270 
DR, U. PRITCHARD OH THE COCHLEA 
The lower surface of the lamina slopes upwards and inwards from the floor of the 
tube, curving round so as to become almost parallel to the upper surface. It also 
terminates in a sharp edge the lower lip of the marginal sulcus ; this lower lip projects 
much further inwards than the upper. 
The free margin of the lamina is deeply grooved, forming the sulcus just alluded to. 
The lamina ossea does not extend to the apex of the cochlea, but terminates before¬ 
hand in a curved border, which forms part of the boundary of the oval communication 
between the scala vestibuli and tympani. As the lamina approaches its anterior 
termination it gradually loses its wedge-shape and ceases to be attached to the outer 
wall of the tube ; so that at- last it simply consists of a plate springing from the floor 
and curving - round so as to become horizontal at the limbus. 
o 
The substance of this lamina ossea is modified bone, like the lamina spiralis ossea of 
typical Mammals. It is not so dense as ordinary bone, and chiefly consists of what 
may be called ossified stellate fibrous tissue. It is well supplied with blood vessels, 
which have a tendency to become tortuous. The substance of the limbus is again 
modified, being more homogeneous in structure and not unlike the matrix of hyaline 
cartilage ; moreover, its surface is deeply grooved transversely, giving it a dentate 
appearance, the free edges of the teeth forming the sharp upper lip of the marginal 
sulcus. 
Through the lamina, close to its lower surface, run the bundles of nerve fibres on 
their way to the organ of Corti ; these will be described further on. 
From this brief description it will be seen that the lamina ossea agrees entirely in 
structure and position with the lamina spiralis ossea of the ordinary Mammal. 
The Scala Media or Ductus Cochlece .—This constitutes the membranous labyrinth of 
the cochlea. Taken as a whole it is a triangular tube rather more than ^ inch in 
length which is coiled round at the apex of the cochlea, and is connected posteriorly 
with the saccule of the vestibule by means of a circular tube with very delicate 
membranous walls, in a manner that I have not completely made out. As already 
mentioned, it is triangular in section. The floor or base of the triangle is formed by 
the limbus of the osseous lamina and by the membrana basilaris, which stretches from 
the lower lip of the marginal sulcus to the ligament of the cochlea. This ligament is 
a large cushion of connective tissue adherent to the inner wall of the cochlea, and 
forms the inner boundary of the scala media. The third and upper side of the triangle 
is formed by the delicate membrane of Reissner, which stretches from the upper part 
of the ligament to the upper surface of the osseous lamina, being attached just where 
the elevation of the limbus commences. 
The whole of the inner surface of the scala is lined with epithelium, which is vari 
ously modified according to its position,—that part of this epithelium which lies on 
the central portion of the floor being developed into that complex form of nerve 
epithelium known as the organ of Corti. 
The membrana basilaris is a strong membrane made up of three layers. The lower 
