DISSECTION OF THE DOG 201 



distinguished by the possession of pigment. The rest of the mucous membrane 

 of the lateral wall, like that of the septum, is divided into an upper olfactory 

 (regio olfactoria) and a lower respiratory region (regio respiratoria) . Though 

 the olfactory mucous membrane is thicker than that over the rest of the wall, 

 the limits of the regions are not recognisable with the naked eye. The 

 nerves supplying the lateral wall of the cavity are similar in origin to those 

 of the septum. 



Dissection. — Remove the muscles which still remain attached to the spinous 

 and transverse processes of the vertebrae. Then open the vertebral 

 canal by cutting through the arches of the vertebrae just within the 

 articular processes. Those parts of the bones which are now removed 

 must be left attached to each other : in other words, the ligaments 

 connecting them must be preserved intact for future study. 



This dissection will expose the spinal cord within its membranes, the roots 

 of the spinal nerves, and certain blood-vessels. 



Little difficulty will be experienced in demonstrating a series of spinal 

 veins emerging from the vertebral canal by the intervertebral foramina ; and, 

 if the spinal cord be raised somewhat from the bodies of the vertebrae, 

 their connection with a longitudinal vein on each side will be seen. Into the 

 longitudinal vessels open veins which drain the bodies of the vertebrae. 



Unless the subject be well injected it will be difficult to detect the small spinal 

 arteries which enter the vertebral canal by the intervertebral foramina. These 

 are derived from the vertebral, intercostal, lumbar, and sacral vessels, and are 

 distributed to the spinal cord, the spinal meninges, and the bodies of the vertebrae. 



Meninges. — Surrounding the spinal cord are three membranous envelopes — 

 the meninges — continuous with those which cover the brain. 



Dura mater spinalis. — A certain amount of soft fat covers the outermost 

 of the spinal meninges and must be removed before a satisfactory examination 

 can be made. 



The spinal dura mater is in the form of a dense, fibrous tube, continuous 

 with the cranial dura mater at the foramen magnum and extending into the 

 sacrum. The tube is of uneven calibre. Wide in the cervical and lumbar 

 regions, it is narrower and much more closely applied to the spinal cord in the 

 thoracic region. Aborally it tapers rapidly to its sacral termination. Along 

 each side the roots of the spinal nerves pierce the wall of the dural tube, and 

 in doing so derive a thin fibrous sheath therefrom. 



Dissection. — The dura mater should now be carefully slit along the middle 

 line. This having been done, the smooth nature of the interior of the 

 tube will be manifest. The interval between the dura and the arachnoid 

 is known as the subdural cavity (cavum subdurale) . 



