430 THE ORGANS 



cially conspicuous arc large, irregular cells with delicate bodies and 

 large distinct nuclei. They lie upon the connective- tissue bundles 

 partially lining the spaces. 



The Pacchionian bodies are peculiar outgrowths from the outer 

 layer of the pia mater cerebralis, which are most numerous along the 

 longitudinal fissure. They are composed of fibrous tissue, and fre- 

 quently contain fat cells and calcareous deposits. 



Blood-vessels. — The spinal dura and the inner layer of the cere- 

 bral dura are poor in blood-vessels. The outer layer of the cerebral 

 dura, forming as it does the periosteum of the cranial bones, is rich 

 in blood-vessels which pass into and supply the bones. The pia is 

 very vascular, especially its inner layers, from which vessels pass 

 into the brain and cord. 



TECHNIC 



For the study of the structure of the membranes of the brain and spinal cord, 

 fix pieces of the cord with its membranes, and of the surface of the brain with 

 membranes attached, in formaUn-Miiller's fluid (technic 6, p. 7) and stain sec- 

 tions with haematoxylin-eosin (technic i, p. 20). 



THE PERIPHERAL NERVES 



The peripheral nerves are divided into spinal nerves and cranial 

 nerves, the former being connected with the cord, the latter with the 

 brain. Each spinal nerve consists of two parts — a motor or efferent 

 part and a sensory or afterent part. Of the cranial nerves some are 

 purely efferent, others purely afferent, w^hile still others consist, like 

 the spinal nerves, of both efferent and afferent fibres. The efferent 

 fibres of the spinal nerves are axones of cell bodies situated in the 

 anterior horns of the cord (see p. 457, and Figs. 316 and 326) and 

 axones of sympathetic ganglion cells. The former leave the cord in 

 several bundles, which join to form the motor or efferent root. The 

 afferent fibres are peripheral processes of cell bodies situated in the 

 spinal ganglia (p. 435 and Figs. 316, 300). These leave the ganglion 

 and join with the fibres of the motor root to form the mixed spinal 

 nerve (Fig. 316, /). The connection of the ganglion with the cord is 

 by means of the central processes of the spinal ganghon cells, which 

 enter the cord as the posterior root. Among the aff'erent fibres of 

 the posterior root are also found, in some animals, a few efferent 

 fibres (Fig. 316, c), processes of cells in the cord. Some fibres from 

 the spinal ganglion and from the efferent roots form the white ramus 



