9 o COMPARATIVE ANATOMY 



It should be noticed that epithelia play a very important 

 part in the economy of the animal, and that the cells of the 

 outer skin, as well as those lining the gut, are modified in 

 diverse ways for the performance of diverse functions. The 

 endothelia lining the ccelom, the cavities of the heart and 

 blood-vessels, the lymph-spaces, etc., must be considered as 

 belonging to a different category from the other two epithelia, 

 since they are formed from the middle of the three embryonic 

 layers, and are differentiated in one respect only viz. in respect 

 of their function as components of a limiting membrane. 



In the nervous system of the frog two principal elements 

 are to be distinguished, nerve ganglion cells and nerve-fibres. 

 The former are found in the cortical substance of the brain 

 and in the so-called grey matter surrounding the central canal 

 of the spinal cord and forming the floor of the fourth ventricle 

 in the medulla oblongata. They are also found in the various 

 ganglia which have been described; such as the ganglia of 

 the dorsal roots of the spinal nerves and the sympathetic 

 ganglia. Nerve ganglion cells, however, may be most easily 

 studied in the spinal cord of a larger animal, such as the 

 ox. In that part of the grey matter known as the anterior 

 horn of the spinal cord a few large cells may be seen in 

 transverse section or may be separated by suitable methods 

 of maceration. Each ganglion cell has a body composed of 

 a finely granular cytoplasm, which sometimes appears to be 

 indistinctly striated or reticular in structure. In the centre 

 of the cell-body is a nucleus, which usually has the form 

 of a relatively large, clear, rounded vesicle with a distinct 

 nucleolus. The intra-nuclear network of chromatin is not 

 usually well defined. The most characteristic feature in a 

 nerve-cell is the prolongation of the cell-body into long 

 branching processes, which show a distinct but fine longi- 

 tudinal striation, the striae of one process being continued over 

 the peripheral parts of the cell-body to unite with the similar 

 striae of an adjacent process. A nerve-cell from the anterior 

 horn of the spinal cord of the ox or other mammal has 

 numerous processes (see fig. 16, A), which break up into 

 branches at a little distance from the cell-body, and the bran- 

 ches sub-divide again till they end in very fine nerve-fibrils, 

 which inosculate with fibrils from adjoining cells. One process, 

 however, is not branched, and may be traced into direct con- 



