PERIPHERAL NERVOUS SYSTEM OF VERTEBRATA. 513 



The pia mater, part of the nervous system, is a layer of con- 

 nective tissue covering the latter, in which the blood-vessels of 

 the nervous centres run. It extends iuto the depressions between 

 the different portions. It sends out convoluted vessels (retia) from 

 the large cerebral cleft, which are connected with the roof of 

 the cleft ; these pass into the interior of the lateral ventricles of the 

 prosencephalon. It is continued over and forms a roof for the sinus 

 rhornboidalis of the myelencephalon, where it often forms a vascular 

 plexus. 



The arachnoid membrane is the most variable. In those 

 Fishes, in which the brain fills the cranial cavity, it is a thin layer of 

 connective tissue which scarcely deserves the name of a membrane, 

 for it is as intimately counected with the pia, as with the dura mater. 

 When a larger space is formed between the brain and the wall of 

 the skull, this tissue is either converted into a network filled with 

 lymph (Squatina), or into gelatinous tissue (Scymnus), or it gives 

 rise to fat cells (many Teleostei). In the higher Vertebrata the 

 arachnoid is generally a delicate layer of connective tissue; in the 

 Mammalia it is differentiated as it is in Man. 



B. Peripheral Nervous System. 



§ 385. 



The nerves which are given off from the central organs are 

 divided into spinal and cerebral nerves ; in the Acrania there is no 

 difference between these two kinds. In Amphioxus only one 

 anterior and larger trunk is remarkable for its course, and its 

 numerous ramifications in the anterior end of the body. It is 

 clearly comparable to one of the cerebral nerves of the higher 

 Vertebrata, but it must be here noted that, compared with the 

 Craniota, the whole organisation of Amphioxus is in an indifferent 

 condition. The other nerves of the medullary tube (with the excep- 

 tion of those for the nasal pit and eyes) are similar in character to 

 spinal nerves, and are remarkable for the fact that they are given 

 off alternately from the medullary chord. The similarity in the 

 characters of these nerves leads to the supposition that the difference 

 between the cerebral and spinal nerves, which is seen in the 

 Craniota, is a condition which was acquired by them, when the head 

 was developed. The nerves of Amphioxus have no ganglia, and are 

 formed by single roots ; this, again, is a point in which they differ 

 markedly from the Craniota. As Amphioxus has no " head," in the 

 sense in which we speak of it in the Craniota, we cannot suppose 

 that the nerves are separated into cephalic and spinal nerves. The 

 most we can do is to regard the nerves which belong to the region 

 in front of the posterior boundary of the branchial cavity, as the 

 indifferent equivalents of the cephalic nerves of the Craniota, and 

 the rest of the nerves behind these as spinal nerves. 



