HEAD AND NECK OF THE HORSE 163 



outer wall of the spiral canal of the cochlea, and on the other to and 

 about the edge of the bony spiral lamina. Both ends of the duct are 

 blind. One, the cceciivi ^ vestihidare, lies in the recessus cochlearis of 

 the vestibule ; while the other, the cwcum cupulare, is attached to the 

 cupula and helps to bound the helicotrema. Near its vestibular end, 

 the duct is joined by the ductus reuniens from the lower part of the 

 saccule. 



In section, the cochlear duct is triangular. The roof or vestibular 

 wall, between the cavity of the duct and the scala vestibuli, is formed 

 by the meinbrana vestibularis (of Reissner) that stretches from the 

 periosteum on the apical side of the lamina spiralis to the outer wall 

 of the cochlea. The outer wall is formed by the ligamentuni spirale 

 cochlece, a thickening of the periosteum of the cochlea. The basal or 

 tympanic wall, separating the cavity of the duct from the scala tympani 

 is formed by the limbus ^ lamina spiralis and the laviina basilaris. 

 The limbus is a grooved thickening of the periosteum at, and near, the 

 border of the bony spiral lamina. The lamina basilaris stretches across 

 from the tympanic (lower) lip of the groove to the outer wall of the 

 cochlea, and supports the essential end-organ of hearing, the spiral 

 organ of Gorti ^ (organon spirale Cortii). 



The term perilymph is applied to a watery fluid between the 

 osseous and membranous labyrinths, and the membranous labyrinth is 

 filled with endolymph. 



There still remain for examination the spinal cord and brain that 

 were laid aside at an earlier stage of the dissection. 



The spinal meninges. — Surrounding the spinal cord there are three 

 membranous envelopes — the meninges * — continuous with those that 

 cover the brain. A certain amount of fat covers the outermost of these 

 and must be removed before a satisfactory examination of the dura 

 mater can be made. 



The spinal dura 'inater (dura mater spinalis) is in the form of a 

 dense, fibrous tube, continuous with the cranial dura mater at the 

 foramen magnum, and extending into the sacrum. Close to the 

 foramen magnum it is connected with the ventral occipito-atlantal 

 membrane. The tube is uneven in calibre. Wide in the cervical and 

 lumbar regions (and especially so opposite the atlas), it is narrower and 

 much more closely applied to the spinal cord in the thoracic region. 

 Posteriorly it tapers to its sacral termination. Along each side the 



1 Gcecus [L.], blind. 



2 Limbus [L.], a border or edge. 



2 Alfonso Corti, an Italian anatomist, 1822-1876. 



4 fXTJuiy^ (nieninx) (Gr.), any membrane, but especially of the brain (Hippocrates). 



