especially on the posterior aspect. The peculiarities of 

 the glandule cerminosae of the outer passage, in which sebaceous follicles 

 are also present, have been likewise dealt with in an earlier portion of our 

 work ( 302). The muscles of the pinna belong to the striped class 

 ( 164). 



The membrana tympani consists of a fibrous plate clothed externally 

 by the cutis, and internally by the mucous membrane of the middle ear. 

 Through the so-called annulus cartilagineus it is continuous with the adja- 

 cent periosteum. The coat derived from the skin presents a thin fibrous 

 layer, quite devoid of glands and papillae, which may be found, however, in 

 its immediate neighbourhood. The fibrous plate consists of an external 

 lamina of radiating fibres, and an inner sheet of circular bundles. The 

 elements are those of ill-developed connective-tissue, exhibiting flat anasto- 

 mosing bands with connective-tissue corpuscles (Gerlacli), and perhaps also 

 a few smooth muscle fibres (Prussak). The mucous membrane of the inner 

 surface possesses likewise a very thin fibrous portion, and usually a covering 

 of simple flattened epithelium. The latter extends also into the several 

 depressions of the tympanum, and into the mastoid cells. The rest of the 

 cavity of the tympanum is lined with doubly laminated ciliated epithe- 

 lium. The vascular network of this diaphragm consists of several portions 

 (Cferlach) of an external web formed of radiating elongated meshes of 

 fine tubes belonging to the dermal layer, an internal with rather small 

 meshes supplying the mucous membrane, while the middle fibrous layer 

 of the tympanum, which was formerly said to be devoid of vascularity, 

 presents a third set of vessels according to Kessel. Lymphatics are also 

 present here in large numbers, and nerves likewise, but the mode of 

 termination of the latter has not yet been ascertained. 



The whole of the middle ear, with its various parts and adjoining cavi- 

 ties, is lined bv the same thin, vascular, probably glandless, mucous mera- 



