202 DISSECTION OF THE DOG 



It is obvious that differences exist between the dura mater covering the 

 brain and its continuation over the spinal cord. The cranial dura mater is 

 closely adherent to, and forms the periosteum of, the cranial bones. The spinal 

 dura mater is separated from the vertebrae by a space in which is a certain 

 amount of fatty tissue. The cranial dura mater consists of two layers, from 

 one of which membranous partitions are formed. The spinal dura mater 

 corresponds to the inner of these two layers only. Venous sinuses are present 

 in the cranial dura mater, while the corresponding membrane of the spinal 

 cord is devoid of them. 



Arachnoidea spinalis. — This, the second, tubular investment of the 

 spinal cord is very delicate and transparent. Continuous at its cranial end 

 with the arachnoid of the brain, caudally it forms a loose investment for the 

 cauda equina and ends by joining the other meninges. Between the arach- 

 noid and the pia mater is the subarachnoid space (cavum subarachnoidale). 



Dissection. — Remove the arachnoid mater from a length of the cord and 

 so expose the pia mater. 



Pia mater spinalis. — The pia mater is a tough vascular membrane 

 closely applied to the surface of the cord and continued into the ventral median 

 fissure. 



Ligamentum denticulatum. — Connecting the pia mater with the dura 

 mater and suspending the cord in the dural sheath, the denticulate ligament 

 is attached continuously along the lateral border of the spinal cord. Its 

 connection with the dura mater, however, is interrupted and in the form of 

 pointed teeth, which reach the dura mater between the apertures of exit 

 of successive spinal nerves. 



Medulla spinalis. — The spinal cord is a bilaterally symmetrical cylin- 

 drical mass of nerve matter extending from the foramen magnum, where 

 it is continuous with the medulla oblongata of the brain, to the seventh 

 lumbar vertebra. Its caudal extremity rapidly tapers, thus forming the 

 conus medullaris, beyond which is the filum terminate. The last named is a 

 thin, thread-like structure, mainly composed of pia mater, but containing 

 some small amount of nervous tissue. 



A certain degree of dorso-ventral flattening is observable in most of the 

 spinal cord. This, however, is infinitesimal in the thoracic region. The 

 thickness of the cord is not uniform. Those parts are thickest from which 

 spring the nerves forming the limb-plexuses. Thus it comes to pass that 

 there are two enlargements, a cervical swelling (intumescentia cervicalis) 

 and a lumbar swelling (intumescentia lumbalis). The longest stretch with 

 uniform diameter is the thoracic part of the cord. 



Nn. spinales. — Leaving each side of the spinal cord are thirty-four or 

 thirty-five spinal nerves, classified according to the vertebrae caudal to which 

 they leave the vertebral canal. The number of nerves agrees with the number 



