STRUCTURE OP THE HUMAN BODY. 51 



it is deficient in fibrin in adult animals, the fibrin pre- 

 dominates, and the gelatine is deficient. 



28. |The peculiar property of muscular tissue is vital, and 

 consists in the power of diminishing its length, or shortening 

 on the application of stimulus J All the motions of the body 

 are performed by means of it, and without its incessant 

 action, respiration, digestion, nor circulation could be carried 

 on for a moment. Tendons, ligaments, cartilages, and bones 

 seem to be mechanical contrivances to aid the muscles in 

 accomplishing their varied purposes, so that the only source 

 of motion in the body is muscular tissue, and the only mode 

 in which motion is generated is by contractility^ 



29. The last primary tissue which we have to describe, is 

 termed nervous. ^It consists of a soft and pulpy matter, of a 

 brownish white colour, and appears to be composed of solid, 

 elongated threads, differing in thickness from that of a hair 

 to the finest fibre of silk^j Like the muscles, the nerves are 

 enclosed in a sheath of condensed cellular membrane, called 

 neurilema, or nerve coat^ and like them also, each nerve is 

 composed of many bundles ; these of many fibres ; and the 

 fibres of many filaments. In one respect the nervous fibres 

 differ from the muscular, and that is, while the muscular fi- 

 bres generally run parallel to each other, those of the nerves 

 cross and penetrate each other, so as to form an intimate in-, 

 terlacement, as represented on the following page. When we 

 come to describe the vascular system, it will be seen that 

 either all the vessels proceed from one large trunk, which 

 goes on progressively to divide and subdivide, until its 

 branches become so minute as to be invisible ; or that they 

 arise by numerous and invisible branches, which unite to 

 form larger and larger vessels, until they ultimately consti- 

 tute only a few trunks. But the muscular and nervous fila- 

 ments never divide and subdivide in this manner. It is the 

 opinion of the best anatomists, that there is a diameter be- 

 yond which they no longer dimmish. That diameter they 

 maintain is quite uniform in each. 



