THE MEDULLA OBLONGATA. 349 



Of the Structure of the Pia Mater. The pia mater is com- 

 monly spoken of as a complete membrane, yet its structure is 

 different from that of membranes generally, inasmuch as it is a 

 net-work, the meshes of which are formed by arteries and veins, 

 and the interstices filled up by a loose, weak cellular substance. 

 Bichat has very justly observed, that the union with the tunica 

 arachnoidea is solely on the part of this cellular substance; 

 whereas, the union with the cerebrum is confined to the ves- 

 sels, which are extremely numerous and very small before they 

 penetrate it, and appear as bloody points when we cut into the 

 substance of the brain. The principal arterial trunks of the 

 pia mater, being the internal carotids and the vertebrate and 

 their branches, are at the basis of the brain ; these trunks divide 

 into smaller branches, on the convolutions and in the fissures. 

 The primary divisions again divide and subdivide into tubes 

 not much exceeding the size of the capillary vessels. In 

 this last state they enter the brain and may be seen very 

 readily, either by a fine injection, or by tearing up the pia 

 mater. 



SECT. II-.-s-OF THE MEDULLA OBLONGATA. 



According to the usage of the best authorities of the present 

 day, who follow in the description of the central parts of the 

 nervous system, the order of their development in the human 

 subject, and also of their appearance in animals, I shall describe 

 the encephalon from below upwards instead of from above 

 downwards. The preference thus shown is, perhaps, princi- 

 pally serviceable in fixing upon the mind the order of growth 

 and appearance, which, according to well established experi- 

 ments, are exactly in the order of importance to life. 



The Medulla Oblongata, also called Bulbus Rachidicus, ex- 

 tends from the superior margin of the first cervical vertebra to 

 the middle of the basilar process of the os occipitis. It becorrfes 

 gradually larger as it ascen-ds, and is about an inch in length, 

 and eight lines wide at its base. It is by no means so cylin- 

 droid as the medulla spinalis, but presents several risings and 

 depressions on its surface. 



The under surface of the Medulla Oblongata is divided longi- 

 tudinally by the middle fissure, a continuation of that on the 



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