454 NERVOUS SY.STEM AND SENSE ORGANS OP AMIURUS. 



PLATE VI.— (Continued). 



the dura mater medullaris. Between these two membranes is 

 the characteristic loose adipose tissue. The roof of the neural 

 canal is here formed by cartilage which has largely undergone 

 ossification both from the centre and the perichondrium. It is 

 invested on the outside by the membrane bone in continuity 

 with the third spinous process. The dorsal musculature lies 

 above the anterior part of the tranverse process of the fourth 

 vertebra and the saccus paravertebralis which contains the mal- 

 leus. ( Vide Fig. 4). 



FiG. 2. — Through the ventral roots of the second nerve (further back than the 

 dorsal) (vide Fig. 8, PI. IV.) which emerge behind the ascending 

 processes of the stapes (right side is somewhat further back- 

 wards). Between the neural arches of the first vertebra is the 

 thickened patch of dura mater which furnishes the membranous 

 wall of the neural canal as well as the dura mater medullaris. 



Yiq. 3. — Shows the dorsal roots of the second spinal nerve emerging above 

 the ascending processes of the stapes. The neural arches of the 

 first vertebra are seen as the articular processes of the stapes. 

 The ventral roots of the second are seen in a canal of dura mater 

 in their backward course towards their foramina. The anterior 

 spoon-shaped process of the stapes is caught just behind the 

 atrium sinus imparis. 



Flo. 4 — Is through the middle of the atria sinus imparis, the partition be- 

 tween which is formed by the thickened dura mater which is in 

 continuity with the connective tissue surrounding the stapes, 

 and that on the outside of the claustra. The separation of the 

 layers of dura mater is less complete, but in the dorsal part of 

 the spinal canal the medullary can be distinguished from the 

 vertebral layers and between them the rest of the membrane is 

 continuous with the loose adipose tissue. The tips of the claus- 

 tra project slightly from above into the atria sinus imparis. The 

 whole of the saccus paravertebralis containing the oily reticular 

 tissue is seen in section, the malleus and incus being connected 

 to the stapes by tendon. 



Fig. 5. — The basi-occipital is here only exposed for a small portion in the 

 middle line owing to the ex-occipitals abutting on it. The section 

 passes through the communication between the cavum and atria 

 sinus imparis, the partition (dm. of last figure) being only 

 caught above. The atria open above into the reservoir (rsi). 

 The lateral wall of the neural canal is formed by the connective 

 tissue separating the claustra from the ex-occipitals. 



Fig. 6. — The reservoir appears in this section, which passes through the 

 foramen magnum, slightly bilobed. The dorsal and the ventral 

 roots of the first nerve are caught in the bony canal through 

 which they emerge. The cavum sinus imparis is cut behind the 

 saccus endolymphaticus. It contains only fluid like the atria 

 and reservoir, no reticular tissue. The posterior surface of the 

 lagena cochleae is just caught. The osseous roof of the cavum 

 thins out before it joins the patch of dura mater. 



Pig. 7. — J U st behind the vagus foramen, and through the strong ventral roots 

 of the first spinal nerve, the membranes of the brain have here 

 the features characteristic of the cranial cavity. The osseous 

 roof of the cavum sinus imparis is somewhat thicker, its walls 

 much thinner. It contains here the thin walled saccus endolym- 

 phaticus. In the foveae sacculi is the section of the pars inferior 



