644 THE CENTRAL NERVOUS SYSTEM [CH. XLIV. 



brain, and between the cerebellum or small brain (B), and the rest of 

 the nervous system. 



The mid-brain comes next (a, I), and this leads into the peduncles 

 or crura of the cerebrum (A), the largest portion of the brain. 



Through the brain runs a cavity filled with cerebro-spinal fluid 

 and lined by ciliated epithelium; this is continuous with the central 

 canal of the spinal cord. In the brain, however, it does not remain 

 a simple canal, but is enlarged at intervals into what are called the 

 ventricles. There is one ventricle in each half or hemisphere of the 

 cerebrum ; these are called the lateral ventricles, they open into the 

 third ventricle, which is in the middle line ; and then a narrow canal 

 (aqueduct of Sylvius) leads from this through the mid-brain to the 

 fourth ventricle, which is placed on the back of the bulb and pons, 

 which form its floor ; its roof is formed partly by the overhanging 

 cerebellum, partly by pia mater. This piece of pia mater is pierced 

 by a hole (Foramen of Magendie), and so the cerebro-spinal fluid in 

 the interior of the cerebro-spinal cavity is continuous with that which 

 bathes the external surface of brain and cord in the sub-arachnoid 

 space. The fourth ventricle leads into the central canal of the 

 spinal cord. Taking origin from the wall of the cerebral ventricles, 

 and running down the central canal of the cord, is a fine thread 

 called Eeissner's fibre ; the function of this thread is entirely 

 unknown. 



Speaking generally, there are two main collections of grey 

 matter that on the surface, called the cortex, and that in the interior, 

 bordering on the cerebro-spinal cavity, and subdivided into various 

 masses (grey matter of cord, floor of fourth ventricle, corpora striata, 

 optic thalami, etc.), whose closer acquaintance we shall make presently. 



But such a complex brain as the human brain does not obtain 

 throughout the vertebrate series. The lower one goes in the scale, 

 the less important and large does the cerebrum become, until in the 

 fishes the cerebral hemispheres are practically absent. It is the large 

 size and convoluted grey cortex of these hemispheres which dis- 

 tinguishes the higher from the lower vertebrates. 



A comparative study of the brain in different animals has been 

 most valuable in the elucidation of the functions of its various parts. 



It is in fact possible to-day to foretell, if one knows the habits of 

 an animal, what sort of brain it possesses The converse is also 

 true ; given the brain of an animal, one can describe its habits and 

 mode of life very fairly accurately. For instance, animals which 

 rely largely on the sense of smell for their prey will have a large 

 olfactory area ; whereas in such animals as the porpoise, which have 

 no sense of smell, the olfactory area of the brain is absent. Animals 

 with keen vision will have a large visual area in their brains ," animals 

 of nocturnal habits, or who live underground in the dark, will have 



