BLOOD-VESSELS OF THE CENTRAL NERVOUS SYSTEM. 439 



interpial spaces, and, by means of the adventitia of the blood-vessels, 

 with the subarachnoid space. Aside from those just described, 

 numerous fine, nonvascular, connective-tissue septa penetrate from 

 the pia mater into the substance of the spinal cord. Wherever the 

 pia mater penetrates the spinal cord, the latter is hollowed out, 

 forming the so-called pial funnels. Just beneath the pia there is 

 found in the spinal cord of man a well-developed layer of neuroglia 

 fibers. The posterior longitudinal septum of the spinal cord consists 

 (in the thoracic region) exclusively of neurogliar elements, but in the 

 cervical and lumbar regions the pia also enters into its peripheral 

 formation. 



In the brain, however, the conditions are somewhat different. 

 Here the external layer of the pia disappears, leaving only a single 

 layer analogous to the pia intima of the spinal cord. 



The pia mater enters into the formation of the choroid plexus. 

 This structure consists of numerous freely anastomosing blood- 

 vessels, which form villus-like processes, the surfaces of which are 

 covered by squamous or cubic epithelial cells. This epithelium is 

 regarded as a continuation of the ventricular epithelium, and is cili- 

 ated, at least in embryonic life and in the lower classes of verte- 

 brates. From an embryologic point of view the whole structure 

 represents the brain-wall reduced to a single layer of epithelium 

 (internal epithelial investment) pushed forward into the ventricle by 

 the vessels and pia mater. 



Since the dura and arachnoid accompany the cerebrospinal 

 nerves for some distance, it is obvious that the lymph-vessels 

 of the nasal mucous membrane (see these) may also be injected 

 from the subarachnoid space (compare also Key and Retzius). 



The pia mater, like the dura mater, receives two varieties of 

 nerve-fibers : (i) Vasomotor fibers, which form plexuses in the ad- 

 ventitial coat of the arteries and terminate in the muscular layer of 

 the arteries. These may be traced to the small precapillary 

 branches of the vessels. (2) Larger and smaller bundles of rela- 

 tively large, medullated nerve-fibers, which accompany the larger 

 pial vessels, forming loose plexuses in or on the adventitial coat of 

 the vessels. After repeated divisions these medullated nerves lose 

 their medullary sheaths and terminate in the adventitia of the ves- 

 sels, in long, varicose fibrils or in groups of such fibrils (Huber, 



9.9)- 



J. BLOOD-VESSELS OF THE CENTRAL NERVOUS 



SYSTEM, 



The blood-vessels of the central nervous system present certain 

 peculiarities which deserve special consideration. 



The spinal cord receives its arterial blood mainly through vessels 

 which accompany the spinal nerve roots and through numerous 

 anastomoses from a plexus in the pia mater in which there may be 



